What plug gaps are you guys running in your race cars?

I have 11.5 CR (#185 cranking pressure) with the HEI ignition, MSD E-coil and I am going to put in new plugs. Curerently they are at .040

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

10-12.3:1 NA- NGK UR5 at .040-.042 gap

Also, heat range with Fat pack and 11-12:1 SCR with AC Delco R44TS to be better color wise and in performance gapped to .045...

Both setups using recurve mopar distributor 1990's era MP Orange Box and Accel Coil without issues.

.030-.035".

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Official Cookie and Mater Tormentor.

Wow, that is a low cranking psi for 11.5:1. Must be a big cam or not so early centerline. I would go for 0.035" or a skosh more. I've been gapping Project V (11:1) at 0.038.

Lou

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Home of Slant6-powered fun machines

.566 / 257@ .050/ 102 intake centerline, installed at 99

So I was thinking a bigger plug gap would be better for performance, but I likely need a little less than my current .040? That is what I will do, but can you help me understand why? Thanks!

_________________
2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

0.040 should be fine, really. If it gets too big, then the spark is not fat enough to fire the mix properly. Higher energy ignitions/coils will allow larger plug gap. In my experience, gap makes no difference in power (ETs...) at least from 0.030" to 0.050", which I've tried on ~250 HP Slants. So, you may as well run no-so-large gaps. Higher comp and more HP mean more fuel in the cylinder and it is harder to ignite. Smaller gaps can ensure everything is igniting. I run 0.030-0.033" on the turbo motor, just for insurance, somewhere around 0.038" on NA. Depends on my mood.

This is my understanding and experience....

Lou

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Home of Slant6-powered fun machines

As Lou said, wider gaps make no more power, but do make it easier to put a plug out and foul it.

When I was spraying I closed them up to .025" just to make sure they would fire, and that was with an MSD.

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Official Cookie and Mater Tormentor.

.035 it is!!

Thanks Guys!!

_________________
2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6