Fuel mix for my 69 Camaro L78

[Chevy454]

I was thinking the amount of timing is determined more by compression ratio/cylinder pressure and flame speed than by "engine size". Bore does play a small role in it, because a larger bore I would think would need MORE timing since it takes longer to burn due to the sheer distance of the big bore? But my experience only goes up to 454ci, so that might not be right?

Anyway, higher compression means higher cylinder pressure, which means a faster flame front speed, which means you need less total timing...due to compression, the charge is more dense, because there is less *space* between the molecules which allows it to burn faster. That's why/how vacuum advance works, 'cause high vacuum conditions can tolerate a crap load of timing. But, your dynamic compression comes into affect, as you mentioned some cams will bleed off pressure or even just change the timing of the cam events...tricky stuff!

Our pair of L72s are a good example compression & timing. One is 10:1 the other is 12:1, yet the higher compression engine likes 5-8 degrees LESS of total timing..same parts, same ignition, in the same car. A supercharged engine is another example...the flame front is faster in the s/c engine because of the increased cylinder pressure, so less total timing is needed. Heck, the LS1 in my '98 SS only needs like 28 degrees of total timing because of the superior flame characteristics of the Gen III heads/pistons...

Zedder: something worth mentioning is that a lean condition can lead to pre-ignition and detonation...how good was the tune on that Z?