The Bourke/VBL(modern modified Bourke stlye) engine

[curtis73]

The problem with that theory is that detonation is counter productive to power. The problem with detonation is that the flame front is slamming into the piston before it reaches TDC which applies force in the wrong direction. Best timing is an advance that meets the piston at the right time to take advantage of peak cylinder pressures at about 23* past TDC. You waste energy with detonation, and I don't care if the pistons are forged inconel steel, detonation destroys bearings, melts pistons, and kills engines.

Someone a few years back did the calculations on detonation and came up with this analogy based on a bunch of typical engine parameters: Average detonation (about the time you can really start to hear it) is equivalent to hitting the piston with a 25 lb sledge swinging at 12 mph. Couple that with the fact that the flame front is burning at about 1900 degrees and you have a white-hot sledge hammer.

[KiwiBacon]

Originally Posted by 534BC

Bourke used a mag on his small engines and instead of retarding spark , he ran (I think up to 90 degrees) even more advance.

That doesn't help.
There's a maximum spark advance which gives the best engine torque. Basically it gives the maximum combustion pressure right after TDC.
If you advance past this, you're using a large proportion of the combustion pressure to try and make the engine run backwards.

As you could imagine, engine efficiency goes downhill fast.

[Steel]

It looks like the power stroke happens when the crank is ~90 degrees to the piston. That probably explains its high torque numbers more than anything.