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My modified 2L with 10.4 pistons pinged a lot on 91 octane gas. In one of David Vizard's numerious engine performance books, he mentioned that Otto cycle engines all tend to have a 2400 rpm preignition phase. Accepting his premise, I decided to experiment. I installed an MSD6A multi firing ignition which fired over 34 crankshaft degrees (now 20 for new ones) until 3-4000 rpm. As I understand it, preignition, is caused by an uneven burning flame-front. The multi-firing MSD design is intended to induce more complete burning of the combustion process. The experiment worked well: the pinging was almost completely eliminated. If I worked hard I could made the engine ping a little but only on hot, humid days.