Algodoo

Forgot to mention that is a spawn engine but you may have discovered it by yourself xD

You can correct some errors still:
When the engine is running at max throttle, the spawner thingies that spin get too unbalanced. to fix this you have to copy them, rotate them 180º and make them collision layer none.
Also you should use circles, this script works like a charm:
keys.IsDown("Up") ? {
scene.addCircle({
radius := 0.09;
density := 500;
collideSet := 24;
pos := pos;
friction = 0;
restitution = 0;
zDepth := 200;
vel = entity.vel;
color = [1, 0, 0, 0.3]
})
} : {
scene.addCircle({
radius := 0.04;
density := 2;
collideSet := 24;
zDepth := 200;
pos := pos;
friction = 0;
restitution = 0;
vel = entity.vel;
color = [0, 1, 1, 0.3]
})
}

You're using sretched axles with brake to make the timing spawner. The problem with this is that brakes are not perfect. After some time they will get unsynced and your engine will fail to run or run really slow and with no torque. To fix this you can try bend:

Triple click one of the stretched axles that connect flywheel with flywheel. This will select all of them, so now right click them
Go to Script menu
Find bend, bendConstant and bendTarget
Put:

bend = true
bendConstant = +inf
bendTarget = 0.7

Now deactivate the brake on the axles. You will see that the pistons arrange and look like a wave now

Now triple click one of the axles that connect a flywheel with a timing spawner
Find bend, bendConstant and bendTarget on the script menu
Put:

bend = true
bendConstant = +inf
bendTarget = 0

Now deactivate the brake on these axles. you shouldn't note much of a change in these

After doing all that I can get 95 rad/s off the engine

If I change density of the spawned balls to 500 (on the script) I can get 180 HP at 600 RPM

Thanks for the advices