Compression ratio and turbo

[mrj82]

Ok I know normally and FA'd car has a compression ratio of around 8:1 plus or minus a little bit. I've been wondering whats the harm and running higher compression ex. 9.5:1 with lower boost? Is it just going to detonate or is it possible that it will run and make more power? This is for a street engine btw. I'm sure it would make tuning a bit harder right? But if you have strong enough internals why not go with more compression and a little more boost?

 

[Defiant]

Boost gives the effect of higher compression.

 

[mrj82]

Yeah well thats my point, so if I run higher compression will it do the same with less boost and have no real problems?

 

[Defiant]

Now you're getting beyond me. The dynamics get hairy, variables include everything from weather conditions to fuel octane. But yes, true high-performance engines use high compression- but they aren't going to be friendly to boosting. You'll have a much sharper knife-edge at the limits.

 

[nine5raptor]

Originally posted by Defiant
You'll have a much sharper knife-edge at the limits.

That's the main thing. Running a higher static compression ratio increases your chance of knocking faster than you gain power. In other words, by keeping the S.C.R. lower and running higher boost (well-supported boost that is) you can make more power than if you ran lower boost and higher S.C.R.

Also as Defiant pointed out, your 'safe zone' where you are making the most power with a little knocking gets much smaller. Knock will be more dangerous to the engine.

 

[a351must2]

You are supposed to be able to get around the boost limits of high compression by running cams with extra overlap to reduce the effective compression ratio. Of course, the extra overlap increases turbo lag and would not be good on a street car, but can work well on a race car.

I run 11:1 compression on my 351W Mustang and have a specially ground cam that lets me run 92 octane gas and still have the advantage of the high compression. I've read about people doing the same with turbocharged cars/bikes that are drag race only.

Every once in a while people on TurboFord build 9.0:1 compression 2.3L turbos and still get away with 15psi, but the only advantage is better response when there is not boost...I never here of them running 20psi or more like everyone else with 8.0:1 compression.

Lower compression and higher boost is always the way to make more power. IMO there's no reason to do it any other way...I feel the engine should be built with as low of compression as can be streetable and just match up the turbo to be at least partially spooled by the time your in your operating RPM range.

 

[nine5raptor]

I wonder how that would affect our cars (I don't really have any experience with cams). As far as I knew these were designed without valve overlap.

 

[mrj82]

So if I just got the hahn racing turbo kit with my stock 420a running like 5-7 psi I'd pull around 200hp, but if I go with lower compression for my turbo engine I'll build real soon, at the same boost I'll most certainly have less hp. My thing is I don't know when I'll get my turbo kit, it will be after I build my engine so I'll probably go ahead and drop it in before I do the turbo, its gonna be slow as crap before I boost. Not as street drivable you know. Well I guess I'll run the lower compression and 15psi or so after a good engine build up. Only time well tell I guess .