how much boost increases compression ratio 1 point

**mudhog** · 03-08-2012, 07:41 PM

would have put this in the forced induction section but dosnt look like anyone looks in there only like 3 posts

anyway how many pounds of boost raise your compression ratio up one point ie: 8:1 comp to 9:1 compression. the reason i ask is im thinking about getting a 5 speed 2.2 s10 and putting a turbo on it but dont know what pistons to buy since i dont know how much the compression will go up win the turbo spools up.

**96ImpSS** · 03-09-2012, 08:50 AM

Compression RATIO is a mechanical calculation that is not influenced by intake pressure/boost.
Without knowing a lot of detail about the 2.2 engine/head/intake/engine cooling/intercooling parameters and how you're going to use it, you won't get any reliable answers of how much to decrease CR for a given amount of boost.
A really WAG would be to go with a 9.0 CR and limit boost to 5 psi. That would give you good low-boost performance and a nice increase under max boost.
Or........going the other way.... a 8.0 CR and 8-10 psi, which would be a bit sluggish on the low end, but a real push under max boost.
Those are conservative suggestions, since I have no idea what your expertiese is and how stout your engine will be built.

**mudhog** · 03-09-2012, 01:21 PM

ok i see what your sayin about it being a fixed mechanical number. i guess what i meant is ummmm... naturaly asperated at a fixed CR theres x air and x pressure, but if you boost it that x amount of air goes up raising that x pressure. i want to rais the air amount with boost but dont want that pressure to go sky high and causing preignition/detonation.

**96ImpSS** · 03-11-2012, 07:40 AM

There are SO many variables other than static CR and "boost" that will influence your max cylinder pressure.
Cam timing will affect it differently at different rpm's. Almost every component in your turbo system will affect it differently at different rpm's.
In short, there is no direct answer to your question.
I'm no expert on forced induction, and I've not seen any indication that anyone on this board comes close to being one either. Even in the Forced Induction section, I didn't see much technical content that could be applied to your situation.
I'm just saying, that to get any better than the rough guidelines I posted above, you're going to need advice from somebody truly competent on spec'ing a turbo motor.

**bored&stroked** · 03-12-2012, 09:54 PM

Most mild forced induction motors tolerate stock compression ratio's. How much boost do you plan on running?

**1989K1500** · 03-13-2012, 08:19 PM

Theres a calculator for calculating the effective compression ratio with X Static compression and X boost. But when i checked it it was way off IMO.

edit: Found it. http://www.rbracing-rsr.com/compression.htm

Again, I think its way off. A friend of mine was daily driving a 5.3 truck with a turbo setup. The static CR was 9.9, and it was run at 12psi daily on 91 octane. That calculator shows that it would be 19.9:1. No way it would have ran on pump gas. And when it was turned up to 19 the calculator shows over 22:1. The truck had no problems on 19psi with race gas, or pump gas and meth.

**chucksrt** · 03-15-2012, 01:03 PM

96ImpSS is right, there is no calculation for compression being raised by boost... not that i've seen. What do you plan to do? Are you planning on keeping it stock but slapping a turbo on it? Is this something you will be building for boost. A stock bottom end can handle 6psi of boost without a problem but you need to retard timing and add fuel, and at 6psi you don't need to intercool it. Your best bet would be to get a smaller mitsu turbo it will boost quickly and still have some top end pull. 8.5:1 is considered ideal for boosted applications so if your starting out a 8.1:1 comp you just need to figure what type of bottom end is in it so you know what level of boost to run. unless you plan on doing a complete build I would stick to a mitsu turbo and 6psi and not have to worry about changing the internals.