The ignition process begins during compression stroke. Increased pressure and temperature causes precursor chemical reactions. Then spark ignites the flame front which takes some time as well to grow from the actual spark to a flame front. Then the flame progresses across the chamber. The last part of the mixture to burn is the end gas which is highly compressed with plenty of time for precursor reactions so it can spontaneously ignite also known as spark knock.
All of the above process is sensitive to engine speed. Most notably the flame front progression and end gas mixing at higher engine speed causes the combustion to occur in less time.
Note that ignition lead mechanical advance curve typically slopes up to a maximum at, say 2500rpm, then does not advance further. This fixed lead above 2500 is because the total time for combustion is less at higher speed roughly in proportion to engine speed.
For determining what ignition lead you need try this: Run some crappy low octane gas like 87 no ethanol if you can find it. Play with your timing gradually creeping up on your map. If you hear some spark knock at certain points then adjust accordingly. Repeat until you are happy. Then put some better gas in it and drive on.
Last edited by strokersix; 02/12/12 01:48 PM.
