# What happens in a turbo at very low boost levels?



## Iceman666 (Mar 26, 2001)

In my research on twin charging, ive come across situations where a turbo might need to flow say 250 CFM at 0 psi (or a 1:1 compression ratio) and flow small boost levels such as 280-300CFM at 2-5 psi(or a 1.2 compression ratio)...this would be on the low boost setting BTW
now on a "large" turbo..something like a TO4e lets say, can the turbo sustain low boost levels like that? Or would they surge around from 3 psi down to 0 and back up at times? Basicly how steady would the boost level be?


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

That's like installing a T04 on a V8 with 350whp and not stomping on the gas, but doing a part-throttle drive to say 5000RPM. Not a problem.
Speedy G


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

ok so if that low flow point is out of the plot on the turbo chart(below the blob) its not a big deal?


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## slappynuts (Feb 27, 2005)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

An improperly sized turbo will have this problem.Too small of a turbine and a large compressor will be a problem.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (slappynuts)*

well that is my problem in trying to size a turbo...
for the low boost setting, it would only need to produce 2-5 psi, but it would need to be a steady 2-5 psi.
high boost would be right in the middle of the blob.


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## slappynuts (Feb 27, 2005)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

So are you trying to run under 5lbs of boost with a large turbo?Why are you trying to do that?


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (slappynuts)*

cuz when the eaton recompresses the air it will be 15-20 psi


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## GTijoejoe (Oct 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_well that is my problem in trying to size a turbo...
for the low boost setting, it would only need to produce 2-5 psi, but it would need to be a steady 2-5 psi.
high boost would be right in the middle of the blob.

then you'll be looking to use a turbo with a small compressor. You wouldn't need any where near a t4 compressor. If you needed a T4 hot side you should use a hybrid t4/t3.


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (GTijoejoe)*

Damn dude, i told you it was too complicated FOR YOU. No offense should be taken by you, But this is not a task for a beginner.
There is no way to feed a device at a constant pressure with a turbo if it FLOWS at a linear proportinal rate to shaft speed, ( your roots SC )
With a device which flow and pressure both are proportional to the CUBE of the tip velocity ( turbo )
This is why, In the first few times i answered your questions i told your about how diesels' supercharger DRAWS (vaccum) the air thru the turbo at low speeds, The turbo does not pressurize anything untill something resticts its FLOW.

_Modified by Vr6Fidelity at 12:23 PM 11-1-2005_


_Modified by Vr6Fidelity at 12:24 PM 11-1-2005_


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_cuz when the eaton recompresses the air it will be 15-20 psi

Where's the EATON, before or after the turbo? The turbo cares about the ratio of its inlet to its outlet, not absolute pressures. It does howerver care about air mass flow. Let's say you have an M60 supplying a T04 60 trim and you have it set so that you get 15psi to 3500RPM, then the T04 spools up and takes over. The T04 will suck the air through the SC and the post-SC pressure will drop. The flow limit will be reached ideally when the post-SC pressure drops to 0psi at redline, and the T04 has a P2/P1 of 15psi at redline.
That's not efficient though. I'd say the best solution is for the SC to disengage with a clutch & vents to the atm when the turbo spools up.
Speedy G


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

the hell is your problem.....the hole piont of this is as a learning experiance.
yeah the charger will suck air through the turbo at first, but once the turbo is up to speed it can start to push air at a rate set by a boost controler.
if it worked the way you say, a compound charger setup would never be able to preduce more boost then the supercharger could put out....and they do...


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

[Edit] I had just said something really stupid[/Edit]
Good Page: http://www.rbracing-rsr.com/turbotech.html
You're forgetting variables which are efficiency and pressure. A supercharger can initially pull air through a turbo, but a turbo can later push air through the supercharger. Air Mass is what makes power, and a supercharger has a fixed air volume that it pushes into the engine. If the air is pre-compressed 800CFM of air can be 800lbs/min of air flow, but if it's not compressed the same 800CFM can be only 400lbs/min of air. Remember neither turbos nor SCs care about absolute pressure, they care about the ratio of pressure between their outlet and the inlet. If an SC receives 800CFM at 15psi from a turbo, it could flow the 800CFM without compressing it further, but you get the disadvantages of both items.
Why super & turbo charge an engine? because you get the efficiencies of each. The supercharger is inefficient at high boost and RPM. A turbo does great at high boost, but needs exhaust flow to spin. 15psi sounds too much for a supercharger btw. Check out the lysholm flow maps. Peak efficiency happens at barely 50% of its RPM range and at less than 15psi. Also, it only achieves 64% efficiency, which is bad. A turbo however, like a GT3071R can take 36psi at the rim of its operational band and still maintain 65% efficiency. So with a dual setup you could possibly get 15psi from 1000-3000RPM only on the SC and 36psi with the turbocharger.
Inefficiency - It would be inefficient for the air to go through the SC and then into the turbo. You'd get the inefficiencies of both. You could do a Y setup like a bi-turbo mazda RX-7, and use a clutch to disengage the SC once the turbo has spooled up and achieved boost pressures of more than say 17psi. 
Speedy G


_Modified by Speedy G at 6:19 PM 11-1-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

see my understanding was that the supercharger would recompress air at its given compression ratio(which BTW wont be 15 psi i was just using that for calculations) So if its getting atmospheric air you get 15 psi out the end, but if it gets 3 psi over atmospheric air(say from the turbo) it takes that and recompresses it at its compression ratio and you wind up with 20 psi.
I plan on running a few different configurations, but to start with the turbo will be run into the charger.
My initial plan was to use the turbo to push the charger so it gets no lag, but further reseach says thats not how it works, due to the set compression ratio.
if i can truely run "lets say" 15psi into the charger and get 15 out which no charger lag, then my low turbo boost problem is gone....but how would i control the turbo boost in this case?


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

BTW holy crap that site rocks!! been looking for something like that forever!
thanks


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## sold on expense (Mar 22, 2003)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*


_Quote, originally posted by *Speedy G* »_Good Page: http://www.rbracing-rsr.com/turbotech.html


Awesome page


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (sold on expense)*

The thing is ice i explained all this to you the first several times you brought this up.
http://forums.vwvortex.com/zerothread?id=2241775

_Quote, originally posted by *Vr6Fidelity* »_
Twin charging started on large diesels, mainly because of problems with both units.
A supercharger big enough to stand alone at high revs draws massive crank HP.
A turbo small enough to have boost off idle is way too small at high revs, causing excessive back-pressure and power robbing cylinder reversion.
The solution was to build a low-pressure-rise, high flow supercharger. The superchargers low pressure ratio removed alot of the crank losses, and the turbo feeds the air into the SC at elevated pressures but not as high a boost as if it was turbo alone.
So the air at 15 psi goes into the turbo and gets multiplied to lets say 25 psi. Then the supercharger compresses it again at 2:1 to get 50 PSI. 
The turbo's top end flow compensates for the SC running out of juice at high rpms, and the SC takes care of business off idle until the EGT gets going to get the turbo up to speed. 
This is not at all how the new VW twin-charged engine works. As stated earlier it simply has a valve and a clutch to shout down the SC once the turbo is singing along.


REMEMBER THAT???
Perhaps you should look up "pressure ratio"
If you really want to make this a viable, good running setup the only logical cohice is two, paralell systems with an rpm or pressure activated changeover. 
Perhaps you might want to read a book before making post after post about your pipe dream you have no plans to actually build.










_Modified by Vr6Fidelity at 12:10 PM 11-2-2005_


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## Scooter98144 (Jul 15, 2002)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

I can understand the idea of wanting to learn about twin charging. But why would you waste your own money on something that more than likley will never work. Especialy when you can just get a nice well tunned turbo kit?


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Scooter98144)*

hmm probly because it will be a fun learning experiance, and maybe Its something that i feel is worth while. 
yes i actaully do plan on doing it i have alot of the parts already, so screw you, i dont jump on your back for buying some ugly couch with your money.
and for the last damn time, i know everyone thinks its a bad idea....duh...how about you be nice, answer some questions a curiouse student has and lighten up...damn


_Modified by Iceman666 at 10:33 PM 11-2-2005_


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## ErosNJ (Oct 21, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

The New Golf Gti will feature a 1.4L Supercharged+turbocharger.
Supercharger operation at off idle and up to 2500 3000somth.. rpm then the turbo will take over. Output? 180PS with a torque curve from low end..
All that out of a 1.4L.








