Are these worth using?
Throw them away!
At the referenced mtfca site, the examples of capacitors to use are listed at 400volts (400v). Value is rated at .47 mfd. as is all the examples shown. The one presented by Ellis is rated at 630 volts same value (.47 mfd) and is, as are the ones on the suggested site, film designated capacitors. Why then is his example a throwaway?
Voltage rating is not the key ingredient here.
It is a parameter called dv/dt.
This is the ability of the capacitor to charge and discharge quickly to keep up with the ignition pulses.
The capacitors from Fun Projects are specifically designed and manufactured to meet the demands of the Model T ignition system.
As with anything that seems simple , it really isn't.
The dV/dt rating is insufficient (too low) for long life service. Google John Regan's post on the subject. respectfully, jb
Is the difference based on manufacture method -- Film vs metalized Mylar or voltage rate? How significant a difference in the dV/dt rate is there between 400 volts and 600 volts at the .47 mfd values? It would appear that the one presented at the beginning of this discussion is the “Orange Drop”?
This link should help,
Dang, sorry for the repost, jb
The dv/ft rating is a indication on the current flow in the capacitor. A regular capacitor has two plates (rolled together to make it small) with a connection at a single point of each plate (side). A high dv/ft rating will experience a higher current flow so the connection is either continuous or at many points.
A final question, since my questions are still unanswered, why are those who rebuild the coils not rolling their own wax paper/foil capacitors?
Because when you "roll your own" the smoke gets out!!
George your questions have been answered.
Royce - How significant a difference in the dV/dt rate is there between 400 volts and 600 volts at the .47 mfd values?
And -- Is the (operational life) difference based on manufacture method -- Film vs metalized Mylar? Knowing that the original capacitor was waxed paper and foil.
And --Is not the capacitor, as located in relation to the points, there to prevent arcing between the points?
The original capacitors are glass and foil. Yes the capacitor reduces arcing at the points. The voltage rating is not the dv / dt rating and is unrelated to that factor.
DV / DT rating is the capability of the capacitor to deal with current. The DV / DT rating is (in a purely capacitive device) is considered roughly the equivalent of current / capacitance or I/C. If the DV/DT rating is insufficient then the capacitor arcs internally or overheats and melts or both.
Royce - Thank you.