Coil resistance
Thread Starter
Drifting
Joined: Nov 2016
Posts: 1,957
Likes: 576
From: Pittsburgh PA
Coil resistance
Ok I think I’m overthinking this one. I’m helping an older guy get his big block running. It had an older electronic points replacement, maybe MSD I’m not sure. Wire rubbed after 30 years ruined it.
he had a new in box, but still slightly older crane xr-I. I think it’s now called Fast XR-I.
so I installed that got car running. But coil was also bad. I tossed a stock GM points coil I had laying around in and the car does run, but ready the directions for the XR-I it says I should have a ballast resistor. This car does not, I think it was removed for the original points conversion.
anyway, since I need a new coil anyway, can I just buy a coil with 3 ohms resistance instead of the typical points 1.5 ohms? Seems to me if the directions say I need a resistor with a standard coil, that’s the same as buying a 3.0 ohm coil, right?
he had a new in box, but still slightly older crane xr-I. I think it’s now called Fast XR-I.
so I installed that got car running. But coil was also bad. I tossed a stock GM points coil I had laying around in and the car does run, but ready the directions for the XR-I it says I should have a ballast resistor. This car does not, I think it was removed for the original points conversion.
anyway, since I need a new coil anyway, can I just buy a coil with 3 ohms resistance instead of the typical points 1.5 ohms? Seems to me if the directions say I need a resistor with a standard coil, that’s the same as buying a 3.0 ohm coil, right?
Tech Contributor
Joined: Jun 2004
Posts: 20,857
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
You can use a 3 ohm coil if necessary to limit the current to protect the electronic module. You'll get the same primary current and energy for all intents and purpose. It's just not the preferred arrangement compared to a ballast resistor and coil arrangement. With three ohms of resistance you're going to get 4-4.5 amps of primary current. With your chosen favorite formula for wattage (Heat!) in the coil, you'll get about 50 instantaneous watts (the average watts will depend on what dwell duty cycle your system runs on). Regardless, this is twice the wattage that the stock setup sees, as it divides the workload/wattage between the ballast and the coil resistance.
Hopefully the manufacturer of the 3 ohm coil did a decent job of designing the coil to operate under those conditions.
Hopefully the manufacturer of the 3 ohm coil did a decent job of designing the coil to operate under those conditions.
Thread Starter
Drifting
Joined: Nov 2016
Posts: 1,957
Likes: 576
From: Pittsburgh PA
Ok that is likely a great answer but a bit over my head.
if you don’t mind explaining
so twice the watts- what is my risk here? Damaging the coil?
Truthfully I originally installed this not even thinking about coils or ohms. It was running with an MSD blaster 2 coil with I think .7 primary resistance. And is currently running now with the stock GM coil, albeit only for like 10 miles.
here is the instruction sheet for the module, and it seemed to me I could use either coil + resistor, or higher resistance coil.
https://static.speedwaymotors.com/pdf/2881710.pdf
if you don’t mind explaining
so twice the watts- what is my risk here? Damaging the coil?
Truthfully I originally installed this not even thinking about coils or ohms. It was running with an MSD blaster 2 coil with I think .7 primary resistance. And is currently running now with the stock GM coil, albeit only for like 10 miles.
here is the instruction sheet for the module, and it seemed to me I could use either coil + resistor, or higher resistance coil.
https://static.speedwaymotors.com/pdf/2881710.pdf
Tech Contributor
Joined: Jun 2004
Posts: 20,857
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Ok that is likely a great answer but a bit over my head.
if you don’t mind explaining
so twice the watts- what is my risk here? Damaging the coil?
Truthfully I originally installed this not even thinking about coils or ohms. It was running with an MSD blaster 2 coil with I think .7 primary resistance. And is currently running now with the stock GM coil, albeit only for like 10 miles.
here is the instruction sheet for the module, and it seemed to me I could use either coil + resistor, or higher resistance coil.
https://static.speedwaymotors.com/pdf/2881710.pdf
if you don’t mind explaining
so twice the watts- what is my risk here? Damaging the coil?