Over here the second stage 1.8T makes 180ps....
2L FSI version is now offered at 200bhp.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (ErosNJ)*

wow they are already selling them??..cool
ill have to see if i can use they check valve off the TSI setup


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

I don't think compound charging is a bad idea myself ... I just don't think it's anything for a first timer to try to accomplish... I'm making an assumption ... I don't recognize your name... I suppose there's a chance you're one of the most accomplished weekend warrior mechanics... 
I don't quite agree with the idea that the only viable option for twin-charging is to have a changeover point. However, it is a waste of time to do all the work to only achieve 15psi ... apart from the fact that we've never discussed what sort of power output your looking for which ultimately the only consideration for the math your needing to do for turbo/sc selection. there are turbo all over the damned place that will push 15 pounds of compressed air into what ever engine you want to choose.
The benifit of compound charging is the ability to achieve rediculous levels of compression with minimal effort from the system or at extremely low RPM levels. 
Large diesel truck are utilizing compound charging at an industry due to ever tighten EPA standards. Because of how diesel engines are build they can withstand stupid amounts of cylinder pressure. From conversations I've had with Corky Bell, he's mentioned that he's seen numbers as high as 250psi from a triple compound turbo setup... this was a tractor pull tractor. However, because they can't simply drop a 20liter diesel into the big trucks any more and not give a second thought to air contaminants or fuel economy, they're forced to find better ways to make more power... so they make a smaller engine, and then increase the virtual displacement by pumping 60-80psi air into the engine at 1000rpm. This is where the compound charging comes in. In most typical setups you will have a small turbo feeding a larger turbo. The smaller of the two is simply regarded as a multiplier. The air is first compressed to 1 bar... 14.7psi ... which is of course a 2:1 pressure ratio ... which is honestly pretty tame in this case. Then the second turbo takes that air and compresses it again to 1 bar above the psi that the air it received was at. After that compression step you end up with a 4:1 pressure ratio... assuming both turbos were set to 15psi... 4:1 ends up at 60psi ... this is the benefit of compound turbo'ing. 
One reason why people nay say so much in regards to compound turbo'ing a gasoline engine is because gasoline engines simply cannot handle the type of pressure a diesel can ... and only about 1% of people who talk about building something like that are honestly going to build the block right to be able to support the kind of numbers that something like that would produce.
Now in regards to feeding a SC with a turbocharger ... because the other way around is just silly (read : the turbocharger in eventually will begin to pull air through the supercharger which is no good, air starvation happens then). All turbochargers and superchargers are are pumps. Nothing special, one just does the job differently from the other. The benefit of compressing the air going into a supercharger is that a: you get benefit of the SC pushing linear power from the start... but then producing exponential gains in air flow as the turbo begins pushing air into the charger. 
Matter of fact, according to theory you could reasonably end up with an engine close to 100% efficentcy... nownow don't laugh ... the turbo runs off the exhaust gases right? Use that power to push air into the supercharger which is create parasitic lose against the crank due to being ran by belt, however, the additional power create by the compressed air being pushed in easily replaces the parasitic loses from the SC belt... more air being pushed into the engine results in more exhaust to push the turbocharger which means more gets pushed into the SC ... and you see the cycle going on and on. 2 things that will kill you a: heat ... b: the pressure inside of the SC ... let's say you pump 15psi into a machine that was designed to pump ambiant air pressure to no more than 17-18 psi, and ends up pushing 15psi air to 50-60psi ... you better do something with the seals on that thing. Heat? Well we all know about heat ... it's just bad no need to talk about that.
For shear practice? Buy a G60 and plug a turbocharger in place and blow something up and rebuild it, make it stronger and blow it up ... each time you'll learn oodles about the engine and how it needs to be built ... 
but first ... build a regular turbo car and get decent consistant numbers without blowing anything up ... then get extravagent.


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

Some good info there! 250psi!!! How is that possible??? Remember P2/P1... A turbo boosts air pressure by say 15psi. So if you start out with 235psi at the inlet, you get 250psi at the outlet? Wrong. That's why compressor maps use the P2/P1 ratio.
Let's say a SC can take 500CFM and is efficient at a pressure ratio of 2. Your 1.8L engine will have twice the air density from 1000RPM and it would normally make 220whp with that setup. Essentially your 1.8L became a 3.6L engine. Now let's strap on a turbo to feed that SC. It would be a turbo meant for a 3.6L engine. Now we're talking supra turbos. Now you want to double that engines power again so you're looking for a turbo that can flow around 44lbs with a pressure ratio of 2:1... So you take a GT35R and set a pressure ratio of 2:1 and you double your HP to 440whp? Nope. 
The truth is you might actually only end up with 180whp with just the supercharger, and 280whp with the turbo feeding the SC due to heat loss and the inefficiencies of the 2 compression stages. Also, that's even optimistic, because as air density increases, the efficiency of the latter stage is decreased. Oh, and you're running 4 atmospheres or 60psi btw. Each compression stage also takes power away from the engine, it's not free.
The multiplicative property of the compound stages also multiplies their inefficiencies. I'm not saying it won't work. I'm saying it's not as easy as pie. Ballpark figures:
180CFM engine = 110whp let's say
360CFM SC set at 2:1 -> engine makes 220whp with 100% efficiency
reality says -> engine makes 180whp because SC is 66% efficient
720CFM turbo set at 2-> engine makes 440whp with 100% efficiency
reality says -> engine makes 280whp because turbo is 78% efficient & SC is 66% efficient
Compression stage efficiency IS proportional to power btw.
Speedy G


_Modified by Speedy G at 11:55 PM 11-3-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

pressure ratio for 235 in and 250 out would only be 1.06:1, not the usual 2.02:1 for 15 psi, but the boost conrtoler for the turbo actually activly changes the pressure ratio dependent on output...right?
BTW thank you for all the information!
For clarification my ultimate goal is to swap in a 16v, run 5-8 psi off the charger and use the turbo to boost to 20-25 total psi. If i do run the charger into the turbo, there would be a check valve parallel with the charger that would open when vacuume is sensed to supply more air to the turbo(like the new TSi motor) actuation is gonna be a bitch though.
Back to my main question. When the supercharger has a set pressure ratio of 1.7:1(boosting to 10 psi) and I only want to run 15 psi(for low boost rain / daily driving setting) the turbo only needs a pressure ratio of 1.2:1(boosting about 3 psi) to bump the supercharger output to 15 psi.
so at this point will the pressure output of the turbo be stable? If not what exactly is happening(stalling/surging)??


_Modified by Iceman666 at 8:28 AM 11-4-2005_


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*


_Quote, originally posted by *Speedy G* »_Some good info there! 250psi!!! How is that possible???

