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MG Midget and Sprite Technical - Another plug failure?

Couple of weeks ago I had an episode of severe misfire. Did the usual checks and traced it to one of the colder NGK BP8ES plugs being duff. Plugs were colder due to sports coils being fitted. Changed it out for one of the originals...et voila.

However, it did exactly the same again last night!

Very difficult to believe 2 plugs - albeit from I guess a similar batch - were duff, but replacing them certainly semes to have cured the problem.

Could it be the coil? Strange that it runs fine enough with the old plugs though.....

Regards

Mark
Mark O

Why does a "sports" coil require colder plugs? And what exactly is a sports coil anyway?

Are you sure that the fault isn't with the plug connector on the HT cable - maybe it is faulty and swapping it around onto different plugs temporarily cured a poor contact there making you think it was the plug?
Guy W

Guy

Sports coil is the Lucas HV thing, thus has a chance of burning out normal plugs early, thus colder plugs to compensate. Allegedly provides a better spark.

Not the connector as I swapped out to different plug position on first occasion. This time around different plug also.

Mark O

I ran a gold 'sports' coil on my A for 3 years, it's been on dad's for 2 more, and has never killed a standard plug. I'm not sure how it has a chance of burning out normal plugs?

I had an issue which was resolved by changing plugs which then went bad, traced it to the coil (this was on the V8)
Rob Armstrong

Mark.

"Sports coil is the Lucas HV thing, thus has a chance of burning out normal plugs early, thus colder plugs to compensate."

No, your understanding is, totally, incorrect.

The Lucas Sports Coil, and its various sister "high performance coils" are a method of selling you a potential--the potential for higher voltage output. But, the potential is seldom, if ever, either needed or used.

The points (or electronic triggering device) open which causes the magnetic field in the primary windings of the coil to collapse and induce a current in the secondary windings--the ones which go to the coil lead. As the magnetic field collapses, the voltage build up in the secondary windings continues to rise in voltage until there is sufficient voltage to cause a spark to jump the electrodes built into the spark plug. Then it releases the voltage in a brief fraction of a second. The most common voltage build up is about 11K volts which I commonly see at idle and steady rate cruise. When under load, there is a somewhat higher voltage requirement to induce the spark plugs to fire, sometimes as high as 19K volts but more commonly somewhat lower than that. But, the coil will only put out the number of volts required to cause a spark, never more than is necessary.

The standard coil offers a potential of 20K volts. The various "high performance coils" offer a potential of 35 to 50 K volts. Both will, in truth, only put out the voltage necessary to cause the plugs to spark and, again, this is commonly less than the 20K volt potential of the standard coil.

Thus, there is no need for using a colder than standard plug when running a "performance coil". But, the colder plug, being more difficult to fire, may require a high performance coil to fire properly. But, this is due to the mechanic making the wrong choice of plugs. When the standard plugs were used, the high performance coil would be operating like a standard performance coil.

Another area to consider is: if the colder plugs require a higher voltage build up to fire properly (especially when the engine is not warmed to operating temperature), the higher voltage output means that all of the rest of the high tension circuit is being subjected to higher than normal stress. The coil and spark plug leads are carrying higher voltage than they normally would. The rotor and distributor cap also. Higher required system voltage increases the potential for voltage leaking from the plug cap and grounding down the outside of a dirty and/or wet spark plug. Thus, if one were to actually operate the engine's ignition system at a higher than normal voltage, the entire high tension system would have to be examined for potential problems due to this higher than standard voltage over longer periods of time. The insulation of the coil and spark plug leads would have to be heavier as would the diameter of the wires if they were to constantly carry the increased voltage. The rotor would burn more rapidly and the distributor cap would begin to deteriorate more rapidly.

The reason that none of these things happen is simple--the high performance coils offer a potential for higher voltage output (with its advantages and disadvantages) but, during normal engine operation, only provide the same voltage that your plain, standard oil coil did.

Install standard spark plugs. Gap them to the recommended gap in your workshop manual. Enjoy driving.

Les
Les Bengtson

As les says, you can't get a larger voltage from the coil, unless you increase the resistance in the secondary (HT) circuit, and allow the voltage to grow. The easiest way to do that, is to open the plug gap to maximum of it's stated range. The larger the gap, the higher the voltage, and the bigger the spark, --- if as Les says, you don't leak anywhere else in the HT circuit.

