compression

[jeremy_283]

can you run 91 octane fuel with a 10:1 compression piston?

[GPracer2500]

yes

[jeremy_283]

what is the highest compression that is safe to run with 91 octane fuel? and will it show a big differnce on a 400ex that has a 9:1 compression stock?

[jsmith2232]

11.1:1 is the highest for an air cooled engine
12.1:1 is the highest for a liquid cooled engine

I just got a 10.8:1 je put in mine and it ran fine on 93 but being right at the border of needing race gas I will try to run at least a 50/50 mix when possible.

[GPracer2500]

Popular opinion/experience indicates that your OK with 11:1 on premium pump gas. In my opinion 11:1 with 91 is iffy. 11:1 may be ok with 91 but you might also get some detonation under certain conditions.

I have a 10.8:1 JE piston and a rev box that advances the ignition timing a little. 91 is the highest octane available at the pump in my area. I'm getting a small amount of detonation under high load, high rpm conditions.

This is on a new build for me so I'm still experimenting on what it will take to eliminate the detonation. I'll likely start running a pump/race gas mix.

Higher compression pistons do make a significant difference vs. the stock 9.1:1 compression. The more compression the bigger the difference.

ETA: I second what JSmith said.