_*edit* Let add here at the beginning the formula for pressure ratio is (PSI+14.7)/14.7. You add the desired or known compression to ambiant (hence absolute pressure) and then divide it by the ambiant._
Here's how ... and I think you're doing your math wrong... the pressure from one does not simply add to the pressure of the second. The pressure of ambient air at sea level is 14.7psi ... colorado and other places muchly above sea level are a little bit different ... say 13.7 <--- severe guestimate. Anyhow ... ambiant is 14.7psi, when you push 1 bar, the measurement is technically an addition of 1 times ambiant... in result an abosolute pressure of 29.4psi. However, ambiant is consider a 0 point, or base line when engine operation is considered so it's simply refered to as 14.7 pounds of boost. 
Now instraments of forced induction are simply pumps, the compressor doesn't care how much AIR is flowing through it but rather to what percentage it are compressing the air, the turbine of course cares about how much air, but that's more a function of powering the pump... That being said everything is relative the compressor's Pressure Ratio that it's being controlled to produce via the boost controller on your system. If you have your boost controller set to 15 psi, you're making the compressor operate at a pressure ratio of 2.02:1, 14.7psi is 2:1 ... for math sake we'll use 15psi... 
Air coming out of the compressor set to a PR of 2.02 is compressed to 15 pounds per square inch. In a compound turbo setup you're are then feeding that air into the compressor side of another turbo or supercharger. Hence the difference between compound turbo'ing and twin turbo, or sequential charging (they refer to to compressors being in parallel, whether by means of utilizing a clutch setup or some sort of Y connection). Compound turbo'ing puts the compressors in a series setup (harking back to some basic electronic concepts ... ie parallel and series circuits)...... back to the compressed air going into the 2nd compressor. Like I said the compressor doesn't really care how much air if flowing through it only by what pressure ratio it's compressing the air. That being the case and to make the math easy, let's say that the second turbo is compressing the air with the same pressure ratio as the first: 2.02:1... first turbo was compression to a PR of 2.02:1 ... when you add ratios you mutiply the 2 sides of the ratio against the other ratio ... ie a:1 & b:1 would look like a*b:1 ... so 2.02:1 and 2.02:1 would end up being a PR of 4.04:1 as compared to the ambiant pressure of the air entering the entire system. That ambiant pressure was 14.7 (rounded to 15) and 15 * 4.04 is approx 60psi. 
With that background let's work the 250psi backwards ... that was with 3 turbo's in series with each other ... deviding 250 by 14.7 will give is the total pressure ratio of the system ... which is 18.00... each turbo is going to compress the air being fed to it ... in this case it would simply be a case of the first 2 turbo's pushing compression ratios of 3:1 and the last one pushing a PR of 2:1. The math would look like this ... 3:1 and 3:1 results in 9:1, then the last turbo of 2:1 and 9:1 results you with a PR of 18:1... I know it's seems rediculous... and it would be on a gasoline engine with the turbo's that we're used to playing with. These guys of course know this crap backwards and forwards and have building this junk for longer than some of us have been living. That's why guys like Corky Bell know about them rather than use . 
To achieve a resultant 25psi ... here's what you'll want to do ... find the relation of 25psi as compared to ambiant ... 25psi is a pressure ratio of 2.7:1 ... I can't remember at this second what you said your SC's PR was set to so for illustration I'll assume the turbo and SC are producing the same PR ... divide 2.7 by 2 and you get 1.35PR ... which means that each compressor is adding a compression of 5.15psi. 
I've got more to add but I'm supposed to be at work in 45 minutes and I've still got a shower to take and a 20 minute drive ... adios... 


_Modified by ExtremeVR6 at 11:26 AM 11-4-2005_


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

I'll try to take a minute at work and do some numbers up and give something more substantial ... 

_Quote »_pressure ratio for 235 in and 250 out would only be 1.06:1, not the usual 2.02:1 for 15 psi, but the boost conrtoler for the turbo actually activly changes the pressure ratio dependent on output...right?

235 wouldn't be going in ... it's be 9*14.7-14.7=Psi in ... and then it's be PSI:in*pressure ratio set by the boost controller.

_Quote »_Back to my main question. When the supercharger has a set pressure ratio of 1.7:1(boosting to 10 psi) and I only want to run 15 psi(for low boost rain / daily driving setting) the turbo only needs a pressure ratio of 1.2:1(boosting about 3 psi) to bump the supercharger output to 15 psi.

I don't think your math is being done right ... What you need to do it decide power output levels that you want to achieve. PSI doesn't equal power ... a T3 could be pushing 25 psi into a V8 but will only add about 50hp ... on the other hand a T70 on a Vr6 needs only 5-10 psi to achieve power numbers north of 300 ... 
Do some reverse engineering, figure the numbers your wanting, and the static components that you already have and are independant variables. Then figure the compression that will be needed to achieve those numbers. 

_Quote »_so at this point will the pressure output of the turbo be stable? If not what exactly is happening(stalling/surging)??

With proper turbo sizing no this won't be a problem... but it will limit higher levels of operation... something you select to operate safely at lower pressure ratios won't be able to maintain their performance or operation if pushed to higher pressure ratios... more to come.


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

Well at least Extreme Vr6 knows what he is talking about. Everthing he said is correct 
Speedy G I have no idea what you are tying to say but your pretty much just wrong all over the board. I also notice you love to toss large words about for no reason. 
Iceman 666 , first of all, AGAIN get a book, read it, post stupid questions later. about 4 people have explaned pressure ratio to you and you stell seem to have not grasped it.
Have you even thought about mass flow? Does your proposed charger stay in the proper range? You do realize a charger that is too big will produce way more than its rated pressure if the engine cannot accept the volume. The oppisite is also true.
What about the turbo? It is going to provide mass flow at a rate proportional to shaft speed^2. Your superchargers mass flow output is linear with speed. This is why they do not work well in series. Thats why i told you go paralell with your little experiment.
You will also not be able to find a 4 psi turbo with an acceptable range of mass flow or on/off performance. This turbine will also be very small, causing backpressure and reversion.
There are simply dozens of reasons why this is a terrible idea. Why do you think its not done???


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*


_Quote, originally posted by *Vr6Fidelity* »_Well at least Extreme Vr6 knows what he is talking about. Everthing he said is correct 
 
Thanks ... I try ... 

_Quote »_Have you even thought about mass flow? Does your proposed charger stay in the proper range? You do realize a charger that is too big will produce way more than its rated pressure if the engine cannot accept the volume. The oppisite is also true.

This is an extremely valid concern, considering the fact that the 16v is a 1.8liter engine, there aren't many SC's designed to operate at that level. There's of course the jackson SC and the neuspeed sc, and the g60 ... but you said eaton ... I haven't studied them very much, but I don't recall hearing about something for a small displacement engine.

_Quote »_What about the turbo? It is going to provide mass flow at a rate proportional to shaft speed^2. Your superchargers mass flow output is linear with speed. This is why they do not work well in series. Thats why i told you go paralell with your little experiment.
You will also not be able to find a 4 psi turbo with an acceptable range of mass flow or on/off performance. This turbine will also be very small, causing backpressure and reversion.

What vr6fidelity is getting at here is the fact that turbo's aren't designed to operate at 4psi... most of the time the intention for them to operate between 1.4 PR and 2.2. That is one huge reason why he's expressing such dismay towards the project. You plans puts you looking for operation in a range that no one designs the turbos for. 
I understand that Mechanical Engineering school is all about taking something and disecting it as much as possible to learn from the process. One part of your project make an illustration of the applicability of design verses build. By that I mean, the differences between the demographic/industrial explanations as to why and what equipment is being used. I believe that what you'll find is that a compound setup simply isn't necessary nor feasible for the levels you're talking about. 
However, I said that I would provide some math for ya ... here ya go... 
You said that the supercharger is rated to 1.7 PR. To result in 25 psi (2.7:1 PR) you need to find the correspondant PR to 1.7:1 to result the 2.7... yup divide 2.7 by 1.7 you need a PR of 1.5 (8.64psi) now you need to find a turbo that is correctly sized for the 16v engine ... I'd recommend something along the lines of a t3-45. especially since you're going into a SC after that. 
One thing I'll challange you on is the reason behind trying to provide a "rain mode" or "daily driving" mode for a school excersize in design? It complicates the entire project and introduces a completely difference set of parameters that need to be adhered to. Keep it simple. Putting together a compound setup, or a twin setup is enough of a challange without worrying about part throttle response.
feel free to email me... 
[email protected] 
or
[email protected]


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

Just curious, What is your job Extreme Vr6?
I work at a Rotordynamics and foil bearing house, Everything we do is tribological or hydrodynamic in nature. I have also developed oil free turbochargers for large diesel engines for the soul purpose of eliminating turbo oil leakage and subsequent burning. This is all to reduce NOX for the new 07 EPA particulate regulations. The turbocharger is a complete 1-off degign including custom cast wheels by schwitzer, and fully custom designed center sections and multi-piece shafting


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

...janitor for a local arena stadium...

Not really... I'm a Business Sales Rep at CompUSA.


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

I think you missed your calling.


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

funny you should mention that ... I've recently been accepted as a stupent at the University of Louisville Speed School (engineering) for mechanical engineering ... I've got a vocational evaluation to verify what I should be doing with my life ... definately my interest is obvious... they'll be looking at my skills and personality etc...


