yup denny has it... to elaborate on the higher RPM part, your coil has to charge a magnetic field to spark, and the wider gap has higher resistance and so it demands more juice to get the spark across it.... the more juice required the more powerful magnetic field is generated, the higher rpms make it to a point that the coil cant get a fast enough charge to dictate a powerful spark..... thats why all newer cars are going to coil over plug... and also why i'm working on converting my race engine on my mustang to coil overplug.... if only i could afford magnetos like top fuel cars use.... that'd be fun....cept then i'd need a fuel cutoff switch