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MG MGB Technical - spark plug gap

i have a 72 b with a std engine (except k+n and peco exhaust) and a sports coil . what should the gap be when using ngk v grooved bp6e spark plugs ?
phil

Phil. This has been the subject of some discussion in the past and a great deal of information should be available in the archives. I am running a 68GT with the factory exhaust manifold, K&Ns and a sports coil with points type dizzy. I find the car performs much better with a spark plug gap of .035"-.040". Coil voltage builds up until the spark plug fires. With the "normal" gap, this happens at about 10K volts. (I have picked up one of the Gunsen's testers and will be playing with it as I have time.) Hence, your standard coil has more than sufficient capacity to spark the plugs with the standard gaps. The Sports Coil, with twice the capacity of the standard coil, seems to like the larger gaps and my car runs very well with them. Les
Les Bengtson

Les -

I've wondered how critical the gap measurement is. In my case - a 72B with a desmogged engine - my NGK plugs out-of-the-box are gapped at .035. Even though the book says to gap them to .025, it seems to run just fine as-is. The plugs have a nice tan color, performance is peppy enough, etc. I'm sure that lacking the stock emissions equipment affects the performance but how critical is the gap measurement? Just curious.
Greg Smela

In the Haynes manual, all 45D distributor applications run with .035", and the 25D run with .025" (that's of course assuming you can believe the Haynes manual in all cases). I've been using .035" with a standard coil (albeit ballasted, but no real difference in output) and a 45D "Eurospec" points distributor. Personally, I don't really think it makes that much difference on an "unsmogged" engine, running at .025 or .035". In fact, the wider gap is probably more likely NOT to fire at higher rpms, although again, I would consider this unlikely (unless your coil output was pretty deficient). If the mixture is rich enough, you'll light it just fine with .025 (and once it's lit, that's all you need anyway...everything else is just unused energy...doesn't make the car go any "faster"). On the other hand, .035 will probably work just fine as well. Having screwed around with my engine about as much as is decent, I tend to think we worry about the nits just a bit too much here. I didn't really notice much difference at either gap setting. In theory, if your car tolerates a larger gap, then you'll get a larger spark (then again, it may be a big "so what", despite what all the "sports coil" advertisements would have one believe...the same amount of compressed fuel will only make so much combustion, regardless of the size of the "match"). On the other hand, you may burn up points a bit faster as well (mine have lasted just fine...on the other hand, if you can't change your points once in a while, buy a modern car and don't worry about it). Everything's a tradeoff. Probably makes a lot less difference than the myriad of postings on the subject (as one can find in the archives) would suggest.

Have fun,

Barry
Barry Kindig

Greg. American cars of similar vintage always used a gap of .035" with points type distributors. My Mustangs and Corvette always ran fine with that setting. I suspect that the root condition that caused the .025" gap to be used for so long was lack of money to test with. The Brits had some rather poor gasoline/petrol after the second world war and set up their engine specifications to deal with that situation. Years later, they have a reason to do some serious testing (US emissions requirements) and decide .035" is better. As Barry notes, it is all rather academic. Open the gaps until you mis-fire on acceleration or at high rpms and then drop down until you have a consistant firing under acceleration and at high rpms. That represents the largest, workable gap. I suspect it is considerably larger than most of us would believe. I have tried up to .045" without any problems except the feeler gauge only goes up to .035" on my set. (You have to use the .022 and .023" gauges, together, to set to .045".) On the other hand, my car runs much better at .035" than it did at .025". This may be due to the fact the engine is getting old, but there was a significant increase in acceleration and smoothness of idle with the larger gaps. Les
Les Bengtson

My 'B's engine has an electronic ignition and, like Les, I tried larger gaps for my NGKs. I stopped at .060, but had no mis-fires or mis-adventures with that gap up to 6000 RPM. Being a basically timid person, I dropped back to .045 (and 5000 RPM) for daily use.

Regards,
Jerry
Jerry Causey

If I remember correctly from high school physics, the breakdown voltage of clean, dry air is 25,000 volts/centimeter. That means that a voltage of 25Kv will jump a gap of 1 cm. Foreigh substances in the air, such as vaporized gasoline, lower the necessary voltage.

1 cm = .393 inch. 1mm = .0393 inch (which is pretty close to the .035 gap we've been talking about). Therefore, it should take 2.5Kv to jump a 1mm gap. Even a cheap ignition coil creates at least 10Kv, at idle.

At high rpm, the coil has less time to charge between firings, so it puts out less voltage. In other words, a good coil should put out plenty of voltage to jump any conceiveable gap you care to put on the plug.

As Barry Kindig said, once the fire is lit, the rest is academic. I don't believe that the spark from a larger gap or even a higher voltage is really any hotter. The reason for high-voltage coils is to provide enough voltage at high rpm to give a consistent spark, given that all coils provide lower voltage at high speed than they do at low speed.

As Les said, the smaller gap in the past may have resulted from inferior fuel, but it may also be that coils have improved over the years, to the point where there is plenty of voltage available at all engine speeds.
Paul Noble

Well, I'll leave it as is - if it ain't broke....

Interesting comments, thanks!
Greg Smela

Very interesting thread. This is the kind of exchange I really enjoy.

Many thanks for the test results from Les and Jerry and the electrical theory to back it up provided by Paul.


Matt K.
Matt Kulka

In practice the MGB engine doesn't rev fast enough to suffer from inadequate dwell/coil charging time, the same technology was used for the V8, and even Jag V12s. High pressure makes it more difficult for a spark to jump a gap, which is why a plug that won't fire in use may spark perfectly well on the bench. Although increasing the gap makes for a higher *peak* HT voltage as soon as the plug fires it drops back to just a couple of kV for the remainder of the arc (the plug is sparking for around 20mS each firing - unless the capacitor has failed in which case it drops to about 1mS, one of the causes of poor running with a failed capacitor). Increasing the plug gap and hence the peak HT voltage will put greater stress on the remainder of the HT components, which may break down under increased load such as hard accelleration.
Paul Hunt

This thread was discussed between 26/03/2002 and 29/03/2002

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