# High compression turbo engines



## skillton (Sep 26, 2002)

I came across this on another forum. It's a long read, and it's about honda engines, but seems to make sense. I always thought higher compression on a turbo engine won't make that big of a difference in terms of hp, but according to this, it does. 
What do you guys think?

_Quote »_There has been a long-standing myth with turbocharged Honda cars. For as long as I can remember, people have been quite concerned about having too high of a compression ratio, on their turbocharged Honda. The myth is: the lower the compression...the better. Well, this is not entirely true. 
Why is a low compression motor good to turbocharge? 
In essence, a low compression motor is good to turbocharge, because it is just easier to do so...and not worry about it. But is that what we really want to do as enthusiasts? Do we want to just slap a turbocharger on a car, and call it a day? Absolutely not. With turbocharging, comes the responsibility of tuning and care. If you are just going to turbocharge your car, and call it a day, then frankly, you don't deserve the luxery of boost! As well, might I add...that simply because it is easy to slap a turbocharger on a low compression motor and not worry about it, does not mean that it is right. I can guarantee you, that if you turbocharge a low compression motor, and fail to tune it correctly, you will end up on the side of the road, with blue smoke coming from your exhaust pipe. 
Generally, a lower compression motor affords you more margin of error, when tuning. A slightly imperfect a:f ratio probably won't lead to the demise of your motor...unless you drive like a total jerk. 
Why should I consider a higher compression motor, while making my decision of what motor will suit my needs? 
Different from a low compression motor, a higher compression motor will not give you a large margin of error, when tuning. As stated before, if you expect to slap on a turbocharger, and call it a day, well then stay away from the higher compression motors all together. A higher compression motor demands slightly more TLC than a lower compression motor. But oh, the rewards are plenty. 
Bottom line...a higher compression motor, pound for pound, will make more power, than a low compression motor. This works along the same lines as naturally aspirated Honda motors. You never see an NA enthusiasts spouting off about low compression do you? No, one of the keys to NA performance is high compression. So, why should it be any differnt for turbocharged applications? Of course, the routes to high compression are different (NA uses lightweight rods and pistons, that a turbocharged application would simply tear to shreds), but the end goal is the same. 
Let's make an observation here. I am going to throw a scenerio for you... 
Car A: 
B18B (stock 9.2:1 CR) 
Rev Hard Stage II 
Thermal 3" exhaust 
MSD 6AL 
MSD Pro Cap 
MSD Blaster 3 Coil 
Holley 255lph in-tank fuel pump 
AEM fuel rail 
RC440cc injectors 
A'pex S-AFC 
Car B: 
B18B (JE pistons, 10.5:1 CR) 
Rev Hard Stage II 
Thermal 3" exhaust 
MSD 6AL 
MSD Pro Cap 
MSD Blaster 3 Coil 
Holley 255lph in-tank fuel pump 
AEM fuel rail 
RC440cc injectors 
A'pex S-AFC 
Now for the sake of argument, let's say that both cars are well tuned, by the same tuner, both have an identical weight, both have the same slicks, both are boosting 1.0bar, and both are being driven by the same guy...at the same time (yeah, yeah, yeah...just roll with me here)! Now, which one do you think is going to come across the finish line first? Mythological thinking says that the guy with the low compression is best suited to win...right? Wrong. The guy with the 10.5:1 compression ratio is going to smoke the guy with the low compression. 
Another point I would like to bring up is the misuse of the term "high compression" when it comes to Honda motors, and turbocharging them. In all honesty...10.6:1, 11.0:1, etc...aren't even really that high. Most NA monsters utilize 12.5:1 or higher...and some even as high as 14.0:1, in cases of extreme race. If you do some snooping around, you will realize that most of the really fast Hondas, and I'm talking sub-10 second monsters, utilize high compression setups, to achieve enourmous horsepower goals. Most of these guys won't openly discuss it though, so you are likely to come across terms such as "undisclosed compression", or something along that line. As I stated before, the myth is that low compression is key...so these guys want to stay on top, and the way to do it is hide the fact that they are using high compression, turbocharged motors...to propel themselves into sub-10's. 
Reliability, and compression... 
This is a regular question that pops up, and is quite valid. Frankly, as soon as you modify a Honda at all...reliability becomes an issue. A lot of us can testify that the proverbial "can of worms" sprung open as soon as we made our first modification. Many times, Honda enthusiasts will spout off the reliability factor, in Import vs. Domestic wars, when in actuality, they fail to realize that we are in just as much danger as those old pushrod V8's are, as soon as we modify our Honda engines. 
Now, what is the solution? I have been saying it for a long time, as have several others such as Tuan, neouser, texan, and st00pid...TUNING IS KEY! That's the bottom line. 
A well tuned higher compression motor will last just as long as any low compression motor. Tuning is not a factor that can afford to be sacraficed. You will not survive with an untuned higher compression motor, just as you will not survive with an untuned low compression motor. 
So, with all of that said, it basically boils down to your own personal choice. Do not fear the B16A, the B18C1, or the H22A...etc. If you have the monitary requirement to turbocharge a higher compression VTEC motor, I would say, do not hesitate to do so. 
I hope that you have a slightly better understanding of how compression and turbochargers co-exist. If you have any questions, feel free to make a new thread, and inquire. Thanks for reading!



