Posted by David G on June 06, 2016 at 19:53:50 from (63.152.119.240):
If you did not know by know, I like to research things, this week it is spark advance.
Spark advance is used so that the flame front reaches the piston when it is typically about 17 degrees ATDC. I would imagine the bore/stroke ratio's would affect this some. The total distance the flame front has to travel is from the spark plug to the top of the piston. The flame front will travel faster on higher compression engines, or in a more rich mixture. This explains why a high compression engine needs less timing advance, I would assume the rich mixture travels faster because it is more dense.
I am going to measure the distance on my tractor from the spark plug to the top of the piston when the crank is at 17 ATDC just for learning. I should be able to back calculate the flame speed to get rough timing numbers.
I was surprised by the amount of total advance car engines have at idle. In the old days, the initial timing was usually set with the vacuum advance shut off. This value would be 8-12 BTDC, then would advance 10-15 degrees under vacuum. I remember them running OK, then picking up an sounding really good when the vacuum was plugged in. The total advance would be between 20 and 30 degrees at idle.
I was also surprised with the total advance at full throttle and light loading.
The extra advance, within reason, will allow for a leaner mixture.
I would think that any engine with varying loads would run better with vacuum advance, especially off idle torque. I am lucky, that I can program in these settings without having to change springs.
I have noticed that my MH44 EFI engine runs much better with the timing advanced to 12 @ idle to 30@ 1500. I have them the same for any vacuum right now, and it did ping under load, so I will assume I can run a little more advance during light loads, but need to back it off several degrees when loaded.
Here is a pretty good link http://www.megamanual.com/begintuning.htm
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