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*


_Quote, originally posted by *ExtremeVR6* »_ 
You said that the supercharger is rated to 1.7 PR. To result in 25 psi (2.7:1 PR) you need to find the correspondant PR to 1.7:1 to result the 2.7... yup divide 2.7 by 1.7 you need a PR of 1.5 (8.64psi) now you need to find a turbo that is correctly sized for the 16v engine ... I'd recommend something along the lines of a t3-45. especially since you're going into a SC after that. 


Those calcs are good, if you really want to have the SC as the first stage. However, the first stage, as I stated above, needs to be able flow the entire air. A 16V at 25psi will probably flow 350whp (let's say). So since the SC is the first stage, it needs to be a big mama, not a good idea since it'll be tough for the little engine to make it move. Also, a big SC will get some lag. The proper way to do it is to have a turbo first, then add the SC. That way the turbo is big to flow the 350whp worth of air (say 600CFM), and the SC is small since the inlet air is pre-compressed by the turbo, so it could flow just 400CFM.
My calculations above obviously have something wrong. The 60psi stated above is not boost, but rather absolute, so they're 45psi of boost. Also, the quick conversions that I did between CFM and WHP are ballpark figures since they assume a VE of 1 which it never is. Turbo engines usually have volumetric efficiencies of more than 1 (of 100%). However, the inefficiencies do multiply themselves like the pressures do:
First Stage:
30psi = efficiency 1 * ( 14.7psi * First Stage P2/P1)
Second Stage:
60psi = efficiency 2 * ( efficiency 1 * ( 14.7psi *First Stage P2/P1)) * Second Stage P2/P1
Or 
60psi = efficiency 2 * efficiency 1 * (14.7psi) * First Stage P2/P1 * Second Stage P2/P1

Vr6Fidelity... whatever, seems you're an idiot and I'm still an engineer. Maybe you need some English lessons for those big words? BTW English is my fourth language... I don't know what your problem is, I put some numbers out there which seem reasonable except for the 280whp at 60psi. Instead of just bashing why don't you help figure this out and tell me what's wrong with my equation? This seems like an interesting idea and worth discussing, even if mistakes are made on the way.
Speedy G


_Modified by Speedy G at 6:24 PM 11-4-2005_


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*


_Quote, originally posted by *Vr6Fidelity* »_Speedy G I have no idea what you are tying to say but your pretty much just wrong all over the board. I also notice you love to toss large words about for no reason. 

Please be careful about who you bash ... I never said anything to you or your abilities or intelligence... 
For the record ... Vr6Fidelity is also an engineer. I may be an idiot, and I'm sure that those who know me personally can easily find many examples in my life to verify that... but I don't think you know me that well. 
as for your rebuttal ... 

_Quote »_For 25psi, the 16V makes say 300whp (no clue but let's say). 

this statement completely nullifies your entire argument... how can you possibly present an argument based on numbers when you make them up?
once I get home tonight I'll get you some real math to chew on... instead of making numbers up...


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*


_Quote, originally posted by *ExtremeVR6* »_
Please be careful about who you bash 


Oops sorry, changed the name.

_Quote, originally posted by *ExtremeVR6* »_
as for your rebuttal ... 
once I get home tonight I'll get you some real math to chew on... instead of making numbers up...

Well, it's not totally a useless number, but I'm sure you can find a 16V with 350whp and 25psi. There's too many unmentioned variables like what turbo, what outside temps, what fuel, what intercooler, what A/F, what exhaust, etc.
From my math above, multiplying the stages yields 45psi of boost with a 2 stage compression scheme. Each working at a P2/P2 of 2. However, since the efficiencies multiply, .66 * .78 = .515 or 51.5%. This was my point, although maybe not clear enough. A turbo that can boost 45psi usually has efficiencies of at least 70%. All I'm saying here is that it's not as great as it sounds in terms of top end. Yes you have a ton of boost, but it's robbing you a bunch of power to get it.
Those diesel trucks probably do the compound charging thing because they can take 250psi, and because even if they only get 40% efficiency, there probably isn't a turbo that can take 100psi and can be spooled at proper RPMs.
Speedy G


_Modified by Speedy G at 6:45 PM 11-4-2005_


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

Due to the efficiency issue, I still think it's better to just use the supercharger to spool the turbo. Once the turbo is spooled, the SC can decouple or vent its outlet to the atmosphere (depends on the SC you have). That would mean having independent inlets and some kind of valve like a wastegate to select whether the turbo or the SC feeds the engine.
Speedy G


_Modified by Speedy G at 6:51 PM 11-4-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

ok wow this got long fast!
First off VR6Fidelity - yes i have though about mass air flow, but im trying to take things one step at a time, i had a question I asked it....if your going to bash on people for trying to learn...just stop posting here...i value the information you have, but your just not being productive at all. you can tell people oh dont go climb that mountain...you will die! all you want some people still just want to do it.
ExtreemVR6, thank you very much for your information, you actually came pretty close to the pressure ratio's i had calculated while bord at work.
Since i dont yet have the eaton installed, i guessed a pressure ratio of 1.7(based on the pully ratios and charts of the charger) which i have been using to work out some quick math trying to get a grasp of whats going on.
I came up with a 1.6:1(rounded up from 1.58) ratio for the turbo in series with the eaton at 1.7:1
also i assume that no matter what the order(turbo then eaton, or eaton then turbo) the pressure ratio's remain the same..right?
Now back to my initial question again, your saying that in order to size a turbo that will work at the 1.6(or 2.7:1 by itself) running it at 1.2:1 would not work?? If so could you please explain what is happening to make this a bad thing?
would it be a better idea to simple use the boost controler to open the wastegate all the time and let the turbo do no work at all(just let the eaton suck air through it) for low boost?
as far as turbo size goes, im comming up with (quick and drity) about 40 lbm/min max for a 16v at 25 psi, which is a bit off the T4-45 chart http://www.rbracing-rsr.com/tu...6.gif
I was looking a bit more toward a 5, or 60 trim, which would cover the initial 8v(at about 20-25 lbm/min) and the 16v later
http://www.rbracing-rsr.com/tu...7.gif
http://www.rbracing-rsr.com/tu...0.gif
Does this sound right?
BTW that site also has all the eaton charts(going off 10,000 rpm at 10 psi for most numbers)


_Modified by Iceman666 at 9:18 PM 11-4-2005_


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*


_Quote, originally posted by *Speedy G* »_
Well, it's not totally a useless number, but I'm sure you can find a 16V with 350whp and 25psi. There's too many unmentioned variables like what turbo, what outside temps, what fuel, what intercooler, what A/F, what exhaust, etc.

Right there are a ton of unmentioned variables... and yes a 350hp 25psi 16v can be found... very true. I've never tried to express things as an absolute situation or that one specific thing were or were not possible.


_Quote »_Those diesel trucks probably do the compound charging thing because they can take 250psi, and because even if they only get 40% efficiency, there probably isn't a turbo that can take 100psi and can be spooled at proper RPMs.

Yes, most definately! Diesel blocks stock operate at compression ratios like 18:1 anyhow ... numbers that a Gas engine would die of trying to simply imagine something like that. 
The other thing about diesels is that they have RPM ranges like that of american V8's ... ever seen a 5500 redline ... that's high for alot diesels. They're range of operation is extremely shallow ... maybe 500-800 rpm range once they're on the road ... ideal operation for a turbocharger is at a single rpm. If you know the RPM that the engine will be at you can make a turbo, piping etc specifically for that specific point. Because the RPM range of gasoline engines are so much wider, compromise must be made, and that's when you try to find the best suited turbocharger for an application. 18 wheelers don't care about max power output, they don't care about acceleration or anything like that. So for a commercial diesel application there is one answer for which turbo to use, because the range of operation is so slim. For performance cars, and definately in a modification situation you must decide where you want the power ... up top, off the line, midrange, etc. These hondas that are being built as drag cars might use a T66 on a 1.6liter engine... yes rediculously oversized and it takes till 6000 rpm to even start the turbo spinning ... but those guys are all about getting the top end out of those things ... they lose the lowend power to be able to free up the high end power ... someone who's autocrossing will consider that kind of performance deadly... literally... in the middle of a slalom and the boost hits, suddenly you find yourself in the grass ... or worse ... Autocrossers need smooth linear power with as much predictability as possible ... so a smaller turbo with faster response... and yes not as much high end it better.

