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MG MGA - Hi voltage coils

I'm waiting for a 40k volt Petronix coil to pair with my Petronix distributer. Should I expect any inprovement on the road ? The car starts and runs very well with the standard 3ohm coil. I carry the origonal distriburer as a back up and wanted a spare coil as well , so I might as well get the match for the petronix. At $33 ex USA why not ? Thanks Sean
S Sherry

Sean. The standard coil has a potential to produce 20K volts. This amount is seldom needed because the voltage in the coil will only build up sufficiently to cause the spark to jump the gap in the plugs. At idle and cruise, this is somewhere between 10K and 11K volts. Under hard acceleration, you will see the coil voltage rise to between 15K and 18K volts, seldom more. Even with a larger spark plug gap (I run .035"), I have never been able to get more than 19K volts out of a coil except when specifically running the test for full coil voltage output. In other words, all of these super coils offer a potential which, in the majority of cases, is never realized.

Now, if you were to do something that would cause the coil's potential to be realized (such as open the spark plug gap to .080" while running cheap fuel, using a very high compression ratio, and performing some form of activity with a lot of slow downs followed by wide open throttle accelerations) and had the coil putting out something near the 40KV maximum performance level, what do you thing is going to happen to the rest of your box stock high tension circuit components? Carbon bush in the dizzy cap being rapidly worn away due to twice the current flow that it was designed to handle. Rotors burning through and shorting out. Rapid deterioration of the contacts inside the distributor cap and carbon tracking between the contacts. Rapid break down of the spark plug lead's insulation. All of the components designed to work well with a standard coil, operating at some 50% to 60% of the max coil output, when subjected to something greater than three times their intended normal operating voltages, will not hold up very well.

The the vast majority of people, in the vast majority of operating circumstances, the standard 20K volt coil is all that is needed and will work well with the factory designed distributor system. If you are really going to need a 40K volt coil, other upgrades to the distributor system are needed if the system is to last any length of time and perform well.

Les Bengtson

Thanks Les, It makes a lot of sense. At least I'll have a spare to get me home if need be... Sean
S Sherry

I fitted the UK version of the Petronix ignition, the Aldon Ignitor. I ran it for sometime very successfully with the standard coil. Later I fitted their Flamethrower coil (same spec as Petronix) and noticed a considerable performance improvement. Of course I cannot say for certain that the original coil was still A1, but I had no reason to suspect otherwise.

Steve Gyles

Sean, I second what Les said about the strain put on the rest of the high tension circuit, having had a rotor arm burn through thanks to a Lucas "Sport" coil and plug gaps set to .035". If you do fit the pertronix coil (they might be better quality than the modern "Lucas" jobs) I wouldn't open the plug gap much above the standard setting. The wider you make the gap, the higher the voltage developed in the high tension circuit. The higher the voltage, the greater the chance of an insulation breakdown and sparks jumping in places other than the spark plug. As Les says, the standard system is adequate. A bigger spark is not going to give you more horsepower, but a badly timed or erratic spark will certainly rob you of it.
Lindsay Sampford

Steve When you fitted the Flamethrower coil what plug gap did you run? Was it larger than the original set up or the same?

John Francis

Lindsay, I take your point and hope that all will be well. I had fitted a new Petronix distributer with carbon ( graphite leads ) ? ?, and am assuming that this modern quality from Petronix will cope with 35 thou plug gaps. Well, I carry the origonal disributer and coil with me. Most places down under are a long way from anywhere else...... Sean
S Sherry

Sean, you will have no problem with it jumping a 35 thou gap (it will probably jump a 1/2" gap!), but do be aware that it will test your rotor arm to the limit. If you get problems with mis-firing or cutting out, look at the rotor first. A tell-tale burnt bakelite-type smell will give the game away! Solid wire cables with suppressor-caps would be better than the graphite.

Lindsay Sampford

John, I set mine at 30 thou.

I run 8mm silicone suppressed leads and do not have any problems. One of the reasons for using them is that many dynamic timing lights (mine included) do not work properly with the old copper wire leads - too many timing marker spikes on the pulley.

I must be one of the few who has never had a rotor arm nor distributor cap problem.

I now run one of the new-build Chinese distributors. It was another great performance booster when first fitted. By accident or design, the advance curve was spot on. 18 months down the line and it is still in excellent condition.

Steve Gyles

I agree with Les. But I still run plugs at .028. The MGA components are so small that if you push them they fail. My MGA is not a race car never will be. If ultimate performance were the issue I would choose a different car. My car stars and runs every time and is fun to drive. I have never had a burned rotor. In fact the only ignition failure I have experienced in 35 years owning 7 different MGAs was a coil. It got hot and would quit. After cooling it got me home and was then replaced.
If you are conservative with the recommendation of the designer things last. On today's modern cars most Asian ones recommend about 100,000 miles between plug changes. I do mine at 60,000. Lots of people run them longer thinking that as long as it runs fine they don't change anything. We then sell $100.00 + coils and plugs when the over voltage starts burning things out.
The coil is not the limiting factor in our ignition systems the insulating ability of the rest of the components is.
R J Brown

Steve, you've obviously got a good rotor arm there, mine was a poor quality modern thing, and as soon as I pushed it with extra voltage, it failed. That was the first rotor arm failure I had in 35 years of MG ownership!
Lindsay Sampford

Sean, I have been running a Pertronix electronic distributor combined with matching Pertronix "Flamethrower" coil for a couple of years now and they have worked really well.
The car starts much easier and runs much smoother than it did with the standard points distributor.
I think this is only partially due to the better spark the coil gives and more a result of the more accurate timing provided by the electronic distributor in comparison to that provided by the mechanical cam and points of the standard set up.

I had to fit higher specification distributor cap, ignition leads and rotor arm to cope with the higher voltage as the Flamethrower coil soon burnt away the carbon button in the top of the original dizzy cover.
The power of the spark was such that the car ran just fine without it and I never noticed this until I happened to look inside the cap.

So Lyndsay is correct in this and I suspect that the standard coil would work just fine with this set up.


Also, fitting this distributor removes the problem of setting dwell angle too.
Colyn Firth

Hi Colyn, could you expand on "higher spec cap and rotor ". And what plug gap were you using. Sean
S Sherry

I too run the lucas high energy lucas yellow coil. Like Steve I run my plugs at 30 thou and have never had a failure of points, dissy cap, condenser nor rotor arm. Before fitting the new extremely cheap chinese dissy a year or so ago I had run basic stuff in an original MGA dissy. Guess it is a pot luck kind of thing!
Robert (Bob) Midget Turbo

This thread was discussed between 06/02/2011 and 09/02/2011

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