_Quote »_One main concern in power production with forced induction is effective compression. Effective compression is the sum of the motors static compression, plus the additional compression added by the forced induction tool. A B18C1 (also B16A) motor will have a higher effective compression than a B18B motor will, on the same boost...therefore, pound for pound, it will make more power. 
The next argument that people usually bring up is that a higher compression is bad for turbocharging. Well, if you understand the concept of effective compression, then you should understand that this statement is entirely incorrect. A higher compression engine makes more power in NA form. So, why do you turbo guys think that a lower compression turbo motor makes more power? Does that make any sense when you really think about it? A turbocharger is a power adder? So why deplete power that was there to begin with? The answer I usually get to that is "So I can run more boost!" Well, sorry to rain on your parade, but more boost does not always equal more power. Check out this mathematical example of effective compression: 
A motor with a 10.0:1 static CR boosting 10psi 
10psi/14.7psi = .68 
.68 + 1 = 1.68 
1.68 x 10 = 16.8 effective CR 
A motor with an 8.5:1 static CR boosting 10psi 
10psi/14.7psi = .68 
.68 + 1 = 1.68 
1.68 x 8.5 = 14.28 effective CR 
Now tell me who is going to make more power? The higher CR motor, or the lower CR motor? 
So, maybe add more boost to the lower CR motor, right? Wrong... 
A motor with an 8.5:1 static CR boosting 13psi 
13psi/14.7psi = .88 
.88 + 1 = 1.88 
1.88 x 8.5 = 15.98 effective CR 
Now you see, even adding 3psi of boost, still does not equal the effective CR of the higher compression, lower boost motor. 
Effective compression is not the only advantage of the B16A/B18C1 either. The B16A/B18C1 has a stronger, better flowing cylinder head. It can rev much higher, making it that much more effective, and it flows great to handle all of the extra volume. The block has oil squirters to help support the bottom end assembly at high RPM. It takes more than a valvetrain upgrade to make a B18B safe at 8k. The higher compression also aids in spooling the turbo faster too. 
Both motors have similar tolerances though. Both motors pretty much top out at around 350-400hp on stock motors, very well tuned. The B18C1 will make it far more efficiently for you though. It takes less boost to do so, it has more safeguards...and the bottom line on any Honda motor is tuning. If it is well tuned, you will be set. That goes for both motors. YOU ARE A FOOL if you think for one second that just because your B18B has a lower compression, you can substitute that for proper tuning. 
A lot of people like to lower their motors compression when they build their motor. I used to think it was a good idea before I understood about tuning, and the positive aspects of compression. In the mathematical representation below, I will show you how a low compression motor must boost more to equal the output of a higher compression, lower boost motor: 
Motor: stock B16A2 boosting 7psi. 
Static Compression Ratio: 10.4:1 
((boost psi / 14.7) + 1) x motor compression = effective compression 
Stock motor (10.4:1 CR) on 7psi: 
7psi/14.7psi = .47 
.47 + 1 = 1.47 
1.47 x 10.4 = 15.288 effective CR 
Built motor (9.0:1 CR) on 7psi: 
7psi/14.7psi = .47 
.47 + 1 = 1.47 
1.47 x 9 = 13.23 effective CR 
You will lose 2.058 points from your effective compression ratio, this translates to a significant power loss. 
In order to gain back that power, you have to do this: 
Built motor (9.0:1 CR) on 10.5psi: 
10.5psi/14.7psi = .71 
.71 + 1 = 1.71 
1.71 x 9 = 15.39 effective CR 
Add 3.5psi to what you were boosting before, and you should be able to make around the same power as before, granted you haven't done any other kinds of modifications port/polish, cams, etc... 
As you can see, considering all things stay equal (bore/stroke/cylinder head/etc...), you must add 3.5psi to make the motors perform similarly. You just spent about $2,500 to build your bottom end, and make your car slow. 
By now we all should understand the positive aspects of compression, and how when teamed with the faster spoolng turbo, more efficient output, better flowing B-series VTEC cylinder heads, better low end spool time, stock oil squirters, higher redline, etc...you should see that turbocharging B-series VTEC motors is clearly not dangerous, and highly adviseable. I love a good turbo B16A!


