from what I understand, the higher the plug number the cooler the spark (longer the insulator, white porcelain piece). cooler sparks are better for high compression engines I believe. also I think the warmer the plug the larger the spark produced. the general go to for china dolls is the ngk 6 series. if you change to a high compression head then a cooler spark might help. also make sure your clearance is good when putting in a new plug. I had an iridium that looked good, it was too long and got smacked. Guess I didn't check well enough, I just went back to a ngk 6.