A demonstration of the firing interval of an I7 with the crankshaft phased to create a perfect fourth between cylinders 1,2,6,7 (firing at 180º intervals) and 3,4,5 (firing at 240º intervals)
The firing order of this theoretical engine is 1 3 6 4 7 2 5
Firing intervals are 0º, 80º, 180º, 320º, 360º, 540º, 560º
This crank configuration should have a smaller rocking moment than an even fire inline 7 cylinder engine, but second order shaking would probably be bigger/worse. I could model it crudely but that is getting way too far into the weeds.
An engine using this crankshaft would need primaries 1,2,6,7 merging into a tailpipe and primaries 3,4,5 merging into another tailpipe for maximum effect. This could make turbo sizing tricky.