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## B4S (Apr 16, 2003)

*Re: High compression turbo engines (skillton)*

There is TONS of power to be made with compression...but the disadvantages are great too. I had an ABA turbo setup with a hybrid junkyard turbo setup that made 191whp with minor tuning on 11psi @ 10:1 CR. It would ping like an SOB if I was not running some real good gas though, due to my aggressive timing curve.


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## scarboroughdub (Jul 8, 2002)

*Re: High compression turbo engines (B4S)*

i never said low compression is bad on turbo motors








hence why my motor is higher comp than normal


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## 2kjettaguy (Dec 24, 2000)

*Re: High compression turbo engines (scarboroughdub)*

Last winter I ran 15 psi in my 2.0 turbo @ stock 10:1 compression. The temps outside were 35 or below and I always had 2-3 gallons of Toulene in the tank. The car was fastest at that point than it ever was. I loved it. 
If I had it to do over again I would have stayed with the high compression / low boost setup. Or, I would have gone all out with a gigantic turbo and standalone. Either way, it was totally fun.


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## Agtronic (May 14, 2001)

*Re: High compression turbo engines (2kjettaguy)*

My next motor will definitely be 10:1. Even if I decide to go forged pistons, it will be 10:1, and shoot for the most boost I can get on that compression / octane. Tuning makes everything possible. http://****************.com/smile/emthup.gif


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## fast_a2_20v (Jun 25, 2004)

no doubt its all in the tune. 
On the other hand the difference from static compression can be smaller too. For instance i just watched my buddies car make 509whp @ 14psi, on 91 octane, and thats only 9:1 
revs to the sky and beyond though


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## blackmkIII (May 18, 2004)

*Re: (fast_a2_20v)*

honda i suppose...........thats gotta be some kind of record for that much hp at 14 psi. if its a street car of course. u see quite a few making 420 or 430 at 14 psi, but 509.....damn!


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## mrkrad (Nov 9, 2000)

*Re: (blackmkIII)*

vw's new 16V is 10.5:1 turbo charged, and will run on 85-87 US octane so figure it out. It's all in the efficiency, management, and tuning.
to push the limits, you must find the limits however.


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## 2kjettaguy (Dec 24, 2000)

*Re: (mrkrad)*

Keep some spare pistons around


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## StevenT (May 28, 2002)

*Re: (2kjettaguy)*


_Quote, originally posted by *2kjettaguy* »_Keep some spare pistons around









amen to that, I've already gathered a couple of extras!


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## scarboroughdub (Jul 8, 2002)

*Re: (2kjettaguy)*


_Quote, originally posted by *2kjettaguy* »_Keep some spare pistons around









how else you gonna learn


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## dohc (Sep 28, 2001)

*Re: (mrkrad)*


_Quote, originally posted by *mrkrad* »_vw's new 16V is 10.5:1 turbo charged, and will run on 85-87 US octane so figure it out. It's all in the efficiency, management, and tuning.
to push the limits, you must find the limits however. 

Ya, but thats with direct injection, much different setup. I believe with direct injection, even 11.5:1.00 is possible.


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## Hammertime (Nov 23, 2004)

Very good article. http://****************.com/smile/emthup.gif


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## Flite (Jul 16, 2004)

I've been saying this for a while. With a good tune compression is your friend......to a point. A good buddy of mine has a 1.7 liter Civic Si that makes upwards of 615 whp and does so with something like 10.5:1 compression. 
For now I'll be running a basic EIP stage 2 kit but as I go beyond this I want to raise my compression to around 9.5 or 9.8:1.


