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Sti compression ratio and why its good

43857 Views 29 Replies 18 Participants Last post by  Trigga
The sti's compression ratio is 8.2:1 running 14.5 psi and the evo is 8.8:1 running 19.5 psi

Why is low compression better for a Turbocharged Engine?
You make horsepower by how much air you move through the motor. A high compression 10:1 engine is more efficient than a 7:1 engine, so the 10:1 engine gives you more bang for the buck. However, because the lower compression is not as efficient, it will move more air through it. So, at 15 PSI of boost, the 7:1 engine will have an effective compression ratio of 14:1, will not be into detonation, and be moving more air, making more horsepower than the same conditions for the 10:1 engine. That engine will be in self-destruct mode, have detonation, and an effective compression ratio of 20:1!
This is why the racers only run 5:1 or even 6:1. All of this is great for a drag car, but because the static compression is lower, you will not have much bottom end torque either. So, since most of us don't drag race every place we go, a good compromise would be 8:1 or 8.5:1 compression. This way you don't loose too much bottom end for driveability, and if you don't run too much boost, say 10 to 15 PSI, you stay away from the gray effective compression area of 15:1 and up.

Remember, that the shape of the combustion area, cam, type of fuel, etc. all play a part of when the engine starts to detonate. It comes down to start with low boost, and sneak it up from there until you run into problems.

Thought this might be an interesting tidbit for everyone.

Please feel free to chime in.

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Those are interesting comparisons you have made. 15psi with 7:1 is equal to 14:1 comp. How do you make that comparison.....I was wondering about that the other day.

I think this is very good news for us. It looks at though the EVO engine is pretty tapped out already while we have quite a bit of room for more power.

Using your "boost = comp ratio" chart, what kind of boost do you think we can run safely with the comp ratio we have?
Yeah, thanks for the numbers. And of course there is more to it....but having an idea is better than nothing.
This is all very true.

And while chipping the EVO will probably take place, I have a feeling it won't be a plug and play by any means. With all the "American" additions, we'll have to see how long it takes.

And of course gas is becoming the big question. The part about the EVO engine that makes me think it's in trouble, regardless of engine control, is that it is already at 19psi (and knocking on 91) to get 271 hp, if that is the true rating. On the other hand, we have 300 hp while only adding 1 psi max boost to the WRX. 1 psi is not taking advantage of the strengthened internals, so that just leads the general thought to be there will be room to work. And, however small the room to work, we are already 29 hp in front of the game.

Time will certainly show what can be done to both engines. I'd like to think I can be pretty neutral when thinking about the two cars as both have very clear advantages. Something about 19psi, only 271hp and knock turning back the timing in STOCK FORM.....that doesn't sit well with me.

And I really wouldn't want to have to buy 100 octane in order to take on some serious tuning.
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Yeah, the turbo will be different. They aren't sure what type yet, but it isn't the rumored VF34, but probably a new IHI ball bearing turbo.

Regardless of what is making the boost, think of it from just the engine side. Pressure is pressure, regardless of what is making it. Compare the compression ratios and the boost pressures...that's all that should matter.

The power comparisons are being made based on the engines having similar compression ratios and boost levels.
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