High or low ?

[Z26-T]

Im jealous :P. With my 12:1 I have to run C12...

[ForcedFirebird]

Im jealous :P. With my 12:1 I have to run C12...

How much boost, though?

I know a Rustang with 11:1 and 10psi on 91 gas --- with IRON heads O.O

Helloooo tuning sessions, lmao.

[IsaacHayes]

Hello 6 AFR LMAO (J/K)

[Z26-T]

No boost, its in the camaro, with a huge carb and bigger cam :P. I run C12 because Its an old built leaded engine, that doesnt like pump gas to begin with, then when you take it to 8k, problems start to occur...

[loner666]

Running a higher CR with lower boost pressure will net you better power but as said its gonna be alittle bit hard to tune for that then it would with a lower CR and higher boost

[timg]

I've run 7-8 psi on a 11.1:1 CR motor with 91 octane gas daily on the street. That was a great setup. I've now dropped down to 9.5:1 and run 16 psi (still on 91 octane). High compression and high boost can do amazing things IF tuned properly. If your tuning or gas is questionable, it's a bad idea.

Compression ratio and its effect on power production have been studied and there are SAE papers on the subject. It does NOT make the turbo spool any faster. However, because your compression ratio is higher, you make more power (torque) before the turbo spools up. That makes higher CR turbocharged motors feel more responsive and like they spool quicker (when in reality the extra power is due to extra CR not extra boost).

Higher compression ratio can limit your maximum boost. If you're planning to run huge boost on an older motor, you're better off reducing the compression ratio and running higher boost.

On AFR, I prefer 16.5:1 or whatever the motor will take at cruise and idle for fuel economy reasons. It's not as emissions friendly as 14.7:1 though and can also result in higher EGT. Around atmospheric, I shoot for 13:1 or so (maybe a half point leaner or richer depending on the motor. As boost goes up, I like to decrease the AFR to around 11.5:1 at max boost. It's rich, but gives a nice safety net without sacrificing too much power. Engines can run at very wide ranges of AFR at lower loads (say 10:1 to 17:1), but as the load on the engine increases, the acceptable range narrows (say 10:1 to 12.5:1). AFR doesn't really affect power too much as long as you're in a reasonable zone, but it can definitely affect fuel economy and reliability.

Tim

[timg]

9:1 versus 10:1 boosted dyno tests:

http://www.evans-tuning.com/dynocomp_9vs10to1.html

Jeff Evans has a ton of tests like this on his website. There are also some great tech articles.

Tim

[ForcedFirebird]

It's amazing how many people are under the misconception that you NEED low CR for boost. With c16 I have seen as much as 16:1 with high boost.

We need all the torque we can get

Thanks for that link timg.

[Z26-T]

With the price of C14 to $12.50 and expected to raise another $3.00 by the end of the year, I'd hate to fuel that boosted 16:1 :S.

Great link timg, lots to think about. But personally I think everything relies on the tune... Anything is possible with the right, smart, tune .