I just finished this project and was out testing it today. I get an occasional ping that seems to come after about 3 seconds at WOT, just at midrange RPM, like 3500-4000, and it happens on the street and when parked free revving. But it's scary cause it's a more like a sharp crack than a low ping.

I hope it's just the gas, but am wondering if its a fuel deliver problem, like a weak pump, cause it seems to be more related to WOT for a few seconds than at a specific RPM. It doesn't do it if I just rev it up and down gently at less than WOT. Perhaps I should go to a colder plug, but this plug is meant for 10.2:1 comp and mine should be very close, even less. Maybe I'm not even hearing it right, but it definitely comes from the engine, not out the exhaust. It's a little hard to hear cause my muffler is going out. Could it be backfiring, not detonating? What does backfiring sound like?

Gas is half 87, half 92 octane. Timing is 12-14 BTDC, 2-4 retarded from stock Z6 spec. Everything is stock, with z6 im and fpr, a6 ecu, injectors, and o2. The plugs are NGK's for the 99+ SI engine, heat range 6. Pulled them and the insulator is darkened except the top which is still really white, and the ground strap is a light frosty color. What's my problem? What's the worst it could be?