_Quote »_Due to the efficiency issue, I still think it's better to just use the supercharger to spool the turbo. Once the turbo is spooled, the SC can decouple or vent its outlet to the atmosphere (depends on the SC you have). That would mean having independent inlets and some kind of valve like a wastegate to select whether the turbo or the SC feeds the engine.
Speedy G

having the supercharger spool the turbo is ony a good way to spool an oversized turbocharger for extreme power levels ... still silly.
I don't want to get to far off topic here... but we're talking about a FWD car ... anything over 350hp isn't only truely usable at highway speeds. If you look at it from that perspective a single turbocharger would be suitable for any smaller displacement engine... and all this is completely dumb... 
I'll post more later on tonight


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_
Since i dont yet have the eaton installed, i guessed a pressure ratio of 1.7(based on the pully ratios and charts of the charger) which i have been using to work out some quick math trying to get a grasp of whats going on.
I came up with a 1.6:1(rounded up from 1.58) ratio for the turbo in series with the eaton at 1.7:1
also i assume that no matter what the order(turbo then eaton, or eaton then turbo) the pressure ratio's remain the same..right?


Yeah, the pressures remain the same if you stick to the series setup. What changes is the air volume flow capacities of the components. A big turbo is cheaper than a big supercharger, so I'd go with the turbo flowing into the SC.

_Quote »_
Now back to my initial question again, your saying that in order to size a turbo that will work at the 1.6(or 2.7:1 by itself) running it at 1.2:1 would not work?? If so could you please explain what is happening to make this a bad thing?


Nothing bad happens. I run my engine at 6000RPM and 0 boost all the time, or 6500RPM and 3psi of boost. Any time I'm talking on the cell phone and trying to drive in the city, I do that.







Also, it runs perfectly fine at 1.4 P2/P1 since that's the default tial wastegate spring.


_Quote »_
would it be a better idea to simple use the boost controler to open the wastegate all the time and let the turbo do no work at all(just let the eaton suck air through it) for low boost?


I think it's better the for SC not to have to pull too hard. Even if the turbo isn't making boost, the exhaust is still making the turbo turn and feed the SC. I see no problem in keeping the wastegate shut. How would you open up a wastegate if there's no boost anyway?

_Quote »_
as far as turbo size goes, im comming up with (quick and drity) about 40 lbm/min max for a 16v at 25 psi, which is a bit off the T4-45 chart 
I was looking a bit more toward a 50, or 60 trim, which would cover the initial 8v(at about 20-25 lbm/min) and the 16v later


Well, this might be the first time I ever say something agains the T04 46 trim (big fan)... I think you need something that likes less boost. Also, I guess you're looking at cheaper turbos (less than 700 bucks). You not only need a turbo that flows 40lbs, but you need one that flows 40lbs efficiently at a P2/P1 of 1.5, so a 60 trim is probably one of the best that you listed. Look into bigger turbos though. How about a 60-1 or some of the T04B's? T04B's are usually good turbos for low boost and a lot of flow.

_Quote »_
BTW that site also has all the eaton charts(going off 10,000 rpm at 10 psi for most numbers)


How about this... Split your project into a couple stages:
1) Stage I - Eaton M42/M60 on a 16V - get it running right
2) Stage II - Figure out what to do with the turbo. The turbo comes before the SC anyway, so you could install this after installing the SC.
3) Stage III - figure out if it's better to do a series setup of if it's better to disengage the SC once the turbo has spooled. You're talking about a max of 25psi anyway, so a 60 trim could do both jobs.
I'm still saying a parallel charging path is better. Otherwise howcome mazda and Toyota both decided to do parallel twin turbo setups instead of in series?
Speedy G


_Modified by Speedy G at 10:40 PM 11-4-2005_


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_ok wow this got long fast!


Iceman666 said:


> ExtreemVR6, thank you very much for your information, you actually came pretty close to the pressure ratio's i had calculated while bord at work.






Iceman666 said:


> <evil grin> no actually you came close to the right numbers
> 
> _Quote »_I came up with a 1.6:1(rounded up from 1.58) ratio for the turbo in series with the eaton at 1.7:1
> 
> ...


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

one thing I want to say here... typically compound charging happens with 2 turbochargers ... in which case the 1st turbo is smaller and simply acts as a multiplier for the second turbo flowing all the air... that being the case you can't quite consider the flow of the smaller turbo to be a limiting factor because the first is normally overspun... thereby allowing more airflow than would normally be flowing through it... here's how that work ... the reason turbos lose their efficientcy is because of the inability of the compressor to continue compressing the air after a certain point... which is why you see the limitation on the upper right side of the compressor maps ... however, in a compound turbo setup the compressor of the 2nd turbo is actually sucking the into it's compressor (being that is it a larger turbo), the the limitation of push from the first turbocharger is removed and it is allowed to spin faster and flow more air... in the end... giving the 2nd turbo all the air it wants ... but compressing it as it goes through. 
that is what makes compound turbo's such a benefit... that is also why feeding a turbo with supercharger doesn't quite work ... the supercharger can't overspin like a turbo can... because it's connected to the crank ... hmmm interesting thought here ... if you were to suck air through the supercharger forcing it to try to spin faster, causing the gearing to work backwards against the crank ... you've just created a belt driven crank, while pushing air into the engine producing power and air flow to drive the entire system... let's take this to a silly level... suck enough air through the supercharger such that it begins to actually accelerate the crank from the suction of the air through the supercharger rather than the combustion strokes of the engine ... and whoops you've just made free, absolutely 100% efficient power production ... equal horsepower is being created to run the engine, which produces exhaust gases which spin the turbo enough that the compressor sucks are through the supercharger fast enough that the compressor is forced to spin faster than the gears are driving it... you then being to apply force back to the crank ... 
yup it's silly...


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

So you're still thinking about a T3 45 trim on an already supercharged 16V. You have some strange thoughts btw. I do like the idea of accelerating the crank with the output of the turbo. Anyway, hat you've described:
a) cone filter--->turbo--->supercharger---> engine
That means cone filter to compressor of turbo to inlet of supercharger
b) You mentioned this (crazy but interesting):
cone filter-->supercharger-->turbine of turbo-->engine
|--------------------------------compressor of turbo-^
It won't work because the turbine needs the mass from burnt fuel and the volume of hot gases to work, unless it's a pretty small turbine.
c) Supercharges make boost not because they're revving, but because while they're revving they move a certain volume of air. If the air that the supercharger supplies is more than the engine can take, the pressure increases. That means that if you have a supercharger that can supply 400CFM @ 12000RPM and an engine that flows 200CFM @ 6000RPM, at 100% efficiency the supercharger will supply the air to the engine at 2 atmospheres absolute or 15psi of boost if a 2:1 pulley is used. Since as the the supercharger is tied to the engine with a 2:1 RPM ratio and their flow is linear to RPM, at 3000 RPM the engine will flow 100CFM and the supercharger will flow 200CFM, maintaining 15psi. What I'm saying here is that the SC will supply 15psi throughout the power band, not only at redline. There's people on this board that run 15psi at idle btw.
Now since I think it's pretty evident that the only option for a serial supercharger & turbo setup is A), I think you'd be crazy to go with something like a T3 45 trim. I've already explained why twice, but I'll do it again. An Eaton that pushes 320CFM (remember CFM is volume), and a T3/T4E 60 trim pushes 40lbs at a P2/P1 of 1.7 and more at 2 (let's say that's at least 600CFM) :
Flow numbers shown on line on top of setup:
600CFM @ 1bar->2X1-->300CFM @2bar--->2x1---->150CFM @ 4 bar
Cone filter------>turbo----------------->supercharger-->engine
That means the turbo has to flow 600CFM, the supercharger 300 CFM and the engine 150CFM. That means the turbo has to be something pretty beefy to flow 600CFM @ 1 bar of absolute pressure.

