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Old 04-15-2003, 11:17 AM   #1
dlowman
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Default 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.
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Old 04-15-2003, 11:29 AM   #2
Golfa
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wow man, thanks for the education...seriously

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Old 04-15-2003, 11:45 AM   #3
dlowman
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Welcome,

this is exactly why we are soo much better off with the 14.5 psi that the sti is running. Way more head room to play with boost. The evo is alredy at 19.5 psi. They cannot play with boost as much. They are pretty limited to making horsepower through other mods such as exhaust, ecu, etc. Some may be able to hit 21 psi but only on 93 octane gas or higher. If they go any higher in psi they are going to have to upgrade the internals. (we are talking only 2 psi here.)

I would imagine that the stock sti will easily take up to 19 psi with out any issues.
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Old 04-15-2003, 02:02 PM   #4
dlowman
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someone on another forum asked me to make some speculations about final compression ratios. So here it is.

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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Old 04-15-2003, 03:02 PM   #5
jaypride
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I've always wondered, how do you calculate that?
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Old 04-15-2003, 03:21 PM   #6
dlowman
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Quote:
Originally posted by jaypride
I've always wondered, how do you calculate that?
there are calculators that will do it for you. I am too lazy to do the actual math and there are alot of variables that you would have to put in to get an exact number.

I just ball parked it. I am sure I am pretty close though.

Do a search on the web for turbocharging and compression calculators.
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Old 04-15-2003, 05:18 PM   #7
Jon [in CT]
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dlowman, do you know for certain that neither the Evo nor the STi leave the intake valves open for a portion of the compression cycle, effectively reducing the "final" compression ratio? This strategy is, of course, a prime feature of a Miller cycle engine. Is it inconceivable that one or both of these engines are, in fact, Miller cycle but choose not to advertise/emphasize it?

And how do you account for the Evo's significantly higher effective compression ratio, relative to the US STi's?
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Old 04-15-2003, 06:03 PM   #8
HunterKiller
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Size of the engine possibly?
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Old 04-15-2003, 06:10 PM   #9
dlowman
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Quote:
Originally posted by Jon [in CT]
dlowman, do you know for certain that neither the Evo nor the STi leave the intake valves open for a portion of the compression cycle, effectively reducing the "final" compression ratio? This strategy is, of course, a prime feature of a Miller cycle engine. Is it inconceivable that one or both of these engines are, in fact, Miller cycle but choose not to advertise/emphasize it?
No I do not know for certain if they are miller cycled engines. I do not think they are though. Just a guess.


And how do you account for the Evo's significantly higher effective compression ratio, relative to the US STi's? [/quote]

As I mentioned before the internals can make a difference on the final outcome, but on the one calculator I used I simply put in psi and compression ratio and atmosphere to get a final "ballparked" number. The real difference in the data was the original compression ratio.


Dan.
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Old 04-15-2003, 07:55 PM   #10
jcochran
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Default Miller Cycle

Quote:
Originally posted by Jon [in CT]
dlowman, do you know for certain that neither the Evo nor the STi leave the intake valves open for a portion of the compression cycle, effectively reducing the "final" compression ratio? This strategy is, of course, a prime feature of a Miller cycle engine. Is it inconceivable that one or both of these engines are, in fact, Miller cycle but choose not to advertise/emphasize it?

And how do you account for the Evo's significantly higher effective compression ratio, relative to the US STi's?
I was under the impression that Miller Cycle engines required a supercharger to constantly provide boost to compress against during the compression stroke. Is there a turbocharged Miller Cycle engine somewhere in production?

James
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Old 04-15-2003, 09:41 PM   #11
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but wait..... I am confused now....

A RT-spec equiped WRX putting ~250whp (340hp) @ 16psi on a 8:1 ratio 2L motor.

ok now...

EVO is also a 2litre car but with 8.8:1 and 19.5psi. How the hell is the EVO making so much less power that my WRX if....

- The EVO's compresion is higher than the RT-Spec WRX
- The EVO's turbo PSI is also higher than the RT-Spec WRX

It is obvious to me that then the missing part here is the amount of air they are both processing (combusting) within a unit of time correct?

But then again, a stock WRX and pushing the stock turbo to 16psi only makes about 280hp. So... what is the hidden variable that is making such big power differences between all this setups??? Please some explain?
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Old 04-15-2003, 10:30 PM   #12
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Default Re: Miller Cycle

Quote:
Originally posted by jcochran


I was under the impression that Miller Cycle engines required a supercharger ...
A miller cycle engine requires supercharging, from either a turbocharger or a supercharger.
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Old 04-15-2003, 10:55 PM   #13
BenHayat
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Hey Dan;

When I see knowledgeable people like yourself buying STi, it makes me feel much better that I have made the right choice!

..Ben
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