To the electrical experts

First, on about all mags Im familair with, its true the drive cup degrees (like 25 to 35 etc) determines where she fires at RUN RPM, therefore, a 35 cup runs at 35 BTDC once the impulse drops out still dependant on (if any) any rotational mag timing settings. On JD and other tractors that were capable of running on distillate or drip fuel etc., those 35 degree cups were about right at RPM. HOWEVER for running on gasoline its my opinion 35 degrees of advance is too much, therefore, they used say a cup having only 25 degrees of advance. If the mag has the capability of retarding the run timing by a rotational location (like on the JD Wico C n X mags Im used to), then if I used a 35 degree cup when using gasoline I would retard the run timing. Best to just use a 25 degree cup if available.

The advantage of a distributor with centrigugal advance curve is the ability to start at TDC then advance the timing relative to RPM with a slow gradual advance relative to RPM versus ONLY the start or run time available on a mag. Therefore, a distributor may show improved performance but it depends on at what RPM a dyno might be used. If at full RPM a mag was running at 25 advance and a distributor were at 25 then also, not much difference OTHER THEN a batery powered coil ignition likely has more spark energy then a mag. Where a distributor shines is at low lugging RPM when the tracotr needs the best spark, the mags energy drops off (cuz of less RPM) PLUS since it has no variation, its still firing at the full advance.

Sooooo oo, the spark advance of a mag is at full advance (and the full 35 if a 35 cup is used)regardless of RPM once the impulse drops out which is at a very low RPM like 250 or so VERSUS the distributor which starts at TDC also, but then advances the timing more at more RPM as it should.

Ole John T, retired electrical engineer NOT an IH mag man.