Does anyone know what is the highest compression ratio you can use with pump gas? What is a safe limit? I have tried to find out and am having no luck, I am looking at running 9.66 to 1 and am not sure if that is ok for pump gas. Any help is greatly appreciated.

 

I could be wrong but I think you can go as high as 11:1 and have to use high octane gas, like super.

 

You have to play around with the timing a lot to find the safe zone where you get max performance but no knock. I think 10.1:1 is the ideal comp ratio for the street on say 91 octane.

 

i have something close to 9.8~:1 Forged pistons...

sometimes on some pumps the 89 midgrade octane knocks

all pumps 91-92 octane runs fine...

i havent tried dputtin the 87 octane in as i also have an aftermarket chip that doesnt like 87's at all

 

static compression is not always going to say how much octane you need to run

a lot depends on your timing curves
the more advance you run the more octane you need

also dynamic compression can make a big difference
you can run 15:1 compresson on pump gas if say you have a VE of 65 though a motor runnning 9:1 with a VE of maybe 120% could have problems

 

there is alot of veriables on that subject. engine load, efficiency, fuel system, etc. running a higher stall converter puts less load on an engine as does more gear. also running a more effecient cylinder head such as vortec, fast burn, e-tec, afr, etc will allow for more compression due to requiring less timing adavance. running say something with a ram air/cold air induction, 3,000 or so stall speed, 4.10 gear, high duration camshaft, small chamber head with flat top or dished pistons, and fuel injection would allow for around say 11.0 to one vs. 10 to one just running a stock type set up. another big deal is pistons, flat top and dished with a small chamber head allow for more compression than domed pistons with a large chamber. longer rods also allow for higher compression. basicly there is alot to the subject to making it all work. a good example of this is the cadilac with 10.0:1 compression and allowing it to run on 87 octane. (this is the newer models only though, past required 92).

 

A properly built engine and a well tuned, electronic fuel management system can go to about 12.5:1 on 92 octane. Realistically it's only about 10:1 but even then there are a bunch of variables.

My race engine is about 10.3:1 and I run 92 octane. If I went to a sea level track I would probably have to bump the octane rating up.