Coil Question

So after the discussion in my sidegapping thread, I decided to check my ignition set-up this morning.

When checking the stock replacement AC Delco coil, I came accross 2.0ohms resitance on the primary side and 11.0 k ohm resistance on the secondary side. Bench tested without running it today.


Secondary is within all specs that I can find. But I ran accross a couple of different specs for the primary...which of course is confusing:


Corvette shop manual: .35 to .55 ohms primary at 75 degrees (room temp)

MSD Blaster 2: .7 primary 4.7k secondary(comes with a .8 ballast resister to increase the primary value to 1.5?) direct repalcement

Accel Super stock: 1.2 primary and 8.7k secondary....Direct repacement

So what is the correct primary value? Based on the all the info, it looks to me like the shop manual is outta whack with the rest of the info.


Also, the accel and msd repacement coils are much cheaper than the AC Delco replacement...what's up with this? Asian coils?

I will keep this simple. You want to keep the primary resistence at 3 ohms total to prevent burning up the points. 12v/3ohms= 4amperes.
Measure your coil choice, and choose a ballast resistor that gives you 3 ohms total.
It's just that simple. As for coil price and origin, I have not a clue.
Maybe Turbo knows.

A question and a couple comments.
Are the Accel and MSD resistances your measurements also (as with the AC coil), or the catalog specs? (Just trying to make sure we're comparing apples to apples.)
Given the lack of complete technical information on each of the coils, I would agree with Bird's advice on choosing your setup. The resistance measurements give you an indication of maximum coil primary current at low RPM, but do not tell you what the current (and more importantly, the energy) will be at higher RPM.

big2bird said:

I will keep this simple. You want to keep the primary resistence at 3 ohms total to prevent burning up the points. 12v/3ohms= 4amperes.
Measure your coil choice, and choose a ballast resistor that gives you 3 ohms total.
It's just that simple. As for coil price and origin, I have not a clue.
Maybe Turbo knows.

Click to expand...

Simple enough. I don't have points. I am using the Breakerless conversion. It works with the stock coil. They recommend a coil with a 1.5 primary. There is no ballast resister in the wiring diagram and I haven't looked to see if there is one there. I think that there is just the stock resistance wire which I believe is .8ohms(not sure).

69427 said:

A question and a couple comments.
Are the Accel and MSD resistances your measurements also (as with the AC coil), or the catalog specs? (Just trying to make sure we're comparing apples to apples.)
Given the lack of complete technical information on each of the coils, I would agree with Bird's advice on choosing your setup. The resistance measurements give you an indication of maximum coil primary current at low RPM, but do not tell you what the current (and more importantly, the energy) will be at higher RPM.

Click to expand...

The Delco is my coil bench tested. The Accel and MSD are catalogue specs.

Based on what B2B wrote, I don't see how the shop manual can be correct.

SmokinBBC said:

big2bird said:

I will keep this simple. You want to keep the primary resistence at 3 ohms total to prevent burning up the points. 12v/3ohms= 4amperes.
Measure your coil choice, and choose a ballast resistor that gives you 3 ohms total.
It's just that simple. As for coil price and origin, I have not a clue.
Maybe Turbo knows.

Click to expand...

Simple enough. I don't have points. I am using the Breakerless conversion. It works with the stock coil. They recommend a coil with a 1.5 primary. There is no ballast resister in the wiring diagram and I haven't looked to see if there is one there. I think that there is just the stock resistance wire which I believe is .8ohms(not sure).

69427 said:

A question and a couple comments.
Are the Accel and MSD resistances your measurements also (as with the AC coil), or the catalog specs? (Just trying to make sure we're comparing apples to apples.)
Given the lack of complete technical information on each of the coils, I would agree with Bird's advice on choosing your setup. The resistance measurements give you an indication of maximum coil primary current at low RPM, but do not tell you what the current (and more importantly, the energy) will be at higher RPM.

Click to expand...

The Delco is my coil bench tested. The Accel and MSD are catalogue specs.

Based on what B2B wrote, I don't see how the shop manual can be correct.

Click to expand...

Was the ohmeter zero'd out prior to the measurement, or did you notice if shorting the meter leads gives you a reading that is not zero? Many meters are not accurate when measuring very small resistances.

69427 said:

SmokinBBC said:

big2bird said:

I will keep this simple. You want to keep the primary resistence at 3 ohms total to prevent burning up the points. 12v/3ohms= 4amperes.
Measure your coil choice, and choose a ballast resistor that gives you 3 ohms total.
It's just that simple. As for coil price and origin, I have not a clue.
Maybe Turbo knows.

