# 12.3:1 compression?



## mattkosem (Apr 29, 2004)

I see pistons available for 12.3:1 compression in an 8v 1.8L engine. Would compression this high be possible with pump gas if the engine was set up with megasquirt?
--Matt


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## mattkosem (Apr 29, 2004)

And would it make a difference if the JH head was replaced with a 2.0L OBD1 head?


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## antichristonwheels (Jun 14, 2001)

*Re: 12.3:1 compression? (mattkosem)*

no


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## mattkosem (Apr 29, 2004)

Would using a headgasket that lowers comrpession by one point help anything?


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## racingvw92 (Apr 17, 2003)

*Re:  (mattkosem)*

With 12.3:1 your going to have to use race gas, atleast over 100 octane. Make sure that knock sensor is working i you got one. You want some thing under 10.5:1 if you want to start thinking about pump gas.


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## mattkosem (Apr 29, 2004)

*Re: (racingvw92)*

A HG that drops compression by one point and an aftermarket cam with extra overlap should achieve that no? I would think those mods would bring the compression down to around 10:1, is this not the case?
--Matt


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## Peter Tong (Jun 17, 1999)

*Re: (mattkosem)*

If you are starting off with 12.3:1 pistons and then trying to drop the compression would it not be more logical and cost effective to start with 10.0:1 pistons to begin with? I would think so... 
Around 10.5:1 and things start pinging when the weather starts getting hot. Best keep CR down a bit so you have a bit of margin with regards to the knock limit.
Peter T


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## snowfox (Apr 4, 2002)

*Re: (Peter Tong)*


_Quote, originally posted by *Peter Tong* »_Around *10.5:1 and things start pinging *when the weather starts getting hot. Best keep CR down a bit so you have a bit of margin with regards to the knock limit.
Peter T

...or even sooner/lower. The ABA/counter-flow hybrid that I have run for years weighs in at 10.3:1(assuming _unshaved_ head). No knock control for me means premium fuel is a MUST - _all year round_.


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## mattkosem (Apr 29, 2004)

*Re: (snowfox)*

Cam + 10:1 comp pistons gives like 8.something:1 effective compression. The engine should get a boost in compression to support the cam no? Cam + 10:1 is better than cam + 8.something:1 effective compression right?
--Matt


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## racingvw92 (Apr 17, 2003)

*Re: (mattkosem)*


_Quote, originally posted by *mattkosem* »_Cam + 10:1 comp pistons gives like 8.something:1 effective compression. The engine should get a boost in compression to support the cam no? Cam + 10:1 is better than cam + 8.something:1 effective compression right?
--Matt

Where did this big cam theory come from to lower CR? I work on GM 305's with 10.3:1 and big cams and to check CR, we whistle the cylinders. Thats slowly turning the motor over by hand and watching the CR come up at TDC. Just because you open the valve more doesn't mean you lower CR. A bigger cam is designed to let more air in and trap it sooner. Now a real sh!ty cam will lower CR I bet. And if your going to turbo your motor, you need a turbo cam, like the ones on the G60 motor.


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## superl8 (Dec 31, 2002)

*Re: (racingvw92)*

What he said /\ - static compression is not something that changes with a cam. It's a simple ratio - volume in the cylinder. BDC to TDC. What your thinking of is IIRC referred to as dynamic compression. Measured with a gage. A huge cam and carbs and this might run on pump gas. I'm guessing on that though. I've never run this sort of setup. My ABA with shaved counterflow with a 270/280 cam has never had any pinging issues though - about 10-10.5 : 1


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## MkIIRoc (Feb 20, 2005)

Matt, you are correct, but I can't give you a solid number as to where you will be ok.
racing92- uh, no. Put a big cam in, check compression pressure. Put a small cam in, check compression pressure. A bigger cam does let air in sooner, but it also holds the intake valve off the seat during the compression stroke, and air is pushed back out of the cylinder.
How did you measure your actual compression ratio with the motor assembled? you need to take into account the measured combustion chamber volume, headgasket crushed thickness, piston dome volume, etc.
a cam with higher duration lowers dynamic compression ratio. Why do you think every cam manufacturer suggests raising static compression when you put larger and larger cams in. 
And a turbo cam for a turbo motor. good one. I've seen plenty of dyno comparisons where a high overlap cam produces significant gains in a boosted motor.
super- why do you think he reffered to it as effective CR? like gross and net.
The amount of ignorance perpetuated in this forum kills me.


