Hi everyone,

I've done some more research on the dwell thing. In a Kettering ignition, which is a non cd type ignition with points or a hall effect/transistor swithing circuit instead of points, dwell is very important. If any of you remember setting the points on the older cars, you remember the setting needed to be correct. This was because that directly affected dwell.

Dwell is the time that the coil is charged with currect. This time must be in the two millisecond range, but don't worry about that at the moment. By the theory of induction, when the current to the coil stops, the magnetic field collapses, and that is when the spark occurs. In the Kettering type ignition, the Hall Effect sensor is "on", and the coil receiving current when the magnet is crossing the sensor. When the magnet leaves the sensor, it turns off, the magnetic field collapses, and the spark occurs. What you want is sufficient time to completely charge the ignition coil, but no more as more time just wastes power, heats things up, etc. This is your "dwell" time.

Now, how to figure out the dwell time in our applications? It can be done completely with math, but there is a simple way. For a two stroke engine the formula is MAX INTENDED RPM X .0075 = DWELL IN DEGREES. Now the trick is to figure out how to have the Hall Effect sensor turned on by having the magnet under it for this amount of "time" or degrees.

You can draw a straight line on a piece of paper, and then with a protractor, draw another line at the dwell degree number away from the first. At the intersection of these two lines, draw a circle of your hub diameter where you are going to mount your magnet, using the intersection as the center point. Now measure where the circle intersects the angle lines. This is how long the magnet must be passing by the sensor to get the correct dwell. You will probably see that it will take more than one magnet to get the correct dwell time. The smaller the hub, the less distance will be needed to create the same dwell. A large hub at high rpms will take a number of magnets to get your dwell time.

If this is confusing, let me know and I will try to come up with a drawing to illustrate.

Thanks, and hope this helps,
AV8TOR