My stroker was Dyno Tuned a few months ago, but just now I've replaced the OEM HEI unit with a billet Mallory which has a much better power output coil ...

Lars was in town last weekend and also fattened up the carb for me. I want to install a set of Taylor Spir-Pros' that I have here, and figured I would bring the plug gap up from .045" to .050" if it's worth the effort. The plugs are Accel Shorties and the .045 is what the Dyno Shop always sets them at with HEI.

Anyone out there think there's anything to be gained a .050 gap? I'm going to take it back to the dyno shop and have another HP pull done, and have them print out A/F readings while it's run through the power band.

I'm thinking of installing the wires and fattening up the plug gap before I go. Any thoughts on this?

 

Last Friday at T&T one of the changes I made was changing plugs from ACCEL274S gapped at .045 to ACCEL 276S (hotter) gapped at .050. The mph in the eighth was 85.04 and 85.05 respectively on successive passes. That indicates a negligible difference, on my car anyway.

 

Paul -
On all testing I have done, .035" is the max gap you want to use on a high compression engine (over 11:1), and .045" is the max you want to use on moderate compression (such as yours). Once you get above this plug gap you increase the risk of misfiring inside the cap due to the resistance across the widened plug gap at elevated cylider pressures.

 

That's 0 for wider gap and 2 for same gap.

Works for me!

 

stay at ,045

 

If the plugs read ok... I'd leave em alone... Next time your replacing them you could go a little smaller and pull em out after 500 miles. If you like the old gap/plug read better... tweak em a bit to bring you back to your current gap!

If a part still has a good bit of life left, and the cars running well, I wouldnt change a thing... but if curiosity gets to you, its hard to hurt an engine!

 

Lars,

Do you have the same thoughts if you have changed your ignition to the MSD6AL unit. They are recommending an increase by .015 to .020, which would increase the gap on my car to .060 to .065.

Bob

 

Bob -
The experience I have is what I have gained from the dyno room and test equipment at Westech Performance (the guys who do all the dyno runs for Hot Rod, et.al.). Westech runs MSD systems as standard ignition setups on their dyno. They also have a cool little distributor machine that allows the spark plugs to be hooked up, and it pressurizes the plugs while allowing you to view them through glass windows.

Westech and Dougan's Engine Service run .035 to .045 plug gaps on everything they run, including MSD-equipped engines. When you increase the secondary voltage (as the MSD does) and you increase resistance across the plug gap (which occurs as cylinder pressures increase and as plyg gaps increase), the added voltage will cause the spark to simply jump the wire insulation or arc inside the cap once resistance across the plug becomes excessive. For this reason, the plug gap on high peformance engines is limited to .045" in order to assure that the plug is where the spark is going to be. Once you exceed .045", the plug may no longer be the path of least resistance, and then you have a problem...

On low compression engines and engines with low volumetric efficiency, this is not a problem or an issue, and the plug gap can be increased.

 

Well I definitely have a low compression engine, a 77, but I think I will leave them were you have recommended. I have about 500 miles with the box and the plugs are a nice tan color with .045 gap.

Thanks Lars.

 

Gap the plugs to .040" and the ignition wires will live a LOT longer than with big plug gaps! Even G.M. backed off the super wide .065" gaps that were specified with the early HEI ignitions.