Speedy G


_Modified by Speedy G at 12:37 AM 11-5-2005_


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

re: turbo... when the SC sucks on the turbo forcing it to flow more air the pressure between the two decreases yes? yes. That means that if the turbo is trying to maintain a set PR and the PR between the two units decreases that it will spin faster to maintain the pressure ratio ... 
here we need to discuss where the limitation of the turbo come from ... the upper right area of a compressor map is the area where the turbine cannot supply anymore torque to the shaft to make the compressor turn any faster due to the fact that it simply cannot compress the air any more. 
If you reduce the pressure while increasing the air flow, the compressor can then increase in speed there by maintaining the pressure ratio ... does this overspin the turbo yes... but the strain that is normally associated with those rpms are not present... which frees the turbocharger to flow more air than a compressor map might suggest is within operation.
Now I honestly don't expect a turbo to flow twice it's normal flow ... no that is true ... a turbo that was designed to flow 300cfm will not flow 600cfm ... but I think reality has been abandoned a while back.
and in my mind option a as you have it listed is the only reasonable way to COMPOUND charge. option b was me trying to make sense of something I thought may have been eluded to in a prior post ... yes it won't work ... certainly at least will not work well. I do understand that.
With option a ... the flow isn't as much of a concern as the pressure ratio is... a turbo can overspin much much easier ... okay a SC simply CAN'T overspin unless a clutch function is put in ... to much trouble for that to happen...


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

Ok, where to start.
The eaton is a M62(actually a M64 off a C320 benz)
OK let me see if im understanding everything here.
Ok so for both layouts the eaton will be forcing air(either sucking or blowing) through the turbo, which will cause the compressor to spin, and thus causing the exhaust turbine to spin chreating a suction on the exhaust, which will help speed up spool time.
Now then, with the turbo before the eaton, once the turbo is spooled from the above effects, it can then pre-compress some air which the eaton will then recompress to achieve a higher boost level then the eaton alone would be capable of. BUT due to the high delta T of the eaton, this layout would require two intercoolers, one between the turbo and the eaton, and another between the eaton and the intake.
On the other hand, if we run the eaton into the turbo, we still get the spooling benifits noted above, but slightly reduced due to the temperature of the eaton outlet air having a negative effect on spooling time(is this right?) but thus not requiring a second intercooler. BUT due to the fixed flow rate of the eaton, once the turbo tries to flow more air then the eaton a suction will be formed between the compressors and effectivly starve the turbo from flowing more air then the eaton could flow.(no more boost over the eaton's)
This is where copying the TSI motor come in. By using a vacuume sensor(re-wired HOBB switch) between the eaton and turbo the vacuume can trigger an electric DV valve (or check valve) that will be plumbed parallel to the eaton, to supply more air to the turbo, and at the same time disengage the eatons clutched pully. This will allow the turbo to fully take over the boost production, and thus yeild no supercharger lag. See diagram








Now since Low end is taken care of by the eaton, which is in turn aiding in the spooling time of the turbo a larger turbo then would normaly be selected can be used. I have been leaning toward a hybrid T3/T04e turbo somewhere in the range of a 45-60 trim.(which would be a bit large for the 8v setup, but once the 16v is installed will be right on) Potential problems are first off, deciding exactly which turbo will work best for this situation, and second making the damn thing fit behind a damn 8v!
Now I have also thought of a good reason for running the eaton into turbo layout, with a checkvalve and clutch involved. Low boost settings would be much easyer to handle. By utilizing a boost controler tapped off the intake manifold pressure, a low boost setting of 10 psi(or the same as the output of the eaton alone) could be set. And with the eaton engaged simple cause the turbo to do nothing...OR better yet with the eaton disengaged(via a cockpit switch to the clutch), or a boost setting of 1 psi over that of the eaton(causing the check valve an clutch system to activate), the turbo could do all the work with no charger lag, thus resulting in better gas milage as a "safer" power level.
EDIT : thought of something else, in the eaton to turbo layout, since I plan on having the throttlebody located before the eaton, and thus before the turbo....do i still need a blow off vavle??
all that sound about right?


_Modified by Iceman666 at 12:25 PM 11-5-2005_


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

FYI that's not a compound charging diagram. That's a parallel charging diagram which is what I've been saying works best because of the efficiency issues in compound chargin. The "RegelKlappe" is not an electric valve as you say but rather a boost controlled pressure valve. If there's pressure at the outlet of the turbo and the outlet of the supercharger is close to 0psi, the valve gets opened (you can probably use a wastegate). This means that when the supercharger can't flow anymore, the turbo takes over and the supercharger becomes irrelevant. If a supercharger doesn't compress the air but just flows air through it, there's only like a 1-2hp loss, so there's no real gain in disengaging it with a clutch. In fact I'm sure the system counts on flowing say 320CFM of air through the SC without compression when the turbo is spooled.
The system only uses one intercooler since there's only 1 compression stage working at the same time. On that diagram the Drosseklappe is your throttle body which is after the entire compression duct. The blue triangles called the Schubumluftventil is the only DV valve there.
Regarding turbo spool, the SC flowing the turbo doesn't really help out the turbo spool, although it doesn't hurt. What really spools up the turbo is the mass air flow on the exhaust. Since more air is flowing into and out of the engine, with additional gasses from burnt gasoline, you can use a bigger turbo than anything meant for a 1.8L engine. 
In a parallel system like in the diagram, you can flow the air through the turbo, or you can add another wastegate type valve to choose which of the two gets to dump the air into the engine. Look for a mazd Rx-7 or a supra turbo engine diagram and you'll see what I mean.
Speedy G


_Modified by Speedy G at 11:53 AM 11-5-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

if it were parallel charging the one charger would never be blowing through the other.
and yeah i realise the check valve is boost controled on that motor, i was just planning on doing it electrically for my own uses.
and could you clarify what you mean ny "SC flowing the turbo" , turbo then sc or sc then turbo?


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_if it were parallel charging the one charger would never be blowing through the other.
and yeah i realise the check valve is boost controled on that motor, i was just planning on doing it electrically for my own uses.
and could you clarify what you mean ny "SC flowing the turbo" , turbo then sc or sc then turbo?


In that engine the supercharger is not compressing air once the turbo is spooled. That's why the "RegelKlappe" or control valve is opened. It lets in supplemental uncompressed air from the cone filter. If post-supercharger air were compressed once the valve is opened the air would rush out, like in a DV valve. However a DV is of no use there and neither is an electric valve. What's happening is that since turbo starts to suck air from the supercharger, the pressure eventually goes to 0psi of boost and without the valve, air starvation would happen at the inlet of the turbo. You could call this compound charging, but the first stage (the SC) is only working at a P2/P1 of 1 beyond the spool of the turbo. It's the same equation as parallel charging.
I'm thinking you want to control turbo boost by opening and closing the electric valve. That's not efficient if the valve is kept closed, since once the turbo is spooled you're taking away the energy from the exhaust, and using it to compress close to vacuum. It's a waste of enery, and the energy from exhaust of the turbo does affect your output power. If exhaust pressures are increased your engine VE decreases and you lose HP. Also, keep in mind that turbos and superchargers have oil seals that aren't meant to work in vaccuum. Most of the seals need pressure to keep the oil from leaking into the intake.
The proper way to control boost is to do it via de wastegate, and matching the supercharger so that when the turbo spools, the flow of the supercharger is the same or less.
Speedy G


_Modified by Speedy G at 12:01 PM 11-6-2005_


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*


_Quote, originally posted by *Speedy G* »_
Second Stage:
60psi = efficiency 2 * ( efficiency 1 * ( 14.7psi *First Stage P2/P1)) * Second Stage P2/P1



Regarding the efficiencies of multi-stage compression; if intercooling is performed between each stage of compression, then the system will be closer to isothermal rather than isentropic. All commercial compressor map efficiencies are relative to isentropic. Isothermal compression is more efficient than isentropic compression(which is why the carnot cycle is the theoreticaly most efficent cycle). Thus, it is very possible to have higher total efficiency in multi-stage compression that that of each individual stage. The more compression stages, with intercooling, the greater the total efficiency. I wouldn't be surprised if the triple stage compression engine had much greater than 100% of isentropic efficiency at the intake manifold relative to atmosperic conditions. It isnt as simple as multiplying the compressor efficiencies to calculate the total efficiency unless there is absolutely no intercooling(which also means no plumbing between compressors). Any single stage forced induction without intercooling is not wise. Any multi-stage forced induction without intercooling is plain stupid.
Also regarding the triple stage compression engine; to minimize compression work, the pressure ratio accross each stage must be the same. If the total PR is 18, then each stage should be the cube root of 18= 2.6, which is also more pallettable than two stages with a PR of 3.
Game on. http://****************.com/smile/emthup.gif


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

OK, so we have a real ME now. Dude, please help! What you just wrote tells us that all of us are just making some assumptions. How can we get to the right equations? Any web pages of theory? I actually can buy an M60 for USD300 and have access to a GT30R for a new 1.8T motor, so this is of very high level of interest. I'm an EE so compression is out of my direct level of expertise.
Thnx!
Speedy G


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## ExtremeVR6 (Sep 6, 2001)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

whew ... I'm impressed ... now I just need to get myself started on that ME degree I'm wishing I had ... heheh ... self study only goes so far ... hehe ... 
I did think that PR of 3 was a bit high, but the math worked out quicker as I did it in my head that day ... heh. 

