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MG MGB Technical - Pertronix Ignitor Revisited

I have installed a pertronix Ignitor ignition module in my D25 distributor, but I thought that it would be better to check out the spark coil before applying power. It is obviously an aftermarket coil, it is called a MagFire high output 40KV coil. The resistance measures out at 2.0 Ohms whereas I note that the Pertronix installation leaflet specifies a resistance of 3 to 3.5 ohms for a 4-cylinder engine. It is going to pull about 50% more current and I remember reading some time ago that someone else had experienced very short life on the electronic module with a non-Lucas coil fitted. Has anyone else had any experience like this?
C.S. Barrow.

Sport coils are typically 2.4 ohms, 2 ohms is getting close to the standard 6v coil of 1.5 ohms, which certainly overheats and burns out points. If this coil can stand the current and consequently the heat fair enough, but you are right to be concerned about the trigger. If that specifies 3 ohms then you run the risk of invalidating the guarantee if it fails, although they would have to be in a position to test the coil of course. The difference fancy ignition stuff makes to the starting and running of a correctly maintained MGB is marginal anyway.
PaulH Solihull

Thanks Paul,
I have subsequently found on the Pertronix site that maximum permissable switched current for a four cylinder engine is four amps, which immediately disqualifies this particulr coil. The decision that I now have to make is whether to fit a ballast resistor or a 12volt coil with a resistance of 3.5 ohms.
C.S. Barrow.

C.S. I have, over many years of experiment, seldom found a situation where the required sparking voltage was 20K volts--the output of the standard coil. Coil voltage only builds up until the voltage is sufficient to cause the spark plug to fire, normally 10K to 11K volts in my testing. Thus, I do not see any great need for one of the "super voltage" type coils as they seldom are operating at their theoretical potential voltage and, if the ever did, would put an additional strain on the high tension circuit that it was not designed to take. One of the reasons Lucas produced the 45D series was to improve the high tension portions of the dizzy system.

So, my recommendation would be to install, if possible, the standard 12V coil having a resistance of 3.1 to 3.5 ohms which was the standard coil through 1974. The reason I say "if possible" is that we do not know the year of your car, only what type of distributor currently resided in it. If you have a 75 onwards car, it may have a resistance wire in the loom which drops the input voltage to the coil and requires a coil having a lower internal resistance.

If you have a 74 or earlier car, I would recommend the replacement of the coil with a standard coil and hooking up the points replacement system as per the factory instructions. If you have a 75 or later vehicle, I would like to know that and to thing about what needs to be done a little more.

Les
Les Bengtson

Thanks Les,
I bought the car (1973 MGBGT) about five years ago with this coil fitted and I have never experienced any ignition problems, so to me it made good sense to change as little as possible. In fact the only reason for fitting the Pertronix unit is the lack of decent accessibility for changing and setting points. Unfortunately I seem to have opened a can of worms. Interestingly, the clamp that holds the coil has an integral heat sink on it with a screw to secure something to it. I have always wondered about this, but now I realize that this is for a ballast resistor which has obviously been removed at some point.
C.S. Barrow.

I had a failure in a Pirhana system, on the vendors advice (one of those "specialists" who talk lets just say "rubbish") I changed to sports coil when I fitted it. I think it fried the module which failed later even after I went back to a standard coil. On points my car gave about 20Kv and so long as everything worked this was fine. So I would go back to standard.
Stan Best

A ballast resistor with that coil wouldn't make sense, unless that coil is supposed to be a lower voltage coil with an external ballast. At first glance even that might not seem to make sense, but if the matching ballast were, say, 0.5 ohm to 1 ohm instead of the standard ballast on rubber bumper cars of 1.5 ohms then it would. In theory this results in better spark performance at higher revs as the lower resistance coils have less reluctance which means the current builds faster and the revs can go higher before spark performance starts dropping off. However that is irrelevant with a typical MGB, the V8 runs perfectly well with half the current rise time of the 4-cylinder, let alone similar era Jag v12s. It's also why dual-point distributors on an 18-series engine are a waste of money and time.

You could add 1 ohm as an alternative to buying a new coil. As Les says the plugs are the limiting factor for actual HT voltage, but whilst you may not measure any more voltage with an HV coil, it's quite possible that you are still getting more spark energy in terms of current which is more difficult to measure. But, to repeat, it shouldn't be necessary.
PaulH Solihull

I raised this query with the local Pertronix agent and after some exchanges in correspondence he advised as follows:
The Pertronix unit in question has an “effective dwell angle” of 30 for an 8 cylinder motor and 40 for a 4 cylinder motor. It appears that the pertronix ignitor unit has a loading factor which determines whether it is overloaded or not. This factor is the product of the effective dwell angle and the coil current, so we have the following if one assumes that I use the 2.0 ohm coil:

With 12V/1.5 ohm on an 8 cylinder = 8 amps x 30 dwell = factor 240
With 12V/2.0 ohm on an 4 cylinder = 6 amps x 40 dwell = factor 240

From the above it would appear that I would not overload the module, but the advice is that I should monitor coil temperature.
Considering that the coil in question has been operating successfully with points for five years or more, I don’t think that it is about to burn out, but I may just go looking for a 0.5 ohm ballast resistor. In theory, this would reduce the current to 4.8 amps, and the voltage across the coil to 9.6 volts. Assuming a linear relationship, this should reduce the output voltage from 40KV to around 32KV. I don’t think that this would be much of a problem!
C.S. Barrow.

