ignition resistors

[Jim in Wisconsin]

The ignition resistors on the early v8s seem to be sort of a mystery, at least to me. A lot of times the original ones are rusty and bad looking, but I've heard the modern replacements are not very good.
The Mac catalog says their's is 40 ohm, which seems completly wrong to me. Any old ones I find measure very little resistance, like maybe a half ohm or so. I heard that they were supposed to increase resistance when they heated up, which would make some sense - for easier starting. I've tried heating them with a heat gun- got them too hot to touch and don't see any change in the reading.
I know they are needed, because I've seen coils melt when they weren't in the circuit. The primary resistance of the coil is only a few ohms, so the resistor would only need to be a small amount to change the current. How can we tell a good one from a bad one? How much current can a coil handle without melting? Any thoughts?
Jim S.

 

[Brendan]

they heat up as the load is turned on

 

[JSeery]

What you are looking for is ~ 3.5A through the points. This is calculated by using Current (A) = Voltage (V) divided by the Resistance (R) in the circuit (A = V/R). If the Coil is 1.0 ohm for an example and the ballast resister is 1.4 ohm the total resistance would be 2.4 ohms. Battery voltage should be ~6.4V, it will be less when cranking the engine and higher when being charged by the generator, ~7V.

If you used a 1.0 ohm ballast resistor the current would be A = 6.4/2.0 or 3.2A and running with the generator charging A = 7.0/2.0 or 3.5A.

These are just example numbers the higher the system current the hotter the spark but the quicker the points burn and the capacitors go. Like everything it is a compromise, 3.5 to 4.0A is a good range to shoot for.

You can measure the coil to see what the resistance is.

 

[the99car]

OK . NOW. what should the voltage reading be (key bon ,not running) going into the coil from the resister? Kenny - Zhills stock '46 flathead

 

[JSeery]

Around 4v - 4.5v.

 

[Curt in AZ]

JSeery, thanks for a very clear explaination. One thing I'm not clear about is how much the coils impedance from induction adds to the circuits total restance? Thanks, Curt

 

[G.M.]

The ignition resistors on the early v8s seem to be sort of a mystery, at least to me. A lot of times the original ones are rusty and bad looking, but I've heard the modern replacements are not very good.
The Mac catalog says their's is 40 ohm, which seems completly wrong to me. Any old ones I find measure very little resistance, like maybe a half ohm or so. I heard that they were supposed to increase resistance when they heated up, which would make some sense - for easier starting. I've tried heating them with a heat gun- got them too hot to touch and don't see any change in the reading.
I know they are needed, because I've seen coils melt when they weren't in the circuit. The primary resistance of the coil is only a few ohms, so the resistor would only need to be a small amount to change the current. How can we tell a good one from a bad one? How much current can a coil handle without melting? Any thoughts?
Jim S.

The ends of the nichrome resistor wire is held against each end plate with
the rivet through the center. The metal ends get tarnished and don't make
good contact. Gently bend the slotted covers up, clean the area where the
wire is sandwhiched, push the first turn against the end plate and solder it
to the end plate. Regular soldering paste won't work with nichrome, you
need to use acid flux. Now you will have a resistor better than new. G.M.

 

[JSeery]

JSeery, thanks for a very clear explaination. One thing I'm not clear about is how much the coils impedance from induction adds to the circuits total restance? Thanks, Curt

Ok, an ignition circuit can be looked at in simple terms or you can dig into what is really going on which is a little complicated unless you understand electronics. The ignition system is a resonating LC (Inductor - Capacitor) circuit. All of this is important if you are designing an ignition system, but for normal put the parts together guys I don't think it really matters much. The induction in the coil is not really a factor that you need to be concerned about. The static resistance of the coil plus any additional external resistor in the circuit is all that matters for simple calculations. If you start digging into to the details there are lots of factors, such as the total resistance includes the wiring between components, connections, the points themselves, and so on. Design engineers have to worry about all of this stuff, but for us users this can normally be ignored if the major components are considered (coil static resistance and any ballast resistor).