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## traderarturo (Oct 28, 2004)

*Re: (Flite)*

Good post.. I see what you guys are saying.
Im running a 8.5:1 2.0l 8v, and the first 3000 rpm feel slugish.
Im going to run onced I finalize the project in january 20 to 25 psi, waiting on conrods and balaced crankshaft.
My friends 2.0T drives a lot better, or even a stock gti can hit 3000rpm way faster... I guess my question would be can I hit 20psi on let's say 9.5:1 to even 10:1 with good tuning, and race gas.


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## dohc (Sep 28, 2001)

*Re: (traderarturo)*

It all depends on what you want. Some people lower there compression in order to bump up there psi because the compressor is more efficient at higher pressure ratios. But in general, higher compression is always good...to a point.


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## dmiller9254 (Sep 8, 2004)

*Re: High compression turbo engines (skillton)*

I won't state my personal opinion too much but I can give a slightly contradicting example. The Indy cars of the 80's used to run what would be considered low compression and ran somewhere in the neighborhood of 45psi of boost. The interesting part about this is that the displacement of these engines was probably similar to that of a mid-large sized motorcycle engine, and the slowest cars on the circuit made over 1000bhp. But, in accord with the original argument, if you could, in theory, run the same engine with around the same boost at even a slightly higher compression ratio, you would get more horsepower. However, I don't know of a fuel that exists that can handle a higher compression engine with anything near 45psi. The trick as a tuner is to optimize compression and boost to get the throttle response and max output that you would like in order to meet your driving needs. http://****************.com/smile/emthup.gif


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## dohc (Sep 28, 2001)

*Re: High compression turbo engines (dmiller9254)*


_Quote, originally posted by *dmiller9254* »_I won't state my personal opinion too much but I can give a slightly contradicting example. The Indy cars of the 80's used to run what would be considered low compression and ran somewhere in the neighborhood of 45psi of boost. The interesting part about this is that the displacement of these engines was probably similar to that of a mid-large sized motorcycle engine, and the slowest cars on the circuit made over 1000bhp. But, in accord with the original argument, if you could, in theory, run the same engine with around the same boost at even a slightly higher compression ratio, you would get more horsepower. However, I don't know of a fuel that exists that can handle a higher compression engine with anything near 45psi. The trick as a tuner is to optimize compression and boost to get the throttle response and max output that you would like in order to meet your driving needs. http://****************.com/smile/emthup.gif 

Bingo! http://****************.com/smile/emthup.gif


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## VW blk Jetta3 (Jan 16, 2003)

*Re: High compression turbo engines (dohc)*

im 10:1 right now and thinking about lowering my compression. im boosting 10psi right now


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## blinkinbanana (Jul 14, 2000)

this is more for blk jetta. but I recently lowered my compression via pistons because I cracked my old ones. I decided to go with 9:1 over 8.5:1. Reading this I would have liked to stay closer to the stock 9.6:1 and worked on my fueling a little better.


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## VW blk Jetta3 (Jan 16, 2003)

*Re: (blinkinbanana)*

since I have sds and can adjust anything dealing with fuel and timing I guess leaving it at 10:1 is the way to go. I should just ease into everything


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## xdavid (Dec 8, 2002)

*Re: High compression turbo engines (2kjettaguy)*


_Quote, originally posted by *2kjettaguy* »_Last winter I ran 15 psi in my 2.0 turbo @ stock 10:1 compression. The temps outside were 35 or below and I always had 2-3 gallons of Toulene in the tank. The car was fastest at that point than it ever was. I loved it. 
If I had it to do over again I would have stayed with the high compression / low boost setup. Or, I would have gone all out with a gigantic turbo and standalone. Either way, it was totally fun. 

Toulene? I've heard of several people using toulene. I've tried it myself, but the increases in octane weren't very obvious compared to using Acetone at about 10%. 
Actually, I got better results using 10% acetone than I did using 20% Toulene. 
Of course, if you use Acetone, you need additional lubrication in the fuel, but that's easily fixed by adding 16oz of 2 cycle oil per gallon of Acetone.


