http://www.i-club.com/forums/showthr...hreadid=101967
or, for the lazy ones:
I posted this in another thread but wanted to give it it's own thread because I
think it is a question that a lot of people have and there are a lot of misnomers
about how Compression ratio and boost affect your engine. So I don't have to
write the whole thing out again I just cut and pasted it. I love that function!
anyway:

This is a difficult question to answer because so many factors play into it.

Say a car maked 300hp with 8lbs. of boost with 9.5:1 compression. Now say that
the person lowers the compression to 8.0:1 and runs 14lbs of boost and makes the
same 300hp.

Boost and compression ratio combined is known as static compression ratio. On a
street driven car on pump gas, your goal is to keep your static CR under 18.0:1 or
you sill start getting detonation. Fort short periods of time, such as a full throttle
1/4 mile pass, you can go up to 22:1 but don't stay there long . And If you run a
timing retard you can run a higher Static CR but I would not recomend going much
higher. Here is a good chart to show what your static CR will be based on your CR
and boost:



As you can see, an engine with a 9.0:1 CR running 4lbs. of boost nes you a static
CR of 11.4:1. Similarly, an engine with CR of 8.0:1 running 6lbs. of boost has an
almost identical static compression ratio! In theory (and it is what happens) these
too engines should make the same hp. because all things else being equal, they are
pushing the same amout of air through the engine.

The problem with a low compression motor is that it relies heavily on the turbo for
its power. An 8:1 motor is definitely not going to to make monster power. Sure,
you can throw 18 lbs of boost on it and get some real power, but why? A higher
compression motor of 9.5:1 will have much more power without the turbo. Then,
with less boost you could easily have the same overall power - only it would be
much more usable. Both of the motors (8:1 with 4lbs boost and 9.0:1 with 6 lbs
boost) will have almost the same effective compression and about the same overall
power. The big difference will be where you see the power, and how much of a
demand will be placed on the turbo. Obviously, the 9.0:1 motor is going to have far
greater torque and low end power as the boost is only starting to come in.

The high compression engine will make more low end power, like stated, and have a
flatter torque curve (As flat as one can be for a turbo car anyway) than a lower
compression engine. Also, it will generally reach it's power peak at a lower rpm
than a lower compression engine.

So, if you build the car right and it is for the street, go for the highest possible CR
that is safe for the engine. Keep in mind also that the less boost you run, the
cooler the charge will be and the less you will work your turbo. Not to mention, the
car will be more drivable.

So why do automotive companies generally go with low compression and higher
boost? One of the reasons is that the parts used in mass produced cars are not of
the highest quality. The companies err on the safe side and use low compression
and medium amounts of boost that are easily within range of a safe CR.

If you build the car right, in my mind the ideal compression ratio for our engine is
9.0:1. Most people will run from 8.0-8.5:1 to be safe I would guess though. But,
for me, 9.0:1

Eric

Tom
try and read the whole thread though, esp the pointed comment about "the honda boys"

 

very good read
thanks

this will help me choose my compression when i rebuild my motor





[Modified by immortal, 9:54 AM 10/15/2001]

 

That link is tiite.

I tend to agree w/ that MattC guy. There's a civic down here in FL that runs lower compression pistons and approx 18 psi (street) w/a straight T04. Does have less bottom, but when that big boy lights up at 5K - hold on w/ both hands....

Depends on what you want out of your car.

 

Proof in point that ignorance does not depend on which type of car you drive.

Apparently mister man 'Imprezinator' forgot that with added boost you get higher compression and are burning more fuel. Torque increases due to static compression and boost are not the same. He also seems to fundamentally misunderstand the difference between pressure and flow.

And somewhere along the line, he decided that static compression and dynamic compression were interchangeable terms. I guarantee you that dynamic compression (what he is actually talking about) is affected by more factors than just boost pressure and static compression ratios.

Dustin

 

I got a beef with the statement that MattC made saying that a honda with 12.67:1 compression will run 15's. Try 12's buddy. Otherwise I agree with everything else he said.

 

good post

I learned a whole bunch of shiznet from reading all that.