Algodoo

A lot of engines are being made lately, odd...

What's wierd, this uses scene.my.thr and scene.my.rpm, and those are the exact same vars I often use in MY engines!

I might build my own engine soon

That might be because I browse your saves a lot. But I wrote all of the scripts myself.

I tried to fiddle with timeToLive, camshaft timing, spawned circle size etc but no luck in increasing the rotational speed.

The main problem is that your timing AND ball despawn is messed up!
You should use stretched axles as transmissions (you put one, then drag one of the objects, then put
hingeConstant = 1e-30
and then put
bend = true
bendConstant = +inf
bendTarget = 0 (or whatever to make the ignition fire at just the right time)).

That will give you a lot of awesomeness! Also you should forget about that timeToLive thing and use a killer box well below where the ball spawns. when it goes down, it gets killed.
And remember that more density means better collisions! So you should leave the balls' radius at 0.5 meters, and change the density instead (from 1 or less to 500 or more).

You should make the piston longer or just a square because it will turn if you accelerate it too much. and you shouldn't use a counterweight, because axles are a copy of real life bearings, but they work rather elastically, and the counterweight will make vibrations only. also you won't need it at all as you have a starter.
And last but not least, use LOTS of DENSITY for the important objects! More density = more accuracy in sim!

Why do you spawn the ball when the piston is at the bottom of the chamber? That causes instability and loss of power!