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Old 07-31-2017, 05:47 PM   #1
Jim in Wisconsin
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Default ignition resistors

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.
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Old 07-31-2017, 06:06 PM   #2
Brendan
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Default Re: ignition resistors

they heat up as the load is turned on
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Old 07-31-2017, 06:21 PM   #3
JSeery
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Default Re: ignition resistors

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.
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Old 08-05-2017, 09:34 PM   #4
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Default Re: ignition resistors

OK . NOW. what should the voltage reading be (key bon ,not running) going into the coil from the resister? Kenny - Zhills stock '46 flathead
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Old 08-05-2017, 10:43 PM   #5
JSeery
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Default Re: ignition resistors

Around 4v - 4.5v.

Last edited by JSeery; 08-05-2017 at 10:57 PM.
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Old 08-06-2017, 12:21 AM   #6
Curt in AZ
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Default Re: ignition resistors

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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
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Old 08-06-2017, 08:33 AM   #7
G.M.
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Default Re: ignition resistors

Quote:
Originally Posted by Jim in Wisconsin View Post
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.
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Old 08-06-2017, 09:34 AM   #8
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Default Re: ignition resistors

Quote:
Originally Posted by Curt in AZ View Post
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).
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