Re: Engine Question (compression vs HP)

["Jonas Karlsson" <jonas85k@hotmail.com>]

>From: "Langford, Cory" <Cory.Langford@icbc.com>

>The second question is:  Is it worth lowering the compression?
>So lowering the 10:1 to close to 8:1 how much HP will be lost?

Well, high compression increases HP in 2 ways (in my world anyway, which 
might be off slightly). First, the air/fuel mixture is compressed more, so 
it will be denser and yield a more powerful combustion. Second, and more 
importantly, the higher compression motor empties out more exhaust and sucks 
in air with higher velocity than a motor with lower compression. This yields 
a more efficient air-pump and more HP is generated.

With a turbo all this changes. You now have a turbocharger to force the air 
into the cylinder so you don't have to rely on the vacuum created by the 
piston to suck air in. In fact, the lower the compression ratio, the more 
boost you can run. This will yield the most HP.

But wait, compression still matters on a Turbo motor. Why? Because until the 
turbo spools up, you are still relying on the pistons to suck air in. 
Dropping compression to, like, 7.3 or so, will let you run massive boost, 
but you'll have turbo lag till no tomorrow. Not a problem on a dedicated 
drag car, but on a daily driver, or street vehicle, the lack of bottom end 
torque will make you hate the car.

This is why Saab (I read this somewhere) runs fairly high compression 
(9.5:1) and fairly low boost (5-6 psi). Not the best HP numbers, but sweet 
for driving in traffic.

If you are putting in a turbo, drop compression. Yes, on the 2.0. Some 
people run 2 headgaskets with no major problems. If you do the work 
yourself, this will be the cheapest way to go since doing a headgasket, once 
you've done it, is no big deal. A copper headgasket will be better, but 
slightly more expensive. You can remove material from the combustion 
chamber, mostly on the head side. But you need to take out a lot, so that 
could be hard. The best, and most expensive way, is to go with pistons that 
give you the correct CR.

HTH,
Jonas
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