Dual Point Distributor Performance

[Doug Flaten]

[Joe C.]

Re: Dual Point Distributor Performance

Doug,

Same principle as single points. Only diff is that each set has 4 cycles/dist rev instead of 8. The points are connected in series, so that only one is open at a time. They are phased 135* apart, and with 4 apexes instead of 8, one is always closed when the other is on its apex.
Dwell angle is same as with single points. Reason for duals was to prevent points bounce at high RPM due to less cycling. With single points using stiffer spring, this problem is solved, using a simpler setup.

Joe

[Doug Flaten]

Re: Dual Point Distributor Performance

Correct me if I am wrong, but don't both points still cycle 8 times per revolution? The distributor cam has 8 bumps on it and right now I have only one set in operation, triggering an MSD box. I can see it improving high rpm operation in that the dual points may not have to open as far in order to provide the same amount of dwell as a single point setup would have to. The single point would experience a bit more acceleration and possibly be subject to bouncing since its gap would be larger. Jack noted the other advantages about it sharing the electrical load and prolonging the life of the points. But I am confused how extending the time the points are open can result in more dwell.

[Chris D.]

Re: Dual Point Distributor Performance

There are 4-lobe designs out there but the Corvette dual point, like yours, has eight. Points are wired in parallel and offset so that after leading set opens for the spark, the second set closes earlier than the leading set, hence more dwell time with the circuit closed. Leading set does all the sparking. Control of point bounce is same as single point distributors, dependant on speed and spring force.

There are earlier posts in the archives on adjustment specs and procedure. I'll stick to single points, higher tension spring, and the occasional dwell adjustment required.

[Rod K.]

Re: Dual Point Distributor Performance

Doug,

The analysis in your first post seems right on...until you got to the last paragraph where you reversed your thinking.

Point dwell is the period of time the points are closed, not open, and the longer they're closed allowing current to flow through the coil, the more the coil saturates and the higher the output voltage when the points open. That's when the coil field collapses generating the secondary voltage which fires the plug. The point gap is somewhat critical in ensuring the primary circuit is interupted cleanly (w/o arcing, aided by the condenser), so simply closing the gap to increase the dwell isn't an option.

Add the second set of points (gapped the same as the first set) offset a few degrees (say 4*) and wire them in parallel and you get the equivalent additional dwell angle assuming gaps and all else is equal. The first set closes on the circuit to start coil saturation, followed by the second set closing 4* later with no noticable effect. The circuit is still closed. Then the first set opens, say 30* after it closed, but the second set is still closed, so you get additional saturation time, and there is no arc at the first set of points. Finally, the second set opens 4* later, ultimately firing the plug. You get the additional 4* saturation time, which may not seem like much, but at 7000RPM, assuming the coil doesn't saturate, it could mean a significant increase in the voltage due to the high rate of rise in the primary.

The points still float at the same RPM whether you're running single or dual, and is dependent only on the spring force and the mass of the moving contact and arm, and the shape of the cam.

The four lobe cam mentioned in another post would, I think, only be applicable in a dual coil set up or something like Mallory's Rev-Pol ('60's vintage race stuff) which used a special coil with two primary windings.

Hope this helps some.

[Doug Flaten]

Re: Dual Point Distributor Performance

OK, I was overlooking the fact that when the first set of points open, the second set was still closed and saturating the coil. So the first set controls the the closing of the points and the second set controls the openning of the the circuit. I knew it had to be something simple. Even an Aggie can learn something if you talk slow enough.
Thank you,
Doug

[Duke W.]

Re: Dual Point Distributor Performance

The dual point system was designed over 50 years ago and has not been used in production for 45 years, and the original design intent behind it has probably been lost - if it was ever known.

Coil "saturation" - the point where primary current achieves steady state DC value will not occur in the upper rev range unless dwell is increased, but increasing dwell tends to burn points. So by dividing total dwell between two sets of points, current through each is less than maximum for a single set at the same dwell angle.

The first one closes, but before current gets anywhere near steady state DC, the second set closes and begins picking up the current load. Then the first set opens, but the lower current has less tendency to cause arcing as they open, but not enough time elaspses to allow much increase in current through the second set before it opens, so arcing across the second set is also minimized.

That's my WAG on the engineering thinking behind the series connected, slightly out of phase dual point setup. I hope we hear from Jack!

Regarding point bounce, since the dual point uses the same contact sets as the stovebolt six, which I think has 19-23 oz. break arm tension - same as the base production set in the single point V8 distributor, I don't see that the dual point is any less prone to point bounce as a plain vanilla single point V8 distributor.

One of the tricks to extend the rev range of the base contact set in single point distributors is to increase dwell to 35 degrees. This saturates the coil to higher revs and reduces the distance that the points open, so the rev range is extended, however, this will increase the tendency to burn points.

Since each set in the dual point is set to 29 they open about the same distance as the single point, whose nominal dwell setting is 30. If you increased the dwell on each set in the dual point to 35, it should increase total dwell to about 40 and since each set is carrying only about half the current they will likely have less tendency to burn than a single point set at 35.

If anyone wants to try this, let us know the results, and don't forget that changing dwell also changes timing, so check timing before fiddling with the dwell, then reset it to the initial value so you get an apples to apples comparison.

On a four cylinder engine, dwell is typically set to 60 degrees, so coil saturation occurs at much higher revs than a V8, and the point frequency is half of a V8 at the same revs. This was the theory behind the Mallery dual point that used a four lobe cam and two sets of points. It was effectively almost two four cylinder distributors, but still only had one coil.

The D&H Ducoil system went the final step by using a second coil, so it was effectively two independent four cylinder systems.

Duke