Sti compression ratio and why its good
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Sti compression ratio and why its good
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.
Dan
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.
Dan
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boy you've got a lot of hate toward the EVO. I agree with you're point but it sounds a lot like you're saying the EVO is about to detonate.
Yes the STi's engine has many great points on paper and looks like it will be very easy to attain more power but....it has yet to prove itself. I'm not saying that it's not capable of producing huge power but we don't know yet. There's a lot more than just taking a low compression engine, adding forged internals, and cranking the boost. I'm sure Subaru did their homework but the fact of the matter is this is totaly new engine. And let's hope that the rumor about the knocking is just that...a rumor.
What we do know is that the EVO engine without changing boost levels can get another 40-60hp with exhaust, SAFR, and intake. We know that the engine has been seen to run with 20-22 PSI reliably.
Yes the STi's engine has many great points on paper and looks like it will be very easy to attain more power but....it has yet to prove itself. I'm not saying that it's not capable of producing huge power but we don't know yet. There's a lot more than just taking a low compression engine, adding forged internals, and cranking the boost. I'm sure Subaru did their homework but the fact of the matter is this is totaly new engine. And let's hope that the rumor about the knocking is just that...a rumor.
What we do know is that the EVO engine without changing boost levels can get another 40-60hp with exhaust, SAFR, and intake. We know that the engine has been seen to run with 20-22 PSI reliably.
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Originally posted by AbusiveWombat
boy you've got a lot of hate toward the EVO. I agree with you're point but it sounds a lot like you're saying the EVO is about to detonate.
boy you've got a lot of hate toward the EVO. I agree with you're point but it sounds a lot like you're saying the EVO is about to detonate.
Obviously I think the sti is better, but that is my preference and the point of my post is to explain how compression relates to psi.
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someone on another forum asked me to make some speculations on final compression ratios so here it is. Again using the evo as comparison just to give something to compare to.
Ok there are ALOT of variables that will effect the final compression rate. But I will give you an example.
I assumed the atmosphere for all these figures was 4500 feet which is where the bonneville racetrack is.
Evo
Stock 19.5 psi at 8.8:1 compression would end up with a final compression rate of 20:47.1
If you boosted that up to 21.0 psi then you would end up with and adjusted for atmosphere ratio of 21.37:1
Sti
Stock is 14.5 psi at 8.2:1 would end up with a final compression ratio of 16.29:1
if you boost that up to 19 psi then you end up at 18.80:1
Again there are other variables that I cannot account for but this is a good Ball park!
Dan
Ok there are ALOT of variables that will effect the final compression rate. But I will give you an example.
I assumed the atmosphere for all these figures was 4500 feet which is where the bonneville racetrack is.
Evo
Stock 19.5 psi at 8.8:1 compression would end up with a final compression rate of 20:47.1
If you boosted that up to 21.0 psi then you would end up with and adjusted for atmosphere ratio of 21.37:1
Sti
Stock is 14.5 psi at 8.2:1 would end up with a final compression ratio of 16.29:1
if you boost that up to 19 psi then you end up at 18.80:1
Again there are other variables that I cannot account for but this is a good Ball park!
Dan
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Ok, here's how it works and why you want lower compression. Putting it simply a turbo motor is a variable compression motor. The more you air you force in (The higher the boost level) the higher the effective compression ratio. Sort of like this. by running 14.5 lbs of boost you are adding a factor of two to the compression ratio because you force twice the amount of air into the cyliner compared to normal atmospheric. So for a normal 8.1:1 ratio, under boost you are effectivley running 16.2:1 compression. This is mainly important in regards to what octane fuel you can run, and is exactly why the evo drops boost at higher RPM, because the effective compression is too high for normally available fuel.
So to answer the comment about power = how much air you flow through let me first say you are correct. The more air that moves through the motor the more power it produces. SO:
Evo. 2.0l and 19.5psi so it is something like 2.3 factor on top of the displacement meaning a peak of 2.0 * 2.3 = 4.6 Liters of air moving through. (Normal atmospheric condition I believe is 14.5 psi, so you techincally always have 14.5lbs of force into the engine. When you are under boost you are actually adding to that, so 19.5lbs ontop of 14.5 = 34lbs total / 14.5 normal and you get the factor of 2.3 times the normal amount of air being pushed in)
STi 2.5l and 14.5psi so it is a factor of 2 onto the displacement meaning a peak of 2.5 * 2.0 = 5 Liters of air moving through.
So, hopefully this helps you understand things a little better.
And guys, please keep me in check. What I've just said is what I've learned myself over the years and in no way is to be considered absolutley correct. Did I screw anything up?
So to answer the comment about power = how much air you flow through let me first say you are correct. The more air that moves through the motor the more power it produces. SO:
Evo. 2.0l and 19.5psi so it is something like 2.3 factor on top of the displacement meaning a peak of 2.0 * 2.3 = 4.6 Liters of air moving through. (Normal atmospheric condition I believe is 14.5 psi, so you techincally always have 14.5lbs of force into the engine. When you are under boost you are actually adding to that, so 19.5lbs ontop of 14.5 = 34lbs total / 14.5 normal and you get the factor of 2.3 times the normal amount of air being pushed in)
STi 2.5l and 14.5psi so it is a factor of 2 onto the displacement meaning a peak of 2.5 * 2.0 = 5 Liters of air moving through.
So, hopefully this helps you understand things a little better.
And guys, please keep me in check. What I've just said is what I've learned myself over the years and in no way is to be considered absolutley correct. Did I screw anything up?
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Originally posted by AbusiveWombat
Here's a quick question (I'm too lazy to dig through the STi info). Does the STi have variable boost like the EVO (19 PSI at 3500 and tapers to 16.5 PSI at redline)?
Here's a quick question (I'm too lazy to dig through the STi info). Does the STi have variable boost like the EVO (19 PSI at 3500 and tapers to 16.5 PSI at redline)?
time will tell for sure.
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