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## racingvw92 (Apr 17, 2003)

*Re: (MkIIRoc)*


_Quote, originally posted by *MkIIRoc* »_
racing92- uh, no. Put a big cam in, check compression pressure. Put a small cam in, check compression pressure. A bigger cam does let air in sooner, but it also holds the intake valve off the seat during the 
The amount of ignorance perpetuated in this forum kills me.

When the engines are whistled, they are complete engine assemblies. They are turned over by hand moving all the internals at the correctly timed locations. The whistler measures the pressure change inside the cylinder by through the spark plug hole.
Why would you want the valve off the seat during compression, thats the spring not doing its job or a poorly design cam. Have you looked a cam curve or going off what others have said.

Drag racing







, learn to turn and practice your downshifts.











_Modified by racingvw92 at 9:20 PM 6-21-2007_


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## racingvw92 (Apr 17, 2003)

*Re: (racingvw92)*

Also your compression ratio will not drop when ram tuning takes affect. Heres some basic calculations and theory to get you started. Then theres the other ram tuning with a scoop or cowl to the outside of the car. 
http://www.phys.unsw.edu.au/jw/Helmholtz.html
http://www.hotrod.com/projectb...ning/
http://speedtalk.com/forum/viewtopic.php?t=1163


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## MkIIRoc (Feb 20, 2005)

I'm not going to argue with you back and forth, so I'll leave you with this. I know how a cam works. I have an 8v on the edge of 12's. I think I know how to build an engine. Read any cam card. Here's one for you. http://www.crespocams.com/Cams/Cards/VW/VM252c.htm Look at where the intake valve closes. 80* of crank rotation after bottom dead center. That would equate to the intake valve being held open WHILE THE PISTON IS TRAVELING UPWARDS ON THE COMPRESSION STROKE, PUSHING AIR BACK OUT OF THE CYLINDER. Every cam works this way. Some hold it open longer, and open it sooner, equating to a longer duration. Thank you, / thread.


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## racingvw92 (Apr 17, 2003)

*Re: (MkIIRoc)*

Real quick incase you missed it. Ram tuning keeps the air for being pushed back up the intake. The velocity and mass of the air helps to equalize or over come the pressure in cylinder. You can get up to 130% VE that way. 
(Mic hitting stage floor)


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## mattkosem (Apr 29, 2004)

*Re: (MkIIRoc)*


_Quote, originally posted by *MkIIRoc* »_Matt, you are correct, but I can't give you a solid number as to where you will be ok.
racing92- uh, no. Put a big cam in, check compression pressure. Put a small cam in, check compression pressure. A bigger cam does let air in sooner, but it also holds the intake valve off the seat during the compression stroke, and air is pushed back out of the cylinder.
How did you measure your actual compression ratio with the motor assembled? you need to take into account the measured combustion chamber volume, headgasket crushed thickness, piston dome volume, etc.
a cam with higher duration lowers dynamic compression ratio. Why do you think every cam manufacturer suggests raising static compression when you put larger and larger cams in. 
And a turbo cam for a turbo motor. good one. I've seen plenty of dyno comparisons where a high overlap cam produces significant gains in a boosted motor.
super- why do you think he reffered to it as effective CR? like gross and net.
The amount of ignorance perpetuated in this forum kills me.

This is the way it's always been explained to me...







