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MG MGB Technical - Plug gaps with Flame Thrower coil

I've installed a new pertronix "Flame Thrower" 40,000 volt coil in my 71 MGB. I have the Pertronix electronic ignition. I also have an alloy head and run unleaded premium. I'm using standard NGK plugs. Any suggestions on the the best spark plug gap? Any other considerations?
Thanks in Advance,
Greg
Greg Peek

Greg
You can make the gap wider and wider until something fails. The wider the gap the higher the voltage going through the ignition system. A zillion volt system only goes as high as necessary to jump the gap.
The wider the gap the higher the voltage. Eventually the weakest link will fail. Be it the distributor cap, a coil or a plug wire. As a practical matter there isn't much improvement in lighting the fire once you get beyond a gap of about .100". Air acts as an insulator. The thicker the air (the higher the compression ratio) the harder it is for the spark to jump the gap. The typical MG is running a compression ratio below 9:1 and is not particularly demanding on the ignition system. A 4 cylinder engine running at 5000 rpm is equivalent to a v8 ignition system at 2500 rpm
Barry
Barry Parkinson

Greg,

.035. Spec is .025. Widen a little for extra voltage.
Paul Hanley

Greg
One interesting side effect, at least on the old style inductance type tachometers '67 and before: The wider gap makes the tach work better. With a smaller spark plug gap and an electronic ignition the tach fades out at about 3000 rpm. Widen the gap way up and the tach works like it is supposed to. It's way optimistic, but then it always was. My supposition is that the larger voltage spike with the wide spark plug gap gives a more distinct signal for the tach. The only other effect I note with a wider gap is that cold starting and idling are improved. It apparently is able to fire a leaner mixture than the standard plug gap.
I note that current GM cars with electronic ignitions are spec.ed for 55-60 thou gap. (1.5mm).
Barry
Barry Parkinson

The factory increased the gap to .035 even with standard coils.

Electronic ignition conversions do often cause problems with the inductive tach, that maybe opening up the plug gap does help. If tachs on standard ignition systems respond to changing the plug gap then I would suspect a faulty tach. Many years ago when a tach was the go-faster gizmo to have but way beyond my impecunious youthful pocket a couple of us had a go at making one, voltage sensing rather than current. Yes the needle did go up and down with the revs, but more noticeably it went up and down with throttle opening even when the revs stayed the same :o) The higher pressures in the cylinder with a wider throttle makes it harder for the spark to jump the plug gap, so the HT voltage rises higher before it does so, and that rise in HT voltage was being fed back into the primary of the coil and hence the tach.
Paul Hunt

Paul
Now that my tach is working I would like to keep it if I can calibrate it. Is there an adjustment pot or coil available to play with, or is it fixed?
Barry
Barry Parkinson

Thanks guys, for all the input; much appreciated.
Greg
Greg Peek

Barry, if you open it up you'll find a pot for adjustment. It's easiest to drill a hole in the back of the case so you can adjust without removing it each time.
Steve Postins

This thread was discussed between 20/02/2005 and 21/02/2005

MG MGB Technical index

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