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MG MGB Technical - Plug Gap

I put an electronic ignition w/ Flame Thrower coil. I am reading folks gap their plugs at .30 to .35. I gaped mine at .28. Should I gap wider to .30 or am I fine? What would be the difference in performance to gap wider?
Thanks!
JOHN
JW Colson

John. The usable limits to spark plug gap, when using the Lucas 25D4 distributor, seem to be somewhere between the factory recommended .025" and .035". In theory, a slightly wider gap causes the voltage within the secondary windings of the coil to build up higher before it reaches the point where the build up is discharged across the spark plug terminals.

Within limits, this is a slightly good thing. But, excessive build up of the voltage is a bad thing because it can lead to problems with the rotor, distributor cap and coil/spark plug leads. A higher voltage build up will put more strain on these parts than a lower voltage build up. Parts life may be somewhat shorter than with a lower operating voltage.

All of the theory being said, my measurements of the standard operating voltage of a high performance coil (Lucas Sports Coil) is less than 20K volts. In other words, the "high performance" coil is being asked to put out no higher voltage than the standard, factory coil is capable of supplying--and does quite well in most cases.

I found that, in my cars, the spark plug gap of .035" seemed to make the engine perform a little better in the summer heat, without making it hard to start in winter. (Winters are not severe in my area.) Why not play around a little with your spark plug gaps and see if you find any difference in your engine's performance? As long as the gap on each plug is the same, testing between .025" and .035" should cause no problems and you might find one gap the engine seems to like better than others. But, for most most of us, any consistent setting from .025" to .035" will work just fine.

Les
Les Bengtson

John-
In simple terms, you must understand that a coil will produce only enough power to create a current that will jump a given spark plug gap. If 20 Kv is required to jump a .024” spark plug gap, as in the case of the Original Equipment Lucas HA12 coil, a 40 Kv coil will jump the same .024” spark plug gap as soon as its charge reaches 20 Kv, thus wasting its higher output potential. The spark plug gap will have to be widened to the point that the entire capacity of the more powerful coil will be necessary to produce a spark in order to gain any benefit.
Steve S.

Electrical energy consists of two parameters - voltage and current. Whilst I fully agree that the measured voltage won't rise above a given point for a given plug gap, regardless of coil design, I suspect the extra energy that an HV coil will generate becomes available as current instead, but whether that has much effect on engine performance I don't know. Whilst opening up the plug gap may get you a bigger spark, again I don't know what benefit that has in our engines - lean burn modern engines maybe. The usual provisos apply - that with a poorly maintained engine or one with faulty or worn out components a shiny new HV coil may suddenly and magically seem to make it start and run better. But whether that is better than shiny new components to the original spec is debateable.
Paul Hunt 2

Not quite what the original post was about, but I'm thinking of trying this out

http://performanceunlimited.com/documents/plugsidegapping.html

It can't hurt anyway!
Martin Layton

That's the same as the multiple electrode plugs. The advantage of those is that the electrode with the best path delivers the spark, and you get many times the life from having the multiple electrodes. I've had a set of 4-electrode on my roadster for several years and there is no sign of erosion.

However a single electrode will only have the same life as in the normal orientation, and it is going to be difficult if not impossible to adjust i.e. close up the gap between replacement intervals, i.e. you may end up having to replace them more often.

You may find 'plug indexing' better, which is where you mark the open face of the gap on the body of the plug then screw the plugs in each hole until you find a position for each plug where the open face of the gap faces the incoming charge, or at least where the back of the electrode doesn't shield it. This is supposed to give better combustion because the spark isn't shielded from the charge by the main part of the ground electrode. It's said that you can usually get an indexed set from any six plugs in four positions, although I've never tried it.
Paul Hunt 2

This thread was discussed between 27/06/2007 and 02/07/2007

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