Roush built 434 C.I. stroked Windsor with Aluminum AFR ported heads running 10.25 compression. Likes 91 octane.
I was running a mix of F&L 111 octane and 91 octane and the car started running bad on the top end. Took the car to my mechanic thinking it was a fuel issue and after calling the person in charge of engine developement for Roush was told that the gas was to high octane and therefor was burning to slow for the motor. Got rid of the mix and went back to the 91 and the car runs perfect now.
Almost makes me want to bump the compression and run straight race fuel (which is easily available here) and see how much more hp can make, but I think at almost 600 I should probably leave it alone.
