I wanted to get some opinions on the root cause of a broken spark plug problem I had today. Here is a pic of the plug:



Background info about the motor:

This plug was in my 86 Civic EP autocross car. The motor is a built up D15a3 12 valve 1.5. The motor is around 12:1 compression with a large cam, head work, etc.

The plugs were NGK BPR6EIX-11 iridium. '6' is the standard heat range for these motors.

I was running an autocross with two drivers, and we were running about every 6-7 minutes. The coruse was right at the top of 2nd gear, so we were bouncing off the rev limiter quite a bit. When the plug let go, I was trying to hold the motor right under the limiter. It hit the limiter (8600rpm), I lifted a little and held the speed, slowed a little for a turn, then got back into it and it sounded like a lawnmower, and obviously had at least on cylinder dead.

Luckly, it looks like the plug only cracked on the outside, so nothing bad fell into the chamber.

Has anyone ever seen a plug do this? Is it just a function of the plug being too hot for the motor? Any recommendation for a cooler plug? I can't seem to find one just like this in a '7' heat range. Is one heat range enough of a change, or should I go to an '8'? Any help would be appreciated.

Thanks - Chris (#11 EP)

 

It was cracked at or prior to instalation.The vibration of the rev limiter finished it off.