[geeteeohguy]

Brad, recently I helped a friend rebuild a '66 389 for his '65 Lemans....he went with custom dished pistons and stuck with the stock small chamber heads. The heads cc'd at 69. With a 3.23 rear, M20 trans, HO manifolds and a Holley carb, this thing runs like a scalded cat....on 87-89 octane. I would recommend looking into a set of dished pistons for a CR of 9-9.5 to 1. With my own '65, I'm in the same boat as you, and I have been running boosted gas for years. My 389 won't run on 91 octane without ping, even in cool weather. On my '67 GTO, I got sick of playing the detonation game and swapped to lower compression heads. If I were going into the motor, like you are, I would research and get the custom dished pistons. There are specific dish shapes to match your combustion chamber shape, too, and this helps with efficiency.