The staggered dual-point arrangement (one set closes the primary circuit and the other set opens it) provides slightly longer coil saturation time (34* dwell vs. 30* with single points).
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Re: Longer duration...
Good question, and John's answer is probably the best. Remember the Single Point system was "new" in '57-'58 time frame, and Chevrolet decided to stick with the dual point on the highest revving engines. The single point can also be made to work okay at high revs, but requires what I call a "blueprint overhaul". For high performance or racing use the Single Point dwell angle can be increased to 35 degrees, which achieves about the same dwell as the dual point, but the higher average current can reduce point life. The higher the dwell angle the longer the coil has to "saturate" i.e. reach a steady state current, which takes some time due to the "inertia" effect of the coil's inductance. The coil is only saturated up to about 2500-3000 revs, so after that, ignition energy falls off because current does not achieve steady state before the points open again.
Aftermarket dual point systems of yore like Mallory and the W&H Ducoil used two sets of points with a four lobe cam. This maintained coil saturation up to about 5-6000 and supported revs up to a good 10,000 since the point operating frequency is only half of both the Dual and Single point Delco distributors.
If they worked like the old Honda Twin motorcycle engines of the era, which has a set of points and coil for each cylinder, you set the point gaps to the middle of the spec range, then rotate the breaker plate to time #1, then adjust the point gap on #2 as required to achieve the correct timing on #2. I could be a real PIA to get the both right and be within the proper point gap range!
Chevrolet engineers finally came to the realization the the high revving mechanical lifter engines could benefit from vacuum advance and the single point could work at high revs - although not that well until the user shimmed up the end play and installed the high breaker arm tension points. With these changes a Single Point System should provide a reliable and accurately timmed spark up at least 7000 revs.
Duke -
I forgot to mention...
IIRC the Mallory dual point distributor used a single coil, so there was still the issue of lack of coil saturation at over 2500-3000. The single primary circuit was opened and closed by two sets of points 90 degrees out of phase.
The W&H Ducoil system had two coils - hence the name - each grounded by a dedictated set of point - essentially two completely independent ignition systems other than shared high voltage distribution via a special cap and rotor.
The W&H Ducoil was quite expensive, but had a reputation for working well on very high revving drag racing SBs.
DukeComment
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Re: I forgot to mention...
The Kong-Jackson distributor on Ford flatheads worked the same way as the W&H DuCoil (now that one REALLY dates me)Comment
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