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Anyone have a guide on how to gap spark plugs on custom engine?

3K views 12 replies 4 participants last post by  jimfulco 
#1 ·
Chevy 350 .040 over/ "double hump" 2.02 heads (been shaved with 59 cc chambers now), forged flat top pistons, compression ratio 11.72:1, XE274H cam (Lift .487/ .490), HEI, Edlebrock 650 CFM carb

any other info I need to provide?


Thanks


ps using accel header plugs
 
#2 ·
Using 0.039" compressed for the gasket, zero deck, 5 cc's in the piston eyebrows I get 11.08:1 static c.r. I don't see how you could get to 11.72 with flat-tops in a 350. Even at 11.08, the cam's too short.

Anyway, with that much compression, start with a gap of 0.035". Widen it a little at a time to experiment, depending on your ignition system and the coil you're using.
 
#7 ·
The squish or quench area is the gap between the piston flat and the head flat @ TDC, which allows the flame to accelerate and cool during ignition. The wider the quench area the more resistance you have to detonation. Also, the quench is a safety net so that your piston doesnt make contact with the head. You cant just deck blocks, shave heads and throw in some domed slugs. Life is not that easy.
 
#8 ·
Well, actually, the narrower the squish is, the more resistant the motor is to detonation. But there is a mechanical limit to how narrow you can go. That's why I was saying I don't see how you can build a 11.72 motor with 59 cc heads and flat-top pistons.

Timothy, how far down in the bore is the piston at top dead center?
 
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