Does lowering spark advance at normal driving speeds allow for the use of a lower octane? I have a 355 with an LT4 Hotcam, forged flat top pistons, pro comp aluminum 64cc heads, and a stealth ram. Currently I'm running an untuned ARAP bin on 93 octane. When I ran the stock APYN bin with 93 octane, I was getting worse gas mileage than with the ARAP and 93 octane setup. How can I use 89 octane with my motor? Is it even possible? On my last motor, I was running lean and ruined a brand new set of catalytic converters. I'm afraid to run the APYN bin with 89 octane because I don't want to damage the catalytic converters again (very expensive to fix).

 

Generally lower timing will help. Gm late models have a high oct timing table and low oct table for different grade gases.

You'll need to tune for the gas you use and keep near stoich air fuel for cats. Your combo sounds like it wants more timing because of the cam and aluminum heads. I would try an arap bin with maybe 2-3 deg pulled to try 89 oct. do some logs and see what motor does

 

Thank you, I'll have to mess around with the timing tables this week while monitering mileage.