Every once in while, if you have an efficient combustion chamber, higher boost, and a turbo that make more boost than back pressure, the waste spark can light the intake track on fire while the intake valve is open. Took us a year to figure out what the heck was happening. What happens is the pressurized intake charge starts to enter the chamber before the waste spark then BANG! We bent up some stuff and embarrassed ourselves many times before we figured it out. We finally figured it out by adding more timing and retarding the camshaft and problem went away.