The Stovebolt Site Forums The Shop Area Engine Shop Spark Plug Gap...?

HI....on our 57/ 235 engine we have a Langdon HEI dizzy. Spark Plugs....what does the gap of a plug have to do with engine performance? At low speeds is a bigger gap recommended and if you are racing around is a smaller gap needed. I always thought that the gap is set and the plug you buy get is either hotter or colder? OK...I need some GAP help....set me straight. T/C Steve

You might as well call Tom Langdon for advice, even if you did not directly buy the distributor from him.

With your HEI ignition you should run a wider gap than what the tune-up specs called for. The HEI produces a hotter spark which likes a wider gap for better ignition. Going from memory here but I think around .045" is about right. Set it and forget it. It's always the same regardless of how you intend to drive the truck. You always have to check the gap when installing plugs since the gap is easily changed by simply mishandling the box during shipping.

Hi Don & TC....yep its hard to get a hold of Tom tried several times via his site and email? The HEI spark plugs are different from the regular tune-up type for Chevy and I think they are supposed to be gaped at 50 to 65??? I was wondering why and what that did for dive ability....spark plugs seem such a minor thing (wrong) I'm trying to learn from you pros. T/C Steve

I have had several GM HEI,the plugs are set at .045",my drag race car has MSD,plugs are .050".With high output ignitions the gap is much more that points.Try .055" at first if you like ,then .045",see what differance there might be

A wider gap helps drivability a little. Its like a bigger match. They claim a smoother idle. I wouldn't go beyond .045 though on a Chev. When HEI first came out they were running very wide gaps, like .060-.080. They had some problems with starting in damp weather, especially Chev. so they lowered it to .045. The heat range is the path the insulator takes to transfer heat to the shell. The insulator should run white to light grey. If it carbons up it or gets a red varnish it is too cold. If it blisters & erodes the electrodes it is too hot.

The bigger you can get away with the better. Not that it's going to matter on these motors, but when you get into really high compressions or above atmospheric pressures you can increase the energy released by your ignition system and then increase your plug gap and give the mixture a better chance at a complete burn.
Longer gaps take more time to jump, and the spark generates more momentum crossing them, so you get a better burn. This only matters with ultra high performance set-ups. On a 235/261 with 9.5 or less compression the stock rating for a hei (.045"),if you have one, will suffice jes fine.

The ultra-wide gaps, either .060" or .080" were only needed for a couple of years right before catalytic converters started being used. The mixtures were so lean from the manufacturers trying to get a 100% burn in the cylinders that it was possible to misfire with a narrow gap simply because there was no gasoline between the plug electrodes, only air. Widening the gap increased the possibility of having some combustible gas in the spark path. Once converters started being used, the fuel system could be richened up a little, and the converter acted like an afterburner. Richer mixtures and narrower gaps came back as soon as catalytic converters came into general use. On a stovebolt, use a .045" gap with an HEI system.
Jerry

make sure if you are going to measure spark gap that you use a round or rod type feeler guage flat ones dont measure the indentation on the electrode, which if you are precision tuning can mess things up quite a bit because depending on gap height in the indentation it could off set your readings by up to .003" of an inch. yet thats just me im a perfectionist and i also go to a school that demands precision measuring. best of luck.