Thoughts on Restoring a Morgan
Coils & Things

©By: John T. Blair (WA4OHZ)
dot_clear 1133 Chatmoss Dr., Va. Beach, Va. 23464; (757) 495-8229

Originally written: circa 1999
Last update: dot_clear March 12, 2009 - fixed email address

The following is an email that was posted to one of the mailing lists I belong to. It deals with ignition coils, points, etc. Chris Moberg does a nice job with his explantion. I thought this might be of help to some of you.

This is published with permission from Chris. John

Date: Wed, 26 Aug 1998 14:58:21
From: Chris Moberg
Subject: Re: Coils, Resitors, and Gaps

A coil is a coil is a coil. There seems to be thought that one coil is good for vintage and another for current generation. This is simply not true. However, it is important to match the coil to the balance of the ignition circuit.

In the old days (vintage cars) coils were triggered by mechanical points. Points are nothing more than a switch, opened and closed in sync with the rotation of the distributor shaft. Points are prone to erosion of their electrical contact surfaces. To combat this, series resistors were introduced in the circuit to cut the voltage to the coil. Most cars of this era have bypass circuits that provide full voltage to the coil during cranking.

As Lewis pointed out, you can bypass the ballast and get a hotter spark all the time on a resistor ignition circuit. Points are cheap, go for it.

On a breakerless ignition there are no points and there should be no need to cut the voltage to the coil, ever. This applies equally to OEM and aftermarket setups. I run a aftermarket hall effect unit in lieu of points in a 1968 V4 distributor. It allows me to run any sort of coil I want with no detrimental effects.

So whats the difference between black, blue, and red coils. The amount of punch and the presence of an internal ballast. Black and red coils have no internal ballast, and blue coils have one.

Coils are simple multipliers, feed them 12 volts and some multiple of 12 will come out. Coils typically come in powers like 100, 150, or 200, hotter coils have higher multipliers.

Another performance trick is to feed the coil something higher than 12 volts on the input side. A coil with 100 power will provide 12,000 volts out with a 12 volt input but the same coil will produce 40,000 volts if you hit it with 40 volts in. This is what (in part) an MSD unit does.

The problem is not all coils are up to the challenge of generating 40,000 or more volts. They lack the insulation and the ability to dissipate the increased heat. Performance coils are oil or epoxy filled.

Remember the old adage that a chain is only as strong as its weakest link. This certainly holds true in an ignition circuit. The plugs and wires are much more stressed carrying 40,000 volts than 12,000.

Higher potential (voltage) also allows for a greater spark plug gap. A larger gap generates a bigger spark which in turn does a better job lighting the charge (or so the propaganda says).

Compression is the adversary of spark propagation. The greater the compression, the harder it is for the spark to jump the plug gap. So it is customary to provide for a hotter spark when increasing compression rates (this would hold for NA and Turbo cars).

Choosing a coil is therefor not as simply as it might initially appear.


Return to the Index of Tech. articles

To email me with comments or questions.