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MG MGB Technical - The correct coil.

71 MGB. I have a Powerspark electronic kit fitted to my distributor 25D and want to fit a better coil to work with it. I don’t think a ballast resistor is fitted to the Bosch coil that’s is fitted at the moment and is obviously not the original coil. I would like to get the best from this Powerspark kit any suggestions on the best coil. Thanks
Trevor Harvey

A 1971 MGB sold to UK spec would not have ballasted ignition, so any 3 ohm coil should be fine. I expect others will have their favourites. Let's wait and see.
Mike Howlett

A very confusing subject overall but fortunately relatively straightforward for CB MGBs (of all specs as it happens) as Mike has said.

The important thing is to measure the resistance of the primary before fitting it, suppliers and even manufacturers have been known to make mistakes in labelling and understanding of what fits what.
paulh4

I am not to good with electrics always thought a coil was a coil but am happy to learn. How do you tell if it’s 3 ohm.
Getting a reading of 2.5 resistance and 9.46 from the high tension output f the 12 volt Bosch coil, are these suitable readings for my distributor. Thanks Trev
Trevor Harvey

You have to measure between the spade terminals with the wiring removed.

Originally the coil for CB MGBs measured close to 3 ohms. You can get 'sport' (higher output) coils that measure about 2.5 ohms and these are fine. Do not go any lower on a CB as the points (if you have points) will burn and the coil will overheat due to excessive current.

If you have electronic ignition depending on type coil over-heating may or may not occur, but it's not generally realised that you get lower HT with electronic ignition compared to a good set of points.

Things get a lot more complicated with RB cars so I won't go there other than to say these can measure about 1.5 ohms or less and are likely to overheat on a CB.
paulh4

Trev
Just be aware not all electronic ignitions are the same.
Powerspark --- is a brand name not a type.
They produce differing versions of electronic ignitions which in turn require different rating coils to operate. The most common type uses a std.2.5-3ohm coil but there is a full on electronic unit also that uses a coil with less than 1 ohm (.75) resistance.
A pic of inside your dist. with the cap and rotor removed would help to id what you have----------

willy
William Revit

The difference between the two that Willy mentions is whether they are fixed dwell or variable dwell systems, which may not be obvious from markings or even manufacturers blurb.

The very low resistance coils should never be used with fixed dwell systems which includes points or it will grossly overheat. The only way to be sure what type you have is with a dwell meter.

Fixed dwell systems give - as it's name implies - virtually the same dwell on the meter over a wide rev range.

Variable dwell systems will display a low dwell at idle steadily rising as the revs increase as shown here https://www.youtube.com/watch?v=1P0BYeDV4Ts
paulh4

Trev. You mentioned your unit is a Powerspark, Have fitted up several and can tell you what you have from a pic. of inside your dist. with the cap and rotor off.
William Revit

Just a quick note. Multi meter reading accuracy, can be iffy, particularly on resistance!!
Always touch together the two probes, and note any reading. This figure will have to subtracted from item's reading (coil), to get a reasonable figure!
Allan Reeling

Allan--most decent meters now have a zero button for zeroing the resistance scale.
If your meter doesn't have one and it's a digital meter----If you hold the probes together when you turn the meter on and onto resistance it will zero the reading---works on 'most' meters. -worth a try
I think it's an issue with the charge level of the battery, as the meter applies a voltage to measure the resistance and the meters can't be set up for varying voltages for different levels of charge, hence the zero button
A good meter for automotive use on modern electronicsshould measure well under one volt across the probes when it's set for measuring resistance

willy
William Revit

This thread was discussed between 02/12/2025 and 10/12/2025

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