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?

 

Why half 87 and half 92? Why not just use good old 89.

 

Hmmm, timing on minime's can be tricky.
every one I have done I have used an adjustable cam gear to set the timing back to stock. If you are getting good timing ( at the crank looking at it with a timing light) and the distributor is set rougly at the middle of it's adjusting range, then try runing 92 octane, no mixing.
I have a Halmeter, TWM industries sells it. I cannot drive my car without it. It's way better than an autometer a/f gauge. It tells you the a/f ratio more acurately and it definitely helps to make sure you are not detonating because of a lean condition.

 

I had old 87 from before the project, cause that's all it needs stock.

 

Thanks for the info and suggestion. I was initially planning on an adjstable cam gear but heard that they all show signs of wearing on the belt teeth after awhile. But maybe I should reconsider. How much are the halmeters you are talking about? don't you need a wideband o2 sensor for accurate a/f measurement?