MTFCA Forum

I was looking through an old 3 volume set of Model T accessories and found a timer that used only two coils and operated on the "wasted spark" theory which has been used on other engines, including Harleys, for many years. It is also used on the True Fire system. Advantages would be only two coils and maybe a simpler diagnostic process. For instance, if cylinders 1+4 are not firing, you might more readily expect a coil problem. However, it seems if it were so good it would be more common and Ford might have even considered it. I would like to hear from the electrical experts on the problems arising from these systems, and any history as well. The first issue might be shorter life of coils and spark plugs. It also seems that if a coil went bad back in the day, one could have simply wired across the proper terminals to create that system, but I have never heard of anyone doing that.

To fire two plugs from one coil requires that you have one plug at each end of the high tension winding. The Onan two cylinder engine works this way quite well.
To do it with a T coil you will need to add another high tension connection (open up the box and relocate the other end of the high tension winding)
I have used the Onan coil with a Nissan cap and rotor to fire a dual plug T head

One Ford booklet, that came with the HCCT, claims the T coil will put out up to 10,000 volts.

Most coils I have tested may struggle to get 2,500 Volts.

Divide that voltage between two .025 spark plug gaps and you will get no spark.

I have been told by a resident EE engineer that theoretically a spark that jumps .25 in free air will jump only .025 under compression pressure.

I do not have a method for actually measuring those voltages to prove the theory.

A major Model T waste spark danger is, if the timing is late, the waste cylinder has started down with the intake valve open and a backfire through the carburetor often has enough energy to remove the hot air pipe and ignite any loose oil or gas debris.

I have seen this happen with extensive engine and cowl burnt paint damage.