One thing I must try one day is to record the sound of a 4 stroking engine the way I've done it with a 2 stroking engine and listen to the difference. I've recorded my son's car engine at full blast doing 36,000 then slowed down the playback by 100 times (the most my sound program allows) and you can hear every individual exhaust pulse, in this case 6 pulses a second. It sounds sort of like a vooompf...vooompf....vooompf. With a 4 stroking engine I expect I'd hear something like a vooompf....flubbb.....vooompf....flubbb .