The Stovebolt Site Forums The Shop Area Engine Shop What for, those dual points?

In the mid-1950's, my father had a very successful sideline to the family shop going, modifying Lincoln Zephyr 12-cylinder distributors to run on the alcohol-burning Ford flathead V-8s most of the local dirt track racers ran. The Lincoln had a much bigger coil than the Ford, and it made a hotter spark. It would also survive being hooked up to an 8-volt battery, one of the few good uses I've ever found for that battery. All the Ford distributors, 8 or 12 cylinder, had dual points. I was about 12 years old when I got my first lesson in setting up dual point distributors.

Practically all d/p distributors have a "leading" and a "lagging" point set. One point closes a few degrees of rotation ahead of the other, and the lagging point opens a few degrees late for the same reason. This allows the "dwell" (the number of degrees of rotation the points are closed) to be increased. A normal single-point distributor for a 6-cylinder Chevy engine will be somewhere in the 38-45 degree range up through 1954, and 28-35 degrees for later stovebolt engines. Since the total dwell possible on a 6-cyl. dizzy is 60 degrees, and the ideal coil saturation point would be 66%, or 40 degrees, the early dwell setting of 38-45 would seem to be the best choice. In reality, the necessity to set the point gap near .016" to get that much dwell would cause points to arc and burn as the condenser tried to discharge. Dropping the dwell to 28-35 allowed for a .019" gap, and increased point life, but gave up some coil saturation and spark power in the process. This was somewhat offset in 1955 by the switch to a 12 volt ignition system.

Dual points, with their overlapping opening/closing cycle allowed the engineers to take the dwell angle back to the ideal 66% figure, without narrowing the gap sufficiently to cause point burning. For instance, the leading point could be set for 30 degrees of dwell, and the lagging poing set the same, but the opening/closing of the points could be set 10 degrees apart. This would result in a total dwell angle of 40 degrees. The increased dwell time resulted in better magnetic saturation of the coil, and a hotter spark.

It's possible to get dual-point performance from single points, but only for a short time, and at lower speeds. As speed increases, the slapping action of the rubbing block on the distributor cam sets up a harmonic vibration that makes the distributor shaft wobble, even on a dizzy with good, tight shaft bushings. A little bushing wear, and high speed "spark scatter", the tendency of the spark to happen at slightly irregular intervals, becomes quite common. Adding a second set of points approximately halfway around the distributor cam tends to stabilize shaft wobble, and reduces spark scatter at high speed. Sharing the primary current between two sets of contacts also makes a dual point system more reliable and longer-lasting between tuneups.

It takes a distributor test bench and an experienced tech to set up dual points properly, but there's a few tricks a shadetree mechanic can pull to get a good point setting. Set the points one at a time with a dwell meter by inserting a piece of business card stock between one set, and checking the dwell on the other point set with the engine running. The idea is to adjust each one to the same dwell reading, and allow the overlap to get the total dwell reading correct. If you're going to do lots of high speed work, cut the spring off an old set of points, and double-spring the ones in the distributor. Be aware that trick will make for VERY rapid rubbing block wear, so use a good brand of high-temp wheel bearing grease (just a little) to lube the distributor cam. We would turn the old flatheads over 5,000 RPM with double-spring points, but the dizzy needed new points about every 5th. race or so. (less than 50 miles, but at wide-open throttle most of the the time).

Dyno tests confirmed better HP figures with a race-prepared dual point distributor, even on the 350 Chevy engines we ran in the mid-70's, just before electronics took over. I think it was mostly due to more consistent timing and less spark scatter. Hope this helps some of you understand the concept!
Jerry