ThE sPaCeCoWbOy

if one lowers compression ratio to get more boost to get a certain HP, the raising the compression ratio will cause you to lower boost but can still get you the same HP

ex. you have a 9.5:1 ratio

lower to 8:1 with 12 psi to get desired power

or raise to 11:1 wtih let say 7 psi to get same desired power

(note i just made these numbers up)

since either way would give you the same power, why do most people choose to lower compression and raise boost levels? is there a reason?

space

Luke

Im sure it has to do alot with the engine managment and how things are fastened together. For example the RUF R turbo is 9.5:1 and runs 1 bar of boost

I would perfer the "less boost-higher comp." situation in a street car just because of off boost grunt that would be avalible in daily driving.

KW951

My understanding is that with a good setup, lower compression, higher boost makes more power due to the fact that the compressed charge is cooled before it enters the cylinder. Thus the more compression that gets done before the intercooler (by the turbo) the better off you are. Of course the downside to using low compression is that you have a limp-wristed low end unless you are using a tiny turbo that spools up at 1500rpm.

Jon-D

More Boost = More Horsepower

Actually, when you introduce boost to the engine you are, in effect, increasing the compression ratio. This equation should help clarify some of your questions:

ACR = [(B / AP) + 1] * CR

ACR = Ajusted Compression Ratio
B = Boost in PSI
AP = Ambient Barometric Pressure in PSI, not Hg! (14.7 @ sea level)
CR = Compression Ratio

So, a stock '86 951 running 0.75 bar boost (about 11 PSI) with an 8.0:1 compression ratio will have an ajusted compression ratio of about 14.0:1 at full boost. So, you can obviously see the danger of over boosting your car!

You can also see that lowering the CR can allow you to run higher boost levels without grenading the engine. Typically, lower CR and higher boost is a good thing becase the boosted air is more dense and thus creates a more 'energetic' combustion (read: more power) than natural aspiration. Think of a turbocharger as a way to bolt on more displacement! This is, of course, an over-simplification, but the benefits of a more dense air charge are similar to the effects one sees when increasing displacement.

Don't lower the CR too much, though. If ya do that, you'll start running too rich and the cat won't be able to do its job (thus causing you to not pass smog ). Plus performance when not boosted will suffer, blah, blah, blah. You've probably heard all this before, so I'll not preach to the choir.

EDIT:

I should've added this before originally posting. The reason you would not want to reduce boost and raise the CR is because, as I said before, the more dense the air is, the better the combustion. If you are operating at sea level the ambient barometric pressure should be about 14.7 PSI. If your CR was 10.0:1, then you're compressing your mixture to 147 PSI. This considers 100% Volumetric Efficiency and other variables... basically, your results will vary. Now, let's go back to our stock 951. It's got a CR of 8.0:1 and runs 11 PSI boost. So, that means we're taking 25.7 PSI of air (remember that your boost level is the PSI above ambient pressure) and compressing 8.0:1 for 206 PSI! That should help you see the benefits of higher boost vs. higher CR.