Simple V4 engine is here.

[Dave]

Originally Posted by karl_1052

Well, those are all big V8s with heavy pistons and valvetrain. Did you expect this 1650cc motor to use the same size valvetrain as a 5000cc V8?
I posted those vids to show with a lightweight valvetrain(which this motorcycle engine could have) can rev to high RPM limits.
It all comes down to the ability to control the valves, and with light valves and good springs, there is no reason that a pushrod engine won't rev past 10K.

valvetrain and cam profile are important, but the real question here is geometry. If the strokes too long the piston speed can be a very limiting factor