Ill over simplify it for you, as the load rate on the engine increases the rate of accelleration of the piston away from TDC tends to slow as does the rate of increase in the number of strokes per second, cylinder pressures build rapidly from the time the spark ignites the compressed mix untill peak pressures are reached,but not nearly instantly, it takes TIME for usefull pressure to build up, as the pressure in the cylinders before or at TDC does nothing to produce power ,in fact it tends to work against the engines efficiency, but because theres a lag between ignition and the useable pressure over the piston you need to advance the
timing, so that the majority of that pressure peaks slightly after TDC and maintains that positive pressure over the piston for as long as possiable, that pressure curve and the necessary required ignition advance required to get it varies with the resistance or LOAD the engines working against and the RATE at which the piston moves away from the TDC point. if that load slows the piston to much it gives the cylinder a chance for secondary ignition(detonation) to occure, as the pressure and time both have an effect. you can get away with a faster advance if the piston moves rapidly away from TDC,IE ligher loads or a higher gear ratio that allows the engine to build rpms faster, factors like, octane used,air temps in the cylinder,quench ,and valve
timing, even the rod length/to stroke ratio, effect results here
things to read
http://hcs.harvard.edu/~jus/0303/kuo.pdf
http://superchevy.com/technical/engines_drivetrain/accessories_electronics/0411sc_crane/
http://www.erc.wisc.edu/modeling/multi_dimensional/ModelingMtng2004/7-NSMBpaper.pdf
look here
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
now compare your cam choices and notice that the valves open to bleed off pressure and the valves close to maintain pressure while the piston is a good deal closer to TDC tham most people realize, most of the effective pressure occures in the upper 1/2 of the stroke, with very little useful pressure in the cylinder after 90 degrees past TDC
heres a cam I use alot
http://www.cranecams.com/?show=browsePar...lvl=2&prt=5
notice when the exhaust valve opens, and compare that to the location of the piston in relation to the peak cylinder pressures as they relate to the pressure curve over the piston, youll quickly notice that the peak pressure occures between tdc and at most 30 degrees past TDC and has dropped off drastically by the time the exhaust valve opens