We measured, with a gauge, a reduction in the compression on a friend's car when a high-lift high-duration cam was installed. There's enough information out there to back up either theory...
--Matt


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## eurotrashrabbit (Oct 25, 2001)

*Re: 12.3:1 compression? (mattkosem)*

It would not be a problem to mix 93 and higher octane fuel did this in my 12:1 comp 8v for 3 years


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## mattkosem (Apr 29, 2004)

*Re: 12.3:1 compression? (eurotrashrabbit)*

Hmm, mixing doesn't sound like anything I'd want to do with the daily driver. Maybe meth injection would make it happen, I'm okay with keeping that going. Windshield washer fluid is cheap, and I'm sure I could fit a big reservoir somewhere.
--Matt


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## g60vw (Oct 3, 2002)

*Re: 12.3:1 compression? (mattkosem)*

There seems to be a bit of confusion on the difference between compression ratio (static compression), which is the ratio of the volume of air in the cylinder at the bottom and top of the stroke, and cylinder pressure (dynamic compression), the pressure built in the cylinder as the piston moves upward.
A "larger" cam, with more lift and specifically more duration, will never change the compression ratio, but it will tend to lower the cylinder pressure a bit, and like the guy said earlier, is why it is common practice to increase cam spec and compression ratio together to see the most benefit.
Garth


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## antichristonwheels (Jun 14, 2001)

*Re: 12.3:1 compression? (g60vw)*

but you can't expect cylinder pressure changes due to cams at cranking speed to be equal at 6500 rpm


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## eurotrashrabbit (Oct 25, 2001)

*Re: 12.3:1 compression? (g60vw)*

I agree in to the fact that compression and pressure are some times thought of as the same


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## mattkosem (Apr 29, 2004)

My my, I think I'll just spend the money that I'd spend on the pistons on a used ABA and rebuild parts. Displacement seems a lot easier to live with than compression on this engine for daily use. Thanks for the input!
--Matt


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## bugdrvr (Apr 17, 2003)

The nice thing about using the ABA is that you get a bump in both compression and displacement (assuming you have the stock JH 8V). 
I've put together two ABA/JH engines and they are really nice, peppy engines. Great bang for the buck in my opinion.


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## antichristonwheels (Jun 14, 2001)

*Re: (bugdrvr)*

I second that emotion
The ABA lower end swap is easy and works great.


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## The Quinner (Dec 10, 2001)

*Re: 12.3:1 compression? (mattkosem)*

There are a lot of truths, half truths, farthest from the truths...and, everything in-between in this discussion...pressures, compressions, whistles...
There are high lift camshafts...and, there are long duration camshafts. Each one will have an effect on the total package and neither one is the ultimate "best camshaft." Just because a camshaft is designed with lots of duration, does NOT mean it's a sh!ty camshaft or that the valve spring is not doing its job...that is ignorance speaking.
*Generally speaking*, smaller displacement motors (i.e., NOT Chevy V8s) are hot rodded with longer duration camshafts. The additional valve opening COMBINED with additional overlap help to keep the air charge moving quickly...the advantages of this design are generally at the upper end of the rev range (at the expense of the lower end). *_Effective_* compression IS lowered with this design...which is very related to, but not directly the same as combustion pressure or volumetric efficiency.
Another design philosophy is a high pop/short duration camshaft. These CAN allow the SAME amount of air/fuel past the intake valve...making up for less opening time with higher lift. Effective compression is not lowered as much with this design, but the air/fuel charge and the exhaust flow must start/stop more abruptly...not the best for high rpm running.
Modern designs and machinery have led to assymettrical lobe designs...allowing for a gentle and longer opening ramp on the intake valve combined with a sudden closing...the exhaust valve does the opposite: opens quickly and closes on a softer ramp. These can have longer durations without too much overlap...preserving some of the effective compression yet still allowing enough time for the charge to build and fill the cylinder.
The best design for a particular application depends on far too many things. Intake and exhaust design is critical to making ANY camshaft design work...afterall, the camshaft is only there to allow the intake/exhaust to do their thing. Combustion chamber design and its effecct on the air/fuel charge (swirl, tumble)...etc. Again, only ignorance would suggest that one particular design is the best for every engine or that something that works best for a big block funny car might also be the best for a Formula Ford road race car.
An engine is a system of very related parts...not a random combination of aftermarket catalog part numbers. A good tuner understands the relationships and exploits them in order to meet a particular end goal.


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