Truthfully I originally installed this not even thinking about coils or ohms. It was running with an MSD blaster 2 coil with I think .7 primary resistance. And is currently running now with the stock GM coil, albeit only for like 10 miles.
here is the instruction sheet for the module, and it seemed to me I could use either coil + resistor, or higher resistance coil.
https://static.speedwaymotors.com/pdf/2881710.pdf
But, as there's no free lunch in this world, there's always a downside to any choice one makes. While the upside here is ease of implementation and time, the downside is that all the wattage/heat-dissipation in this circuit is in the coil winding. More wattage means more heat. More heat means higher temperature. Higher temperature means more physical expansion of the mechanical construction of the coil (higher thermal cycle magnitudes). A good design will minimize these effects, but can not eliminate them.
The stock configuration separates the wattage between the ballast and the coil. This keeps the part temperatures down, which helps reliability. (No free lunch downside: the ballast wire is generally more costly.)
As it's a Corvette, and not a daily driver, most likely this coil will not be thermal cycled as often or as intensely as in a daily driver. If the coil is from a reputable manufacturer, then you're probably fine going the three ohm route.
Intermediate
Joined: Feb 2025
Posts: 45
Likes: 4
From: Pittsburgh, PA
If I could add my recent experience:
My 72 coupe SBC would run well, but after about 45 min - 1 hr, it would stop running. After cooling down, it ran fine. The last time it happened, I noticed that the positive to the coil was smoking hot (literally burning the wire insulation). As I inspected it, I found a wire that had been spliced, and the splice was broken. I came to find out that the wire was the resistor wire. I replaced the coil (still have "old" points) and reconnected the wire. All is good. It is my understanding that the old coil was overheating due to the resistor wire being disconnected. The coil gets 12V during engine cranking, and then steps down to 6V when the starter is released, thanks to the resistor. Otherwise, the coil is getting 12V all of the time, and overheats. Bottom line, the resistor is the better long term solution.
Am I stating all of that correctly?
My 72 coupe SBC would run well, but after about 45 min - 1 hr, it would stop running. After cooling down, it ran fine. The last time it happened, I noticed that the positive to the coil was smoking hot (literally burning the wire insulation). As I inspected it, I found a wire that had been spliced, and the splice was broken. I came to find out that the wire was the resistor wire. I replaced the coil (still have "old" points) and reconnected the wire. All is good. It is my understanding that the old coil was overheating due to the resistor wire being disconnected. The coil gets 12V during engine cranking, and then steps down to 6V when the starter is released, thanks to the resistor. Otherwise, the coil is getting 12V all of the time, and overheats. Bottom line, the resistor is the better long term solution.
Am I stating all of that correctly?
Drifting
Joined: Jun 2025
Posts: 1,679
Likes: 586
If I could add my recent experience:
My 72 coupe SBC would run well, but after about 45 min - 1 hr, it would stop running. After cooling down, it ran fine. The last time it happened, I noticed that the positive to the coil was smoking hot (literally burning the wire insulation). As I inspected it, I found a wire that had been spliced, and the splice was broken. I came to find out that the wire was the resistor wire. I replaced the coil (still have "old" points) and reconnected the wire. All is good. It is my understanding that the old coil was overheating due to the resistor wire being disconnected. The coil gets 12V during engine cranking, and then steps down to 6V when the starter is released, thanks to the resistor. Otherwise, the coil is getting 12V all of the time, and overheats. Bottom line, the resistor is the better long term solution.
Am I stating all of that correctly?
My 72 coupe SBC would run well, but after about 45 min - 1 hr, it would stop running. After cooling down, it ran fine. The last time it happened, I noticed that the positive to the coil was smoking hot (literally burning the wire insulation). As I inspected it, I found a wire that had been spliced, and the splice was broken. I came to find out that the wire was the resistor wire. I replaced the coil (still have "old" points) and reconnected the wire. All is good. It is my understanding that the old coil was overheating due to the resistor wire being disconnected. The coil gets 12V during engine cranking, and then steps down to 6V when the starter is released, thanks to the resistor. Otherwise, the coil is getting 12V all of the time, and overheats. Bottom line, the resistor is the better long term solution.
Am I stating all of that correctly?
I agree, a dedicated resistor is the better long term solution. Congrats on self-solving your issue.
We'd be hard-pressed to find a more experienced-knowledgeable ignition pro on this forum than 69427.
