Compression Ratio
Turbo Mini Forums

Have any of you clever chaps determined how to calculate the optimum compression ratio yet?

I am planning a road engine with say a 266 cam. To get the best BMEP without detonation or any water injection etc on 95 unleaded fuel, I reckon on aiming at an effective CR of 11:1.

With a boost of say 15 psi, we have a pressure ratio of 2 and a density ratio of 1.5 ish.

So, should we aim for 5.5:1 or 7.3:1 ????

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Theoretically, 5.5:1 would probably be closer to the optimum to allow ignition advance to give best power. However, as always, it is a trade off to give some sembalance of performance off-boost. With a 5.5:1 compression ratio, I would guess you would even possibly be into problems getting the engine to fire up, and off-boost performance would be very poor. I've always made an educated guess as a compression ratio to aim for, based on other engines people have built in the past etc.

Given that it is very difficult to achieve much less than 8:1 using standard parts (and spending lots of money), I would agree.

As this is going top be a road engine, running off boost most of the time, fuel consumption will be crap with much less than 8:1!!!

Sorry to mention fuel consumption (no one else does on this forum) but I am looking to do at least 10k per year!

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

the 1980s formula 1 turbo cars ran 6.9:1 with a witches brew of fuel

boost pressures reached 45psi and deliverd a
stagering 1400BHP for a 1500cc engine @16000RPM

so .....does this tell you something!?

as one famouse turbo expert wrote ...

dont be too eager to lower the compression too low and turn your engine into a low compression slug!

a trade off is disired and..... this would be around 8:1 or 8.5:1

this C/R ratio for the A series is just about spot on with a large intercooler

& if the engine is set up correctley you should have no problem of boosting the motor to over 20 psi & thats more than enough power to make youre arse drop out in a mini