Dual Point vs. Single Point

[human-potato_hybrid]

I was reading up on the differences between a dual point and a single point distributor.

 

 I think I understand how a dual point distributor works but it's not obvious to me why a single point distributor can't have the same functionality in terms of dwell timing.

 

 I would appreciate if someone could provide some insight on this.

[edinmass]

This topic was recently discussed here in a thread whose title I can’t remember. It got fairly technical. It was an interesting read. I think Bloo may have commented on it extensively.

[m-mman]

In very short simple explanation. 
When the points are closed, electricity is charging the coil. (When they open the spark is produced for the plugs)

 

Well the longer you can keep the coil grounded (points closed) the higher the charge the more powerful the spark.  
 

Dual points in a high performance engine produces a hotter spark. 

The thread that discussed this concluded that outside of a highly competitive situation, dual points add nothing to the operation of your car.  
 

1920s early 30s cars (my 1929 Cadillac) used dual points but it wasn’t for performance.  In these situations one set of points operated the odd cylinders and one set the even cylinders with the goal of making the ignition system last longer without maintenance.  Fewer contacts per point set per miles driven.  

Multi-cylinder cars (12-16) mandated dual points because putting 12 or 16 lobes in the distributor with one set of points would make them almost always open and little to no spark.  

[Bloo]

2 hours ago, human-potato_hybrid said:

I think I understand how a dual point distributor works but it's not obvious to me why a single point distributor can't have the same functionality in terms of dwell timing.

 

As RPM increases, or the number of cylinders increases, eventually you run out of time to charge the coil in between firings, and weak spark or no spark is the result.

 

As the dwell gets higher, the points gap becomes less. At some point it becomes impractical to go any narrower with the gap. Dual points (as commonly used in the postwar era with one coil and the same number of lobes as cylinders) extend the possible dwell by using one set of points to do the opening and the other set to do the closing.

 

Dual points with two coils gets confusing because there is more than one way to do it. In a big picture view, if you fire every other cylinder with the opposite coil you gain a bunch of coil charging time (dwell) because you are firing half as often.

 

Imagine if you will that you can make a points distributor for an 8 cylinder reliable at 5000 crankshaft RPM. You can, by the way, but it's fiddly and most 8 cylinder distributors never had to deal with speeds like that. The implication is if it were a four cylinder distributor you could turn 10,000 crankshaft rpm. If it were 16 cylinders, maybe you could turn 2500 crankshaft RPM. More cylinders and or high engine speeds make it more likely you see some exotic solution like dual points, or dual points with dual coils, or dual magnetos, etc. As m-mman mentioned, sometimes it was done when not strictly necessary for the number of cylinders or rpm involved.

 

Edited 5 hours ago by Bloo (see edit history)

[human-potato_hybrid]

Thanks @Bloo. The engine in particular is a Packard 320 V8 which has a redline around 5k RPM. Packard used a single point setup for them, but for some reason Hash used a dual point in their cars that used this engine.