Here is the link to the forum that (I presume) Stephens is talking about: http://www.rcuniverse.com/forum/m_23...tm.htm#2311892
Excellent explanation on the CH Ignition. Looks like this system CAN in fact determine the RPM on the first flip, right down to 200 RPM. Even the wimpiest flip should be able to get that!

As Fence and others have previously stated, the size of the magnet and distance from the centerline of the crank would affect the dwell angle, and therefore affect the RPM "reading" on that first flip. I'm not sure how the CH unit would deal with that. tkg states "The Syncro spark is "calibrated" for a 3/16" dia magnet and will work OK with 1/8"-1/4" magnets"; though this does not address the issue of distance of the magnet from the centerline of the crank. The instructions on the CH site do not talk about this, but perhaps the written instructions that would come with a new ignition unit would.

Ace: sorry- I'll have to disagree on that. Yes - one revolution will take exactly the same time regardless of the distance from center; but the time that the magnet is under the hall sensor (dwell time) will be less for a larger diameter.

Again, easy numbers that I can work with in my head...

If we have a 1/4" magnet that is 1" from the center, it would be under the sensor for almost 1/25th of a revolution (just under 15 degrees or 2 milliseconds at 1200 RPM). This would be a "dwell angle" of 15 degrees. At 2" from the center, the dwell would be about 1/50th of a revolution (just under 1 millisecond).

I don't know if "THIS IS ROCKET SCIENCE" as tkg states, but it is obvious that some pretty smart people have done a lot of work to design & build these units for us.[8D]

Cheers
Bill