"Neil" wrote:

What Ken means is that the armature in the magneto runs at crank speed. In a wasted spark engine (fires every rotation of the crank but the spark at the overlap between exhaust and intake does nothing useful), and in two-cycle enginess, the distributor rotor also rotates at crank speed. In a non-wasted spark four-cycle, the distributor runs at half crank speed. There may be mags where the armature runs at a different speed to the crank so if someone's aware of one, we'd all be interested I'm sure : )

=in the gasoline, EFI world that is called Wasted Spark, works fine even though a spark is not required every rotation, hurts nothing..

My concern when thinking of building a tester was that running a mag at what I thought might be twice its working speed could damage it. I looked in my Cat 22 parts book and the gear in the governor that runs the shaft to the mag is the same number of teeth as the crankshaft gear, so the mag does turn at engine speed. It seems illogical to me but obviously it works. I am guessing that turning it at crank speed gets the mag up to the speed it needs to actually produce spark at a lower rpm (ie hand cranking) and the 2 to 1 reduction at the rotor cuts down on wear. I completely agree with edb about Old Croak Magneto having a wonderful amount of information on their web site.
Thanks to you all. Grant.

Naylorbros correctly pointed out that in most 4 cylinder mags the armature turns crank speed because there are two poles and two possible sparks per revolution and the rotor turns half of that but if you have ever played with a TD18 or other 6 cylinder mag they run 1.5 crank speed and have a 3:1 reduction in the rotor and an 8 cylinder mag runs 2x crank speed with a 4:1 and a 12 cylinder runs 3x crank speed and a 6:1, it is all based on two sparks per revolution. ''UNLESS;; you happen to be playing with a Vertex mag of hot rod fame, they run .5 crank speed and spark the necessary number of times for the number of cylinders in the engine, got one on the bench right now, and they will definitely light up your life.

Thank you for everyone explaining what I was trying to say. I was trying not to go into waste sparks. The earliest starting engines (D9900, early D7700 and D11000) that used the Eisemann GL2 mag had a waste spark at the plug on the exhaust stroke. When the starting engines were up dated and Cat went to the Eisemann GV2Q no more waste spark at the plug. The waste spark was taken care of in the distributor cap