[mjannusch]

You've probably got the car tuned too rich. 10.xx A/F ratios are much richer than where you need to be for a safety factor. Overly rich mixtures have a higher percentage of unburned fuel, and on cars like ours with lots of exhaust backpressure there is some exhaust gas reversion back into the cylinder at high loads and rpm. When that happens, you get a puff of hot exhaust gas into the intake charge before the exhaust valves close at the beginning of the intake stroke.

So you get fresh intake charge, with fresh fuel as the primary charge in the cylinder... But you've also got some hot exhaust gas that contains hot vaporized fuel that didn't burn in the last cycle because there wasn't much oxygen left in the mixture. When that goes through the compression stroke there is a higher probability of detonation. Sometimes that's what we'll see as knock even though the mixture is very rich.

I would shoot for wideband O2 sensor readings more towards the 11.2-11.7 range on pump gas. 11.2 to be extra safe, 11.7 to get a little more power.