The compression ratio on the evo is 8.8:1 running 19.5 psi and the sti's compression ratio is 8.2:1 running 14.5 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.

 

agree 100% "lower" compression for boosted engines are better. Supra runs 8.5:1 thats why it can run 2 bar of boost
The STi will have a good compression ratio as well.

The Evo sacrifices lower compression for better low range accelleration. This means it wont take boost as well as a lower compression motor, comparitively.

 

I disagree.

It depends highly on what you want to do with your car. I've seen Civics running over 1.4 bar (20psi) with a static compression of 10:1. A car like that, has very fast spool up, good off boost torque and is a very "drivable" car, day to day.

A car like that also had to have the head ported and polished and the inside of the cylinders and the top of the pistons polished to prevent detonation, and many many hours on the dyno tuning it to make it reliable yet powerful.

Let's just say you're not happy with the 8.8:1 compression with 19psi boost and you wanted to lower it to your "racers only run 5:1 or even 6:1".

Let's do some maths:

Effective Compression = Static compression x (1 + Boost/14.7)

E.C. = 8.8 x (1 + 19/14.7) = 20.17

Working backwards to see how much boost we need:

20.17 = 5.0 x (1 + x/14.7)... and isolating X
x = ((20.17/5.0) - 1) * 14.7 = 44.59psi

Yep. You need to more than double your boost to make the same effective compression. Now think about this:

How big a turbo do you need to make 44psi on a 2 litre engine? How much lag will you have? Considering your compression is already down off boost (and therefore less exhaust gas is being expelled) how long before you got to full power?

A car like that, would be a true race car - that is, it would need a rolling start, because it certainly wouldn't be idling real well, and it'd have to be revving very high before it got any speed.

 

I thought if you have high compression...you wont be able to handle that much boost

If you have low compression, you can boost a lot.

 

with higher compression you will not need to run as much boost to make the same power. This is were good tunning comes in. There are a few s2000's here that run 14 psi on the stock motor.

on the other hand if you are running a large turbo where its efficiency range is in the 18+ psi range then low compression high boost is where its at.

 

Generally true,

But there comes a point where dropping compression any further is self defeating...

Likewise, if you're willing to spend big bucks on ensuring the combustion chamber is in perfect shape, the car is well tuned and the engine block is kept cool, you can generally run high compression high boost cars too. All a matter of money.

Let's just say that 8:1 and 1.5bar of boost is the most "economical" way to deliver power, where raising or lowering the compression can also yield bigger power, but at higher cost and different characteristics.

Also, I just realised the effective compression formula is slightly flawed when you're talking about such a huge drop in compression. The principle is still the same, but the numbers will be slightly off.

 

I was going to run my S2000 at 11:1 @ 10psi with a T03/04B.

I figured for the money I was going to spend, I might as well get an Evo instead

So now I have an S2000 sitting in my garage and a lot of eager anticipation of my just ordered Tommi Makinen Evo VI, direct from Japan !

 

Some great input so far guys.
someone on another forum asked me to make some speculations about final compression ratios. So here it is. I got this by simply running the psi, with compression ratio and atmosphere through a compression calculator to come up with these numbers Not very scientific but gives a ball park.

There are obviously ALOT of variables that I have not taken into conscideration on both these cars. I use the sti and evo as a comparison.

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

 

Let's not confuse compression of intake air versus compression of the air/fuel (A/F) mixture.

More air allows more fuel to be mixed within, creating a more powerful feed for the engine.

More A/F compression brings the fuel mixture closer and closer to spontaneous combustion, which is more commonly known as detonation.

When you turn up the boost, you're increasing the amount of A/F mix that enters the combustion chamber.

When you increase A/F compression you're only bringing the A/F mix closer (or past, which is bad) to it's combustion point.

The point of having less compression in a forced induction engine is to reduce the need to compress the mixture. The engine's compression cycle generates heat and robs the power stroke in the other cylinders. Since turbos shove a bunch more mixture in the cylinder, there's little need to compress it that much.

Now to raise A/F compression in just about any engine you will need to do one or more of these things:
- Compress/Buy head gasket
- Shave/fill the head
- Shave the block
- Lengthen the connecting rod (provided it won't make the piston hit the head)
- taller piston (provided it won't hit the head)

If it seems like a lot of work it is! Doing these things will give you bigger compression, but you will lose cylinder volume meaning that less air will be able to get into the cylinder. Consider that.

It's much easier to turn up the boost a little rather than increase cylinder compression. I did some off the cuff math and was able to figure that bringing compression up to about 10:1 is the equivalent of bringing boost up to about 22 psi... given that the volume of the cylinder doesn't change!

high A/F compression + high air compression = heat problems and fuel sensitivity issues at high revs
low A/F compression + high air compression = good (typical forced induction engine profile)
high A/F compression + low air compression = good (typical normally aspirated engine proflie)
low A/F compression + low air compression = no power

Whatever you do it's a real good idea to tune your car on a dyno. Now I just wish Pruven was closer than 2 hours away for me...

Sorry Sunder, your Effective Compression equation fails to account for volume. Check out dlowman's math and it doesn't really make sense how the STi ('final compression' of 16.29:1) makes more power than the Evo ('final compression' of 20.47:1), unless you factor in volume. My WS6 has a A/F compression of 10.5:1 with no forced induction (Ram Air doesn't really count)... so how could that engine (340HP) surpass Evo's (271HP) and STi's (300HP) engine power? More volume. (8 cylinders at 5.7 liters of total displacement)

 

http://store.yahoo.com/speedupgrade/efcominchoos.html

they have refrences to b16's and b18's but still gets the point across

 

It's not entirely wrong but it does not account for a couple of things.

First, the stock EVO's effective boost at 6500 RPM is only 16.24 PSI making the final compression @max HP 18.42:1 (The 20.47 final compression @ 3500 rpms WOT explains the cars excellent mid range responsiveness)
Now let's factor in total displacement with the final compression ratios:
EVO = 18.42 *2 liters = 36.84 liter capacity @ 6.5k rpm=(271 HP)
STi = 16.29 * 2.5 liters = 40.725 liter capacity @ 6k rpm=(300 HP)
WS6 = 10.5 * 5.7 liters = 59.85 liter capability @ 5.2k=(340 HP)

This should serve to relate the comparisons. Remember these numbers reflect the peak power capability to pump air at the rpm where the each motor develops peak HP.

 

General explanation....................The reason turbo cars have lower compression is because when the turbo is at full boost, there is a surge of compression to the engine that raises the ratio to like 18:1, 20:1, etc. That's why we can run 19 psi. When you stick a turbo on an NA car you can only run like 4 psi because the compression is already higher on the NA engine. The only way to run more boost would be to lower compression.....head spacer, cams, etc then add more fuel.

 

For those of you who'd like to point out I was wrong, I'd like to point out my disclaimer, in my original post:

Yes, I realise that the forumla doesn't take into account the extra space in the combustion chamber from decompression (the forumla was originally used when trying to figure out how much to decompress an engine to aftermarket turbo it). When talking about small drops, this increase is neglible. However, I do recognise that it does need to be taken into account, when comparing engines which have been designed differently.