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## xdavid (Dec 8, 2002)

*Re: High compression turbo engines (xdavid)*

I've used Acetone for a while now. Started with my '87 5000CSQ about 10 years ago. It made a huge difference. 
On that car, if you turn up the boost very high (over 20 psi back in 1994 was A LOT of boost on a stock motor with stock CIS fuel injection), the engine will not detonate, because the ECU adjusts the timing according to the amount and quality of fuel available, to avoid detonation. 
It's a very good safety measure, but the car simply does not have the expected power output with over 20 psi that you would expect. 
By either running racing fuel (over 100 octane), and huge power increase is easily felt, because the computer does not retard the timing as much to compensate for insufficient fuel or octane. 
I tried Acetone, and have used it ever since, (whenever extra octane was required).


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## mrkrad (Nov 9, 2000)

*Re: High compression turbo engines (xdavid)*

hmm these octane adders increase EGT, not having sufficient fuel increases EGT. Both would not be desirable.

toluene 100% is like 114 effective octane up to 20% mix
xylene is 118 effective octane up to 10% mix.
you also need to think bout the folks you drive behind as some of these chemicals you guys put in your cars are probably not enviro friendly. I've used liquid freon to clean the carbon off my pistons when i was 16 (87) and well know that i actually know the byproduct of doing this, i would probably never do so again. Thank god the dog wasn't tied to the bumper of the car lol. or any small kids around.
race gas like 114AV or C/VP-16 are great, the sweet smell of lead poisoning your brain is intoxicating


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## MarcoVR6SC (May 3, 2003)

*Re: High compression turbo engines (mrkrad)*

One must not forget the valve overlap, I don't now how much valve overlap those Honda’s with VVT have, but valve overlap decreases in a certain way the CR (especially with FI).


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## xdavid (Dec 8, 2002)

*Re: High compression turbo engines (mrkrad)*

Xylene was another additive I tried, but still no camparison with Acetone when both were at 10%. 
Acetone has an octane rating of 150. Yes, ONE HUNDRED AND FIFTY. 
So pure xylene has an octane rating of 118?


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## Vr6Fidelity (Oct 4, 2001)

*Re: High compression turbo engines (xdavid)*

Ill admit i ponder my future compression ratio daily. I have a VR and id like to make 350-450hp Daily driven. Now i know all about turbos and and power boost fueling etc, But where is the 300+ hp C/R sweet spot? I know higher comp= more power off boost but less total boost can be used... So for a max hp daily driven car what C/R do all of the HIGH POWERED VR guys recommend. I don't want a low compression slug but i do want huge power under the curve.
Surely there is a formula for cylinder power based on moles of fuel, air, and heat and pressure.
Now if i had that i would plot that function in excel, taking into account octane available, compression ratio, and PSI and find the most area under the combined curves. That should be the best combination of all the variables.


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## gltuner (Oct 7, 2004)

here is my take on compression with FI:
If you have FI, you need high octane, or else your motor will knock and ping and then you could run into possible detonation. 91 seems to be the average/common where I live, some places sell 93. So you run 93 octane and running good boost pressures (10-12 psi or more). You want more power, so you turn up the boost (like we all do). Knocking and pinging occur since charge temps are getting really high on the compression stroke causing predetonation. To fix this problem you can A)Run higher octane gas or B)lower your compression. Since higher octane is not readily and easily accessible to most, lowering the compression is the ony suitable alternative. This is all assuming that your motor can handle those pressures. You will get more power from a higher comp. motor, but you have to get a fuel that also has a higher ignition rating (or octane for anyone lost in my speal http://****************.com/smile/emthup.gif)


_Modified by gltuner at 3:27 PM 12-1-2004_


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## bobqzzi (Sep 24, 2003)

*Re: High compression turbo engines (dmiller9254)*


_Quote, originally posted by *dmiller9254* »_I won't state my personal opinion too much but I can give a slightly contradicting example. The Indy cars of the 80's used to run what would be considered low compression and ran somewhere in the neighborhood of 45psi of boost. The interesting part about this is that the displacement of these engines was probably similar to that of a mid-large sized motorcycle engine, and the slowest cars on the circuit made over 1000bhp. But, in accord with the original argument, if you could, in theory, run the same engine with around the same boost at even a slightly higher compression ratio, you would get more horsepower. However, I don't know of a fuel that exists that can handle a higher compression engine with anything near 45psi. The trick as a tuner is to optimize compression and boost to get the throttle response and max output that you would like in order to meet your driving needs. http://****************.com/smile/emthup.gif 