Click to expand...

Simple enough. I don't have points. I am using the Breakerless conversion. It works with the stock coil. They recommend a coil with a 1.5 primary. There is no ballast resister in the wiring diagram and I haven't looked to see if there is one there. I think that there is just the stock resistance wire which I believe is .8ohms(not sure).

69427 said:

A question and a couple comments.
Are the Accel and MSD resistances your measurements also (as with the AC coil), or the catalog specs? (Just trying to make sure we're comparing apples to apples.)
Given the lack of complete technical information on each of the coils, I would agree with Bird's advice on choosing your setup. The resistance measurements give you an indication of maximum coil primary current at low RPM, but do not tell you what the current (and more importantly, the energy) will be at higher RPM.

Click to expand...

The Delco is my coil bench tested. The Accel and MSD are catalogue specs.

Based on what B2B wrote, I don't see how the shop manual can be correct.

Click to expand...

Was the ohmeter zero'd out prior to the measurement, or did you notice if shorting the meter leads gives you a reading that is not zero? Many meters are not accurate when measuring very small resistances.

Click to expand...

It's an Actron III. Shorting it gives me a .2ohms readout. Does this mean that it is reading .2 too high?

If shorting the leads together gives you .2 ohms, that's the resistance of the leads. Add that to whatever your reading is. And make sure when you short the leads you're not touching the tips of the leads- your body can induce stray resistance. I'd think that .2 ohms is too high for the leads, I have a meter from Sears (knockoff of a Fluke 77) and it reads about .5 ohms, and I know that the leads are beat pretty bad. So I add that to whatever. The best way to get an accurate reading is still an old Simpson analog.

TimAT said:

If shorting the leads together gives you .2 ohms, that's the resistance of the leads. Add that to whatever your reading is. And make sure when you short the leads you're not touching the tips of the leads- your body can induce stray resistance. I'd think that .2 ohms is too high for the leads, I have a meter from Sears (knockoff of a Fluke 77) and it reads about .5 ohms, and I know that the leads are beat pretty bad. So I add that to whatever. The best way to get an accurate reading is still an old Simpson analog.

Click to expand...

Thanks for the info Tim. I replaced the tips on mine last year. They are not in bad shape at all. So I guess I'll add the .2 to the readouts.

SmokinBBC said:

TimAT said:

If shorting the leads together gives you .2 ohms, that's the resistance of the leads. Add that to whatever your reading is. And make sure when you short the leads you're not touching the tips of the leads- your body can induce stray resistance. I'd think that .2 ohms is too high for the leads, I have a meter from Sears (knockoff of a Fluke 77) and it reads about .5 ohms, and I know that the leads are beat pretty bad. So I add that to whatever. The best way to get an accurate reading is still an old Simpson analog.

Click to expand...

Thanks for the info Tim. I replaced the tips on mine last year. They are not in bad shape at all. So I guess I'll add the .2 to the readouts.

Click to expand...

You need to subtract the .2 ohm reading. Example: If you measure a ballast resistor or coil, and the meter says 2.1 ohms, the actual part resistance is 1.9 ohms. (Tim is correct regarding the old Simpson meters. You just short the leads and dial the meter back to zero prior to measuring anything. It was pretty neat.)

69427 said:

SmokinBBC said:

TimAT said:

If shorting the leads together gives you .2 ohms, that's the resistance of the leads. Add that to whatever your reading is. And make sure when you short the leads you're not touching the tips of the leads- your body can induce stray resistance. I'd think that .2 ohms is too high for the leads, I have a meter from Sears (knockoff of a Fluke 77) and it reads about .5 ohms, and I know that the leads are beat pretty bad. So I add that to whatever. The best way to get an accurate reading is still an old Simpson analog.

Click to expand...

Thanks for the info Tim. I replaced the tips on mine last year. They are not in bad shape at all. So I guess I'll add the .2 to the readouts.

Click to expand...

You need to subtract the .2 ohm reading. Example: If you measure a ballast resistor or coil, and the meter says 2.1 ohms, the actual part resistance is 1.9 ohms. (Tim is correct regarding the old Simpson meters. You just short the leads and dial the meter back to zero prior to measuring anything. It was pretty neat.)

Click to expand...

Ah yes...you are right, subtract! Thanks!