_Quote »_Now class the vocabulary words for today are:
*Isothermal Compression* : _isothermal compression is a compression in which the temperature of the gas remains constant_
*Isentropic Compression* : _Isentropic compression. An adiabatic compression with no increase in entropy; a reversible-adiabatic compression_
Sub Definitions
Adiabatic Compression : Adiabatic means no heat enters the gas from the surroundings
Entropy : a thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work

So to give a gross sumerization, no heat is added to the gas from the surroundings, but no heat is added to the surroundings by from the gas? Am I right? 

_Quote »_*Carnot Cycle* : _A cycle (of expansion and compression) of an idealized reversible heat engine that does work without loss of heat_

Since the Carnot Cycle is ideal, is it applicable? If so, in what senario is the carnot found?

Okay... here's my questions is it considered an isothemal compression as a total result or consideration of the entire system? In which case having cooling between the 2 compressors maintain near ambiant temperatures?
In an ideal situation I would presume that what you're suggesting is that between each stage there is intercooling, as well as heat retentive coatings/procedures to prevent heat either escaping or being added... Is this correct?
Practically speaking, intercooling the first stage can be done pretty easily... alot of air can be directed to the back of the engine compartment by a naturally occuring back flow is air from the windshield. You could get quite a bit of air flow from there... the second stage can simply be intercooled as you would normally from the front of the car.
Would something like water injection or nitrous injection provide cooling on par with intercooling? Or would the side effects of adding moisture to the compressed air going through another compressor be more detrimental?
_sidebar_ : On part of me feels as though we're discussing something that could be getting towards something real innovative ... on the other hand I realize that there have been people far more intelligent, experianced and educated have most likely already ground these things into the ground. Is there something to all of this? Would there be feasibility to a semi belt driving/turbine driven centrifugal turbocharger where at lower RPM's the crank would provide torque to the shaft and compress air into the engine and at later rpms as the exhaust gases increased in velocity and flow would begins to apply greater torque. My imagination suggests that since "clutches" and the like are in consideration, the integration of one to the inards of a turbocharger can't be far off ... 

_Modified by ExtremeVR6 at 10:50 AM 11-7-2005_
seems to me like between 2 ME's 1 EE and a would be theorist something could ceom from it all hehehe


_Modified by ExtremeVR6 at 10:51 AM 11-7-2005_


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

The causes of heat in the turbocharger's compression of air is mostly Kinetic, That is one air molecule impacting another at a high velocity, This comes from compression but mostly shear. Also some molecules take a reversionary path thru the wheel and are heated multiple times. Perhaps a perusal thru the Japiske "centrifugal pump design and performance" and its explanations and quantitative solutions on the phase diagrams for various regions of the volute would be helpful. It explains the change in energy on a molecular level. 
I would first read "maximum boost" by corky bell though. The other book is written at a PHD level and assumes the reader to know all the mysteries of the universe beforehand.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

yeah i dont need to be pondering a book at PHD level, just saying smacking the air around heats it up is good enough for me at this point 
Ill pick up maximum boost though, havent read that one yet
he is basicly saying the more you cool the air between compressors the greater the effeciency of the compressor due to a decrease in losses from heat.
the eaton by itself is a pretty good example of that. the hotter the air comming in, the less effecient it is at compressing the air thus the hotter the output air is. 
I have a good section of my thermo book i can scan for ya that explains the carnot cycle pretty well(its more of an idealised cycle used to study things like a/c units without looses)
but here is my main question, obviously on the turbo to eaton layout a intercooler would be needed in between, but if i run the eaton into the turbo, wouldnt intercooling between compressors actually have a negative effect on the spooling of the turbo?




_Modified by Iceman666 at 3:08 PM 11-7-2005_


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

Turbo spool has mostly to do with the turbine of the turbo, not the compressor side. That means it doesn't matter what temperature the air comes into the compressor.
Anyway, the efficiency bit of info really changes things. 
Speedy G


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (ExtremeVR6)*

The reference to the carnot cycle was simply proving that isothermal compression is the most efficent. This discussion has nothing to do with carnot engine. So, we'll set the carnot thing aside.
Thanks for the kudos, but i'm not a ME. I'm an IE, engine nut, and currently taking thermodynamics as an elective.








Back on track...
What are the "right" equations to calculate efficiency? I dunno. "You're results may vary."
I still haven't decided for myself which arrangement is better, Super into Turbo or Turbo into Super. In either case, the supercharger will aid in spool because of increased exhaust gases. I doubt in either case if the supercharger would significantly aid the turbo spool(PR, not shaft speed...more on that later) on the compressor side.
Let's check out a few past attempts:
The Lancia S4








1.8L, Turbo into Super with bypass(looks like a throttle activated by a wastegate actuator), and an intercooler between each stage. 
Output 550hp, 0-60 in 2.3s on gravel(!)
More info: 
http://www.lanciadelta.org/delta_s4_motore_img.htm
http://rallye.stars.free.fr/autos/s4/S4T.htm
The previously mentioned VW twincharger is similar except Super into Turbo. It also has the advantage of FSI to prevent knock.
Toyota with both arrangements, only ended up with 180hp though:
http://www.geocities.com/Motor....html
On the Diesel side, there have been many successful Turbo into Super 2-stroke engines:
http://www.deltahawkengines.com/
and of course the Detroit Diesel:
http://www.almiller.com/_wwii/detroit12v.html
As well as a sequential twin turbo cummins dragster:
http://www.mrtruck.net/engine26.jpg
However these may not be applicable for this discussion.
On a side note, those diesels don't have any intercooling. Why? Maybe packaging issues. But without a doubt, extra power is waiting to be unleashed.
If it were me building my ideal gas engine with multiple stage compression, I would go with Turbo into supercharger to keep the velocities(both shaft and air) in the turbo higher. The supercharger pulling air through the turbo will make the air density in the turbo less and thus less resistance to spool(shaft speed). Additionaly, i think the compressor blades would be less likely to stall(surge). Then when it comes time to give the supercharger a break and bypass it, the turbo is spinning quickly and ready to make boost.
Packaging all of this complexity in our tight engine bay would be difficult to say the least. I would much rather transplant the engine to the rear before tackling something like this(which i am debating doing...yes...in my jetta







). I hate to be a downer on this cause I love innovative ways to tackle complex problems. Best of luck and make sure to post results.
Having multiple stages of forced induction is a great idea...for a diesel. I dunno about for spark ignition though.


_Modified by revensonjr at 12:05 AM 11-8-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

Fitting it in the engine bay actually isnt that big a problem with anything other then a damn counterflow 8v(cant fit the turbo easy).
Once everything is at max boost with both chargers, the turbo will still only be boosting a very small amount in either layout. and once the charger is disengaged, the turbo would have to catch up, and quick. This is one of the situations im worrying about.
Now with a turbo into supercharger layout, how to go about bypassing the supercharger.


