I'm just about finished rebuilding my 360 into a 390. Depending the accuracy of my measurements I put my CR at somewhere in the range of 9.5:1 to 9.75:1. I used 390 crank and rods, with 360 pistons (flat top with 4 valve reliefs), a felpro head gasket, and stock heads (no decking). I want to get started on the right foot and figure that I may need to replace the 87 octane that is sitting in the gas tank. I assume the timing will be somewhere between 12 and 15 advanced. Any suggestions on which gas and timing should work out well? By the way, I live in the flatlands of NC, so no major altitude, and the pumps here offer 87, 89 and 93.

Thanks,
Mark

 

Set your timing at around 10-12 to start and then work up till she runs well. Do the opposite with the fuel. Put in the good stuff for the startup and then work down. Unfortunately there's no way other than trial and error to make these determinations, too many variables.

-Scouder

 

I wouldn't go to the trouble of replacing the gas. Consider that tank your break-in tank and take it easy. If it pings, back-off the timing until it stops. Do all the tuning adjustments when that tank is empty, the engine is a little more broken-in, and you have the gas in the tank that you intend to burn in the future.