Posted by pete 23 on January 05, 2014 at 16:45:04 from (74.33.68.83):
In Reply to: Mag vs distributor posted by Farmall A Tractor on January 05, 2014 at 15:58:45:
A battery ign unit gives you a variable automatic spark advance. In other words, with the engine running at slow speed you have zero spark advance and when you speed the engine up the spark advances gradually in relation to engine speed. This is controlled with weights and springs,. Each battery ign unit have a maximum advance limit. Most original equip battery used on the letter number IH tractors, ign units were 40 degree units so as you increase engine speed the spark keep occurring earlier in the cycle up to the max limit which should occur at rated engine speed. With a magneto, the spark is retarded (lag) so at cranking speed the spark occurs at or just after top dead center. That is what the impulse coupling does. Of course it also give mag that extra zip for hot spark at cranking speeds. Once the engine starts the spark is fully advanced. Most are about 35 to 40 degrees before top dead center. So, you see, even at low engine speeds the spark plug fires those 35 degrees before that piston gets to the top. This is too early for slow speeds but they get by with it if engine is not working hard at that speed but it makes the engine run less smoothly. Gasoline needs time to ignite and that is why you have to have a advance in timing as engine speeds increases. You always want that charge to have maximum push on piston when piston is going back down, not still coming up. Magnetos have the advantage that no external battery power is needed to run an engine. Some older tractors had a spark control lever to adjust the timing of spark and that was operator controlled.
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