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

Why not just copy the lancia setup exactally? This car is perfect, You could never have a setup as well thought out as the one above. If you truly plan on doing this the best advice anyone could possibly give you is to copy that setup exactally. 
Btw what car is this going in with what motor and drivetrain?
Have you purchased any parts yet?
Do you currently own a turbo car?
I just want to know, not being a dick. which i am excellent at.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

I agree, but that link he posted is as in depth as any tech writeup i have seen on it. If you have any more info on the workings of the bypass on the supercharger, or specs on the turbo or charger itself, please do tell.
This will be going in a corrado, current setup is a ABA bottom end, 8v counterflow head(the problem at the moment) at 9:1. Will be swapping it over eventually for a aba16v. Stock O2A tranny. Will be putting in a custom O2A with custom gearing(cryo-REM treated) and a LSD when the 16v goes in.
For the supercharger, I have a M90 pushing 17 psi ATM, but am working on a M62 kit, which would allow me a bit more room to relocate the TB, as well as the electric clutch. Plus for the lower boost level i want to run(10 psi) its more efficient for this size engine. 
I dont own a turbo car personally, but my brother has a 944 Turbo, putting down over 400whp(last dyno was 392, but we swapped injectors since) and i get to screw around with it.
Plus i have the advatage of having jwatts 10 min away and well he has a very nice turbo aba16v, and he gets to do all the chip tuning http://****************.com/smile/emthup.gif 


_Modified by Iceman666 at 10:19 AM 11-8-2005_


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_If you have any more info on the workings of the bypass on the supercharger, or specs on the turbo or charger itself, please do tell.









Again, it looks like a normal throttle body with a pneumatic wastegate actuator attached to it. The throttle body just connects to the input and output of the supercharger. This seems like a very cost effective and simple solution. If the pneumatic acuator references from the supercharger input, then when the turbo is producing boost it will automatically open up and allow bypass.
If you are going to use an electrically actuated clutch on the supercharger, you could use a basic hobbs switch for deactivation. Maybe add a delay circuit between the hobbs switch and the clutch to prevent any unforseen system interactions and keep the supercharger running between transmission shifts.
With proper intercooling, I am beginning to like this project more. http://****************.com/smile/emthup.gif


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

hmmmm, thats a damn good idea. And that would work for either layout as well.
ill have to draw some stuff up to see how that would fit.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

Took alot of searching but I finally found out the lancia used a Abarth Volumex supercharger, which is basicly a roots type blower. Im still not sure exactly which model was used or how much boost the charger was setup to run. And Im still looking for some info on exactly how that charger bypass is activated.
Good stuff. Few more parts i get to make with the CNC








No one has answered this yet though. If i have the TB on the charger inlet, and run the charger into the turbo...do I need a blow off or DV valve??
Or hell just with the charger alone and the TB in front of it, will i need a DV or any kind of surge protection?


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

I have a feeling there is a reason why placing the throttle after the turbo is the the choice 99.9% of the time(carburetors being the exception). Starving the compressor is probably a bad thing.
Why feed the turbo compressor with a supercharger and not vice-versa?


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

could you elaborate a bit more?


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## Speedy G (Apr 1, 2002)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

Yeah, I agree. starving the compressor, could lead to overrevving the thing.
Regarding setups, I'm thinking of building a '50s high powered VW bug with an 02J tranny and a 2.0L block. That would be a mid-engine setup btw. I have access to a cheap O2J tranny, an M60 and fabrication for the bug suspension. My current setup is a Skoda Octavia 1.8T with a T3 S60 and a chip for the original turbo. I'm really just doing research, but since I can get the M60 and a T3/T4E 50 trim for the same price as a GT30R, this project could be pretty interesting. All I need is 400whp but low end torque would be nice too. The problem may be the drivetrain though... BTW, anybody know if you can mate a 20V head to a 2.0L block? 16Vs are rare and expensive here.[edit] Just read the head swap sucks. I'll probably just go with a clone of my current 1.8T AGU big port engine.[/edit]
Speedy G


_Modified by Speedy G at 4:09 PM 11-9-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*

still want more info


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*









Supercharger mounted to a 1.8T...which still has it's turbo in place. Looks like an Eaton m45 or m42.
I found this here: http://club.corrado.free.fr/prepa/eaton/eaton.htm while doing some research on Rotrex superchargers. The original from Vwvortex is towards the bottom here:
http://vortex3.rely.net/galler...rt=24
I also found this: http://www.koenigsegg.com/thec...ing=2 
"The Koenigsegg RBC-system replaces the traditional by-pass valve with an extra throttle, mounted before the compressors. This throttle is operated by an automatic vacuum sensor, which shuts it immediately at off throttling"
They are using centrifugal superchargers. If designed properly they will not overspeed due to being mechanically attached to the crankshaft, which was my initial concern with a turbo. However, after further consideration, overspeeding a turbo due to throttling pre-compressor may not be as prone to overspeed as I previously thought. Yes, the turbo will probably run at a higher shaft speed due to less resistance from the compressor, but if it is in that condition then the engine is not producing large amounts for exhaust gases anyway. So, throttle before turbo may actually be beneficial at keeping the turbo at a more constant shaft speed, thus decreasing lag without other negative side effects. Hmmmm......

_Modified by revensonjr at 12:15 PM 11-18-2005_


_Modified by revensonjr at 12:16 PM 11-18-2005_


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

great info thanks!!


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## GTijoejoe (Oct 6, 2001)

*Re: What happens in a turbo at very low boost levels? (Speedy G)*


_Quote, originally posted by *Speedy G* »_anybody know if you can mate a 20V head to a 2.0L block? 16Vs are rare and expensive here.[edit] Just read the head swap sucks. I'll probably just go with a clone of my current 1.8T AGU big port engine.[/edit]
Speedy G

_Modified by Speedy G at 4:09 PM 11-9-2005_

yes you can mate a 2.0 block with 20v head
http://forums.vwvortex.com/zerothread?id=2278569


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## Issam Abed (Feb 12, 2004)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*


_Quote, originally posted by *Iceman666* »_
No one has answered this yet though. If i have the TB on the charger inlet, and run the charger into the turbo...do I need a blow off or DV valve??

You dont want the Throttle Body on the charger inlet.Ideally the best set up is using Pressure release Solenoid's.The most simple system would be pushing air through the eaton but this also proves restrictive.
As I told you in a pm.Its not as simple as bolting a Supercharger and Turbocharger and saying "abra-cada-bra".You have to do ALOT of calculations....I hope you gave up on the 8V idea.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Wizard-of-OD)*

no 8v is still there but it is a test bed. I fully plan on ditching it down the road.
and as for calculations...you should see my pads....this crap is starting to rival my machine design notebook in size








and why wouldnt I want the TB before the eaton?? damn near every setup ive see(including the lysholm silencer kit) puts the TB on the charger inlet. Using a recirc. system like the stock G60 has just seems redundant and dumb, when you could just solve all those problems the other way...i just dont see it.


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## DAMACGON (Nov 22, 2004)

I made this with my 8V.
a G60 - TURBO.
Look :

Then here are some pics of my motor :



Works sweet. Now i have my 16V-G60


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

I've since heard that running the compressor under vaccuum will suck oil out of the bearings(center section) and into the intake.


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (revensonjr)*

It is possible, but i believe that would require a lot of vaccum
1psi = 51mm of mercury about your max vaccum signal. 
so you would be limited to -1psi in the turbo. There are also two seals between the compressor voulte and the journal bearing which would reduce that -1psi. I do not think that the oil would be a major issue.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

hmm interesting. At what vaccume would you start to worry about oil leakage?


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## Vr6Fidelity (Oct 4, 2001)

*Re: What happens in a turbo at very low boost levels? (Iceman666)*

I would not worry. How much vaccum can your engine make is the question.


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## Iceman666 (Mar 26, 2001)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

well with a supercharger sucking on it it could be alot more then your thinking


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## revensonjr (Nov 6, 2000)

*Re: What happens in a turbo at very low boost levels? (Vr6Fidelity)*

If the throttle is before the compressor, then it's not "compressor vaccuum" that is pulling oil, it is engine vaccuum which can be quite high. At idle, an engine produces 16-21 in Hg which is about 8-11 psi of vaccuum. At high-speed closed throttle(engine braking), there will be another 1 or 2 psi of vaccuum. This all totals up to around 13 psi of gauge vaccum, which is only 1.7psi above absolute. You can't get much more vaccum than that without a purpose built vaccuum pump. So, yes, there would be a lot of vaccuum in the compressor at closed throttle conditions.
How much oil would that pull if any? I Dunno. Try it.


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