Clarification re Spark Plug Gap

Hi Again,

After replacing the headers, I thought it would probably be good to put in new plugs.

My Bronco has a MSD distributor and coil. It is not fuel injected.

It gets a little confusing reading about the gap recommended with HEI systems... at times it's as if that term and fuel injection get lumped together.

If I am reading things correctly, the gap should be between .045 and .050. Does that fit with what anyone else is using? Starting out, would it be better to stay at .045, or increase to ,050?

Thanks again.

Jeff

I would say the smaller gap would be a better starting point, especially without knowing exactly which MSD system you have. Which distributor is it, by the way? Which coil?
Each engine combination has it's own little likes and dislikes, but it has to be a super stout system to reliably jump an .050 gap under all situations anyway, so in my opinion, smaller is better. Somewhere between .040" and .045" then, would be my recommendation.

If you know it's been running good, and can verify what the current gap is when you take the plugs out, use that as your guide instead. If not sure, when in doubt, compromise is good. Say, .042" instead? ;D

I just wouldn't go over .045" at this point, unless you've got a ignition system known to jump that .050" gap reliably, or you find out that's what you've been running all along anyway.

Paul

The stronger the spark the wider the gap. Points are a bit weak so the gap is narrow, around .030". Hotter spark (electronic ignition) can run a wider gap, .040~.050".

The other factor in gap is cylinder pressure. Higher cylinder pressures (higher compression, cam timing and especially boost) put a lot more compressed molecules between the electrodes for the spark to push through, so the gap gets narrower. But a stock engine isn't high in cylinder pressure so it can run the fatter gap.

So what is the right gap? I remember a spec once that was as high as .080", but I wouldn't try that myself. I would try mid to high .040s. At this point it is try something and see how it works. But this will get you in the ballpark.

.40 is good for MSD

You should be able to run about .045 easily you can play around and see if more actually does anything but normally you cant really tell. Another reason to stay on the smaller side is as the engine runs over time the spark plug gap increases. The larger the gap the more the igintion system has to try and overcome that gap. While your aftermarket setup should be able to handle large gaps it will still strain it as time goes on. Its never good to strain any system.

Im running .040" with the duraspark i dropped in my '73 and it seems to like it. If you have a strong coil .045" may help give you more complete burning of fuel at high rpms. Just remember the wider the gap the harder your system needs to work to jump that gap It also has an effect on the rotor to cap current flow through the terminals, wires and works these components hard as well. A lot of the older GM HEI systems ran a .060" gap but I think that is a bit excessive for the needs of a Ford engine.