My 903 in my coupe has the stock pistons with a compression ratio of about 9:1. They are the pistons with the little step, not flat tops. The engine is supercharged with about 9 psi of boost and burns E85 and is managed by a Microsquirt. In my other post you can see the problem I had recently with detonation. I have organized a set of rings to be manufactured but the specialist reckons I need to reduce the compression ratio to at least 8:1 by machining a dish into the piston crown. This I can do easily enough with the spare pistons I have. What I need to know is how much material do I machine off? I was thinking of just machining the pistons flat then making a dish and keeping the edge as large as possible. Thoughts?