That doesn't make sense, a trigger on a 4-cylinder at 4000 rpm is making and breaking the circuit at the same rate as a V8 at 2000 rpm. V8s typically have the same coil resistance and hence current as 4 cylinders, and it is the instantaneous current that is going to affect an electronic trigger more than anything I'd have said. Even though the dwell time i.e. the current flowing time of a V8 is less (about 50% less with points, can't comment on the relative dwells for most electronic triggers) so is the 'off' i.e. no current flowing time, the effect being twice as many pulses in the same elapsed time, and hence the same heating effect.
PaulH Solihull

One of the big selling features of the distributor, that I fitted after installing a supercharger, is that the module is capable of increasing the dwell by about 20% over stock. It fires a 50,000 volt coil that is mounted in the distributor cap. They don't recommend trying to set the timing using a dial back timing light, as the increased voltage throws the calibration of the light off. The system allows for a .055" spark plug gap. RAY
rjm RAY

All this electronic language and mathematics continues to stump me! So I experiment 'by the seat of my pants'! Years ago when I had a morris mini minor I experimented with Capacitive discharge ignitions. The sales blurb was bigger,fatter sparks for more power. Alfa Romeo and some other car companies declared the vitues of double sparks as a way of extracting more power. Indeed the very simplest experiments on some cylinder head designs tended to suggest that a double spark plug arragement produced significant increases in performance. What it actually meant, was that the flame front became more efficient. So one could say perhaps some primitive cylinder head desings could be 'hotted' up by have two plugs or perhaps a bigger fatter spark maybe longer duration? Anyway the mini did perform better with the capacitive discharge system. Perhaps it highlighted the in-efficiency ( or lack of tune? ) of the old system. Still in all, in the US you have several 'High energy systems' available and quite powerful coils as well. The warning that my old Camaro had clearly stamped prominently, was 'do not touch the ignition wires while engine is running!' I did once, my wife came out and said,'what are you doing on the other side of the street?' They pack a punch. Why bother? because every car manufacturer is using high energy systems, so why not on our MG's? You have to bear in mind that the standard MG system even in peak condition will perform as the cars did 38 years ago. Can they be made to perform better? And especially, as Ray says if our new pre-ignition charge in our cylinder heads is very dense, because of our good porting and efficient cylinder filling, perhaps supercharged, then we need a good spark to set it off. Look at the dragster scene they use huge voltages and really wide gaps! Crumps I'd better end this essay. Mke
J.M. Doust

Hi,

I'll join this discussion with my thoughts, experience and knowledge of electronic ignition systems.

Way back in my youth I ran cars for around 20 years on a Capacitor Discharge System (CDI). The same home built unit moved from car to car. The initial car was a Morris 1500, based on the 1100 body, east - west engine, but I don't know the engine series, but it had an over head cam. Without the CDI I had to run on choke for at least 3 miles, in Australia's mild climate, ie never below freezing, before being able to push the choke in. With the CDI it only required choke to the end of the street, about 1/3 mile.
The instructions that accompanied the CDI unit were to open the plugs to 40 thou. This recommendation is based on statistics, ie the chance that a spark will ignite a fuel mix. With a long duration spark a small gap has a good chance of igniting the mix, but with a short duration spark, like from a CDI, the chance decreased, so opening the gap increased the chance of the spark igniting the mix as there is a greater "area" of mix to be ignited.
When I set up the system at 40 thou it worked well. Many moons and miles later, I decided to check the plugs. NOTE that there was no deterioration of performance in the car. To my surprise the plug gap was over 80 thou., due to erosion of the plug electrodes.
As said, there was no deterioration in the performance, no failure of coil or HT leads, though I suppose the peak HT would have been much higher than with the standard 25 or 35 thou gap.

My two bob's worth.

Herb
Herb Adler

I had CDI for a while, a Sparkrite unit built up from a kit. Made absolutely no difference I could detect on my Mini, didn't want to lose it when I swapped the car but never fitted it to anything else, I still have it in my garage over 40 years later. Must be worth something as an antique now ...
PaulH Solihull

I have an '84 Honda 700cc V Twin Shadow motorcycle with a CDI ignition system. It uses 2 spark plugs per cylinder as well as 2 intake valves and one exhaust valve. It has one of the most complex bike engines that I have ever worked on, but it has never failed to start right up. That being said, it uses 2 CDI boxes, one for each cylinder, and they go for almost $300 apiece. RAY
rjm RAY

This thread was discussed between 25/01/2011 and 28/01/2011

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