"Here is the chamber on the Indy 360-2 cylinder head. Its heart-shaped combustion chamber has a well-placed spark plug. It finishes out at 63 cc, but it can be milled down to 53 cc. Since we will use a flat-top piston, this will be how we adjust the compression ratio. How much will we run? Everybody knows a well-cooled dyno room with the engine temperature kept around 160 degrees could support 12 to 13:1 on 92-octane pump fuel. You would have to run a lot of camshaft exhaust duration to prolong detonation, but you could not abuse the engine at all. I'm sure you guys are going to abuse the Challenge engines in your testing, so safe compression should be about 10.5:1, with 11:1 pushing it."