Why a Dual Point Distributor

Number 2 example says ----"This offers no performance gain"
For the standard engine -27.5 degrees closed, 17.5 degrees open
For the supercharged engine -34 degrees closed, 11 degrees open The 6.5 degrees increase does result in better spark with a performance gain.

Hi Gary,

that is a great question and I was curious about the answer as well. I searched on the internet and found the following on JalopyJournal:

1. First there are 2 styles of dual points - each claiming different advantages.
Speaking on a typical V8 you can have a 4 lobe distributor or a 8 lobe.

On the 4 lobe the advantage claimed was increased RPM since each point set opens 1/2 as much (speed wise) so where you might get point bounce at 7,000 rpm you could now theoretically extend that to 14,000. I could also see extended life since each one works less times. On the 4 lobe - each point only fires every other cylinder.

On the 8 lobe you get increased dwell. I believe this is about being physically able to get the points open and closed quicker - meaning the ramp portion. Simply re-grinding the lobe probably has practical limits where you can no longer increase the "steepness" of the lobe without risking point bounce. By staggering the point sets and overlapping them - you extend this without having to get super agressive on the lobe angle. Same reason why they go to larger diameter roller lifters when you get to super high lift cams.

2. There are two kinds of dual point systems. In one kind each contact set serves four cylinders, which in theory the points will last twice as long before needing replacement. This offers no "performance" gain.

The other kind uses both points in tandem for each cylinder. The points act at nearly but not exactly the same time. There is overlap or a "handoff", meaning that the contact made between the power supply and the coil stays longer, leading to a higher saturation or "charge" in the coil, leading to a higher voltage spark. (by the way, there's no such thing as a "longer" spark, if "longer" means time, but a higher voltage spark can travel a longer distance over a bigger gap).

Based on this it would seem that the advantage of the Cord dual point distributor is higher RPM capability and/or longer point life since it is a 4 lobe dual point distributor.

Regards,
Bill

The supercharged 37 Cord engine has a dual point distributor. Why is this? What are the advantages of this? I understand installing a supercharged cam shaft in a non supercharged engine makes them come to life. Why?


Gary Parsons