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MG MGB Technical - MSD Ignition - spark plug gap?

' Just installed MSD on my '73 B/GT tonight. It has also been converted to pertronix. It started and ran fine in the garage just now; no problems so far. MSD instructions say you can run the spark plug gap at as much as .050, but to experiment. I'm running 8.7 pistons in this car. Any suggestions regarding spark plug gap?

Many thanks,

Allen
Allen Bachelder

Allen,
While the MSD may fire very wide gaps, the voltage actually developed is a function of gap width. Too wide and the voltage has too much chance of jumping inside the cap, or though the rotor. The biggest problem with the 23/25D distributors is the close spacing of the contacts due to the cap size. That's why the 45D and many other modern caps are bigger. My feeling is that anything over .035 is looking for trouble with little or no benefit, most especially under damp conditions. Big spark is good for lean mixtures, another reason they are used on modern stuff. It is vital that plug wires are good and that you do not crank the engine with IGN ON and plug or coil leads disconnected.
FRM
FR Millmore

Thanks! I have a 45D around here that I used at one time with Crane/Allison (that car was a '76). I got better performance when I converted to 25D. I attributed that to the advance curves. ' Will set these plugs at .035.

Allen
Allen

I don't like the 45D on the 72/73 engine the performance does seem to suffer. Provided that your engine is to the 72/3 spec I'd be inclined to fit the correct spec 25D for that engine. Re points gap the setting of .035" is spot on. Remember that in theory at least gaps erode at a rate of .001" per 1000 miles which means that by the time you have done 5000 miles the gap is starting to get too wide for comfort.
Iain MacKintosh

Thanks, Iain, I agree. This car is now set up with Petronix and MSD with the plugs gapped at .035. I had no problems setting up the MSD and there is a marked, if not stunning, improvement in performance. Pulling up a modest hill in third, starting around 2000 rpm, the engine started coming on much stronger at about 2200. While it didn't break my head restraint off, the first time I experienced this, the way revs started picking up, it felt for a moment like my clutch was slipping. It wasn't, as the speedometer proved.

In a world where extra horses usually cost a LOT of dollars, I think my $180 for the MSD was well spent.
Allen

Allen

This would be a nice thing to dyno. I would love to see the differences. Always interested in things that can improve performance but don't cost a whole lot.
Bruce Cunha

Bruce,

Yeah, I'm thinking the same thing. There is a dyno in nearby Roanoke. I may see about it.

I'm not going hog wild on this car, although I did rebuild the engine with 8.7 pistons rather than the 8.0 ones it had to begin with. Also, for other reasons it was bored out .030. The cam, I'm told, is a "BL-1" grind. None of those mods appeared to make any significant difference. My subjective view, however, is that the MSD did make a meaningful difference.

The one other thing I'm looking to do is to build a new head for this car that has had some good flow work done on it. I've read quite a bit about how a good DIYer can do this at home, but I'd rather not take the time to remove the head, work on it and replace it only to find my novice porting didn't help. I'll save the experimenting until I have more time and perhaps an expert looking over my shoulder. In the meantime, anybody want to recommend a good machine shop for this?

Thanks all,

Allen
Allen

This thread was discussed between 26/09/2005 and 28/09/2005

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