Indy car engines of the 80s were 2.65 liter Alcohol burning V8s that ran extremly high static compression (@11.5-13:1) turned huge RPMs for the time (10,000 to 13,000 up by the end of the decade) and ran VERY low boost of about 7 pounds, depending on the year. Most common was the Cosworth DFX, but later in the decade the Chevrolet Illmor ingnited a technology war that pushed revs higher,and caused USAC to lower allowable boost (Stock block V6s were allowed about 80% more boost). The max boost was determined by the regulations, not what the engine could take. Compression was raised to the maximum that was practical. They made @700-800 horsepower, althought the stock block Buicks were rumored to make about 1000Hp, which was why they always qualified well, So, your example is completely wrong.
As for the original post- the Honda author makes an erroneous assumption- that effective compression= horsepower. The point is to get as much fuel and air into to the cylinder as possible before detonation is encountered. Sure, increased compression helps to burn the mixture more effectively, but in the trade off between boost and compression there is a sweet spot determined by the engine design and intended use. A turbocharged street engine is going to make a heck of a lot morepower with 9:1 compression than it will with 11:1 because you can stuff a lot more a/f in the cylinder before detonation.
Don't get me wrong, tuning is of paramount importance, but it certainly doesn;t mean that more compression means more power.


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## TAIVW-boosted dubs (Nov 30, 2004)

*Re: High compression turbo engines (bobqzzi)*

Compression is something that will always be discussed,but may take on it is this;IF you are building street engines,in a street car that you have to drive everyday on 92 octane,why not lower the compression........this is simple physical things at work here.......knock limit...you will be knock limited by your compression ratio and fuel used,as well as ambient temperature and IC/turbo efficiency.......I live in a hot climate year round and the best thing to do is have lower compression.........I run 7.2:1 and make approx 330 whp on 23 psi boost,which is not too bad for that low compression.....EVERY car that I lowered compression on does not:blow ringlands,ping,and NOT make power........as for Hondas.......yes THEY make great HP at high compression,but most are running on ragged edge of blowing up......
In Hawaii every year we have "Hawaiis fastest street car shootout.......we have 10 and 9 sec Hondas here.......BUT this year 1 Honda showed up,and 2 240's with Sr20DET's won the import class with low 12 ET's...if I entered I would have got 4th place,lol......you know where the Hondas where? They had all blown up and were in pieces getting fixed..........I told myseolf if I enetered it would be to win,not get "4th place"...but that tells you somethimng about the RELIABILITY of high comp,and turbo.........There are 8 Honda shops here........and 2 VWshops,and I would have taken 4th.only because my car is RELIABLE.......low comp baby!


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## dohc (Sep 28, 2001)

*Re: High compression turbo engines (TAIVW-boosted dubs)*

Well, the thing that some people seem to forget is that forced induction and NA motors (if all else being equal) do not require the same ignition advance. The reason turbo motors make more torque (not power, you can't measure power) is burn more fuel for longer, providing more harnessable pressure during the power stroke. The denser air/fuel charge of a boosted motor will burn way faster then a charge an NA motor would have. This means you can dial back the timing and still still get the maximum cylinder pressure at the optimal crank angle.
Like mentioned above, choosing comp ratio totally depends on what you want and how your gonna tune it. Sure raising compression will increase knock if you don't make any changes to compensate for it.


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## jimivr6 (Feb 18, 2003)

*Re: High compression turbo engines (MarcoVR6SC)*


_Quote, originally posted by *MarcoVR6SC* »_One must not forget the valve overlap, I don't now how much valve overlap those Honda’s with VVT have, but valve overlap decreases in a certain way the CR (especially with FI).

http://****************.com/smile/emthup.gif very true


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## Vr6Fidelity (Oct 4, 2001)

*Re: High compression turbo engines (jimivr6)*

Bump so anyone have a guideline for vr6's using 91-93 octane?
aka compression= boost/3(pi)*heat of the sun? 
im gonna get eith low comp pistons and go 20+ psi or 10:1 at a girlish 8 psi and no blow up.
I dont have the money to blow stuff up either. id like my Peloquin clutch gears axles and motor to last another 100k.


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