^ Ya honestly I think people have insensitive butt dyno's.

Lower compression a couple points FOR SURE LOWERS HP as well as torque. I'm not sure how so many people can argue otherwise....

Especially when everyone talks about RAISING compression in a N/A motor a half point and "feeling" the difference. Now a half point isn't much but a couple points, that's a lot. I've raised the compression a single point in a motorcycle and it was night and day. I'm not sure how this could be any different.

Compression ratio w/ 2mm or 3mm
https://www.s2ki.com/forums/index.php?showt...0&#entry5991746

Dropping from 11:1 down to 8.9:1 is a very big difference. I estimate somewhere around 12hp per compression point. Drop a 2.2 points by using a 3mm and you'll loose a little under 30hp I believe.

People sometimes use the very rough estimate that +1psi=15hp for our motors. If you go off this estimate and calculate the amount volume that must be made up for it would no doubt be noticeable.

* just found this. JDM = 11.7:1 @ 247hp
USA = 11.1:1 @ 240hp

^ this is just a .6 difference and we're seeing a 7hp difference, this would support my estimate of 12hp quite well.

In any case YOU SHOULD DROP COMPRESSION ANYTIME YOU GO F/I END OF STORY!!!!!! Please don't think I'm fighting to keep it stock. I'm not. Mine I'm going to build to 8.5:1 unless plans change. PLEASE ALSO KEEP IN MIND... general rule of thumb is to keep compression as high as possible on your setup prior to detonation occurring, the other thing to consider is that if you have a small size turbo or you are supercharged you'll end up wasting some of the compressor flow capacity to make up for the decrease in CR. For example DO NOT use the Greddy kit and then lower compression with a 3mm gasket... I'd laugh when it goes on the dyno and makes 250whp because the tiny turbo can't flow enough to create enough psi, and it'd runner hotter which = bad news.

There is one last thing to consider, lower compression = more lag because the motor is flowing less exhaust gas which is what spins the turbo. This is rather minor and is made up for on the top end quite easily but it's something to consider, its why I believe Stock gasket <350whp, 2mm gasket for <450whp, and 3mm for >450whp but that's only a personal opinion. That would come down to the usage of the car partially too.

-Greg

 

^ maybe some will agree/disagree.

-G

 

dont get me wrong - i am not disputing that there is some sort of power loss. i totally agree. i'm just saying that for daily driving and putting around town i dont feel anything.

 

Even a large turbo will spool enough soon after throttle application to make up for the drop in compression, if not instantaneously. If you measured engine torque with a stock HG and 3mm HG immediately after going full throttle, of course you could see a slight advantage with a stock HG. In real world driving, it is more or less imperceptible.

The much bigger change in driveability is the off-throttle response. Engine braking is directly related to engine compression and a 3mm headgasket results in an approximately 20% drop in compression. You WILL be able to tell the difference.

Particularly at low speeds, where a high compression ratio can make even a well-heeled driver lurch to and fro, you will notice a change.

 

I forgot compression braking, that was noticeable in motorcycles in a annoying way when increased.

Even sooo .... F/I SHOULD drop compression to 8.5:1-9:1 IMO.

for DD its not like you need 500whp anyway. When we need the hp we'll be on boost anyway.

-G

 

I got a 2mm HG in, since this past February. Can't really feel the difference, but I do get black soot all over the rear of the car if I go close to redline.

I had is dynoed (with a cold air intake, 2mm HG, and stock everything), 156HP on dyno dynamics! I'm about 20 hp weak on that dyno, the tuner said that it's because I'm too rich up top. I'm onto lower 11s after 6k, with a dip into the 10s territory momentarily. So since I'm in the process of putting the turbo on, and hooking up the Emanage (w/ a Harness), about 7 psi, I'll be tuning, so the tune will take care of the A/F mix.

 

With tuning, you should make just as much power out of boost as you did stock with the higher compression.

My car felt more responsive even before the turbo spooled.

 

It's all about the tuning. With a lowered compression you can advance the timing to get the power back.

Most people who has the head gasket installed has a tuned ECU, so it's not exactly a fair comparison.

 

...wait WHAT?


Your saying with compression dropped it should feel better???? Even OFF the boost?

-G

 

Here is mY question, everyone asks about drivability. What does anyone do about the additional timing? IE, lifting the head 3mm is stretching the timing chain while advancing cam timing.. Is this too minimal to be concerned with? Technically it would change at least 2* timing..