I think the point of a Sports coil, is that it will last longer under the conditions, that you are running a greater voltage at the plug.

I'm not sure though, that a so called "cold" plug, will require a greater voltage to fire. Why does it?

The terms hot and cold when talking about spark plugs, refer to their heat range. This simply means the heat the plug can dissipate away from, or keep in the electrode tip. Too hotter tip might cause pre-ignition, and too cold a tip, might cause fowling. It's a balance.

The reason your plugs keep failing, might be because you are using cold plugs. If the plug is too cold, electrically conductive deposits may form on the insulator, causing a loss of spark energy or the actual shorting-out of the spark current. Since the current is now taking a path to earth it wasn't designed to do, it can cause the insulation of the plug to break down.



Lawrence Slater

Les,
Thanks for your explanation. My questions regarding the sports coil and cooler plugs were semi-rhetorical - as I thought both likely to be unnecessary on a standard engine. But my knowledge didn't go beyond that, and I couldn't have given the detailed explanation that you have provided.

At one time I switched to a hotter running plug, but that was because I was getting oiling up of standard plugs. The hotter plug cured this and it kept them clean of oil but did induce detonation so I reverted back to standard, and just drove faster to keep the plugs clean!
Guy W

To clarify, I am running with both types of plugs in the block! All the cold BP8 plug gaps were initially opened up in an attempt to improve the spark - for which I required the "sports coil" - but since BP8 plug failures, I now have 2 open gapped BP8s and 2 BP6s - thus a potential for the BP6s to burn out.

Of course I shall rectify asap with all of one type or another.
Mark O

Mark

I'd run the 6 rated plugs and see how you get on - I suspect your plug failures are as a result of plug fouling due to running an uneccesarily 'hard' plug for your set up.

NGK 6s are perfectly adequate in standard and road tuned engines - I've only ever needed 8s in race engines...

Regards

JB
James Bilsland

James

Indeed so...Still do not quite understand why they failed - when I intially ran all 4 opened up BP8s with a Lucas HV coil that should have coped with the extra voltage requirements. Car has only done 300 miles or so and has now incurred 2 suspected plug failures. Plugs incidentally look clean enough....

However, I shall revert to 100% standard as you suggest.

Mark O

Mark,
IIRC you're running a 123 dissy which might be kinder to the wrong plugs being in

as already said a Lucas sports coil will probably be no better than standard but I've been running a Lucas sports coil fot 3 years I think

I'm with the rest in suggesting you try a set of standard 6 plugs and you could try setting them at widest of range

Note - once changed you might want to also check/adjust your timing and mixture
Nigel Atkins

BP8... Is a really cold plug....i made the mistake along time back in my toyota truck and the kid at autozone gave me BP9s...we didnt know the differance...and it was a bear to start and keep running in the dead of winter and had alot of misfire

Id go to a BP6. Unless you have a real race engine....make sure your threads are really clean and shinny in the cly head for the spark plugs...if you got old excess anti siezure compond or wear and rust it will cause alot of extra resistance on the spark plug and that cant be good

Prop
Prop and the Blackhole Midget

"Still do not quite understand why they failed - when I intially ran all 4 opened up BP8s with a Lucas HV coil that should have coped with the extra voltage requirements."

Hi Mark. Not sure what you mean there. The coil generates the voltage, and is obviously capable of handling the voltages it generates -- unless it's damaged. The plugs too are easily capable of handling high voltages -- unless the ceramic insulation is damaged, which it could have been, because the plug was too cold. This would allow a build up of either fuel, oil or carbon. This accumulation allows the high voltage to travel along the insulator nose to ground rather than jumping the electrode gap.

Since the current is now taking a path to earth it wasn't designed to do, it can cause the insulation of the plug to break down.

You say the plugs are clean. Can you post a close up picture of the nose?

How about lasers then?
http://www.bbc.co.uk/news/science-environment-13160950
Lawrence Slater

sounds like it's a good job you had those nice hefty plugs leads too :)

que further posts :)
Nigel Atkins

When I used BP6ES they ran too hot and had a colourless bleached appearance, and gave the occasional cough when I switched off - swapping to BP7ES put a little colour back on the plug with no run on. I would consider 8's to be a step too far unless you have a very high CR.
F Pollock

This thread was discussed on 05/10/2012

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