Hi Guys,

Looking into building a 2.2 engine and wondering what people think is the best compression ratio. I'm supercharged and will be running about 20psi. A few options what have been said to me was 10.5,10. or 9.5:1. I was leaning towards the 10:1. I will be running a motec ecu as management and the engine will still be rev'ed to 9-9.5k rpm. This will be on road fuel so 99uk/93us.

Cheers

Matt

 

If it were my build, I would run something around 10:1.

 

That's a respectable amount of boost for pump gas. Are you sure you're effective compression ratio is below the knock threshold for your octane? While I understand at higher rpm/higher boost you don't need as much timing and egt's are less of a worry with timing retard and supercharging I'm concerned your goals may be optimistic for pump. I'd go 9.5 or 9.0:1 at that boost level. Advancing timing is much easier than lowering compression...

Look at the oem CR and boost ratios.

 

13-1 on alky ,do it

 

Its common to see 500-550whp/20-23psi on 92 pump with 9:1 comp in my local club on turbo cars. The same pistons on a 2.2 puts it at 9.6:1 due to the added stroke and thats probably what I would go with for you running 93 at 20psi with a SC. So in the 9.5-10 range is safe doable id say.

Being SC is a bit more forgiving, since your not seeing that 20psi until redline, you have a lot of area in the rpms that are much lower pressure. With a good tune I wouldn’t be surprised to see you get away with running stock 11:1. But I wouldn’t risk that unless your tuner feels confident.

I’m running 9.6:1 myself, SC as well. Still breaking in the new motor so no boost yet.

 

Thanks for the replys. I'm running 18psi at the moment on stock compression with the same fuel. Tbh the 93us fuel is more like 97 over here but we can get 99 so helps a lot. I think 10:1 will be the target.

Cheers

 

Nice, yeah the best we have in our state is 92. Higher can be found, but like E85 its not available enough to make for practical D use. 10:1 Should be no problem.

 

I don't really deal with super chargers but for turbos its really comes down to the type of fuel your going to run. In general for pump gas you want to run as low as possible. 9:1 is good for pump gas. For c16/q16/e85 or any type of fuel which lets you not be octane limited the higher the compression the better.

Remember the higher the compression the more HP you will make per pound of boost. A high compression turbo engine will Have a much better low end and mid range power.

Now on straight 91 octane pump gas you can only run so much boost espically on high compression engines. A general rule is 11-13psi is the max you can run on 91 octane on a stock 11:1 compression engine. But with a built low compression 9:1 engine you can run about 20-23 psi on straight 91 octane. 20-23 psi on a 9:1 engine will make more power than a 11-13 psi 11:1 compression engine.


Now on race gas/e85 you have the luxury of running as much boost as it takes to max out your specific turbo/supercharger/engine's capabilities. So with a high compression engine you will run less boost to max out the turbo. The low compression engine will have to run much more boost to max out the same turbo. Thus the high compression engine will make more bottom end/mid range power and have better off boost power and response.

 

99 mon and 99 ron are two different things, fwiw.