Technical > Matt Shaff's Engine Shop

Plug Gap

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spitzerscott:
 Just pondering the best plug gap for my application. I use Autolite 3924 plugs, recommended by my sprint car buddies, I have a 336 SB chevy, injected alcohol, 13.2 cr, aluminum heads, Msd 7al2, msd pro power coil, msd pro billet distributer.  Powerglide, with 4200 stall, in 225" rear engine dragster.   

dusterdave173:
Plug gap generally is determined by type of ignition
Vertex style mag tight at .018 range
MSD's .035-.060 but most like around .040  I have never seen any advantage to more on our dyno--
the stronger your ignition the bigger gap you can run but real world we see little/ no  advantage to bigger( my two cents worth)
Remember--If your lawn mower will not start--tighten up plug gap and it will usually fire :)

GlennLever:
Is blown, if it is you have to be careful about a wide gape as the blow will try and blow the spark out. I have as MSD 6 AL and run .028

ricardo1967:

--- Quote from: GlennLever on January 03, 2015, 09:12:36 PM ---Is blown, if it is you have to be careful about a wide gape as the blow will try and blow the spark out. I have as MSD 6 AL and run .028

--- End quote ---

Sorry, can't agree with that... (not to mention that the intake valve closes before spark takes place).

The concern around spark plug gap on supercharged (or turbo) applications is because of the higher dynamic compression ratio, which makes more difficult for the spark to jump across the air gap.

GlennLever:

--- Quote from: ricardo1967 on January 03, 2015, 10:35:34 PM ---
--- Quote from: GlennLever on January 03, 2015, 09:12:36 PM ---Is blown, if it is you have to be careful about a wide gape as the blow will try and blow the spark out. I have as MSD 6 AL and run .028

--- End quote ---

Sorry, can't agree with that... (not to mention that the intake valve closes before spark takes place).

The concern around spark plug gap on supercharged (or turbo) applications is because of the higher dynamic compression ratio, which makes more difficult for the spark to jump across the air gap.

--- End quote ---

I my reasoning is wrong, are you saying the concern about the wide gap is correct on a blown application.

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