Anyone have a guide on how to gap spark plugs on custom engine? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-18-2007, 07:38 PM
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Anyone have a guide on how to gap spark plugs on custom engine?

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

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Old 08-18-2007, 08:08 PM
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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.
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Old 08-18-2007, 09:31 PM
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With that much static you have more than plug gap to be concerned with. I would immediately change the cam to the comp 292 to bleed off some of that cranking pressure.
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Old 08-18-2007, 10:14 PM
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metal shim gasket with a compressed thickness of .015, and the block has been milled
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Old 08-18-2007, 11:41 PM
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So, what does the squish measure?
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Old 08-19-2007, 06:47 AM
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What is squish?
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Old 08-19-2007, 07:04 AM
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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.
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Old 08-19-2007, 12:15 PM
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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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Old 08-19-2007, 01:04 PM
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.010 in the hole at TDC



we did check the valve to piston clearance with some clay and there is plenty of clearance
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Old 08-19-2007, 02:31 PM
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With the piston 0.010" in the hole and a 0.015" gasket, the piston to head clearance is only 0.025". GOOD LUCK.
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Old 08-19-2007, 05:44 PM
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Use felpro #1003 .041" gasket to tame the motors cr for use with pump gas.
Your cr will be 10.72:1. Use non -projected tip sparkplugs with a heat range slightly cooler than stock 3 to 4 heat range levels. Go easy on ignition advance 32 to 34deg should do it. give it a good bit of initial advance at idle but limit and fine tune the vacuum advance rate and amount.
May have to up the carb jetting a few sizes to keep 'er cool. being slightly rich helps.
If that fails you'll have to remove the heads and relieve the chambers to lower the cr. Opening up the chamber to 65cc using the same gasket gets you 10:1 even. you'll gain some flow and power to boot.
Gap your plugs at .035". Make sure the ground strap does not go past the center pin. Trim back the ground strap length if nessessary.

A water/methanol injection system would be nice.
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Old 08-19-2007, 06:32 PM
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The heads are already bolted down and I sat the engine in the car yesterday. This is my first real build. Watched and helped many times but this is my first time to do it and even then I had a friend that is familiar with Mopars help out. SO it is all new to me.
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Old 08-19-2007, 09:05 PM
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Quote:
Originally Posted by techinspector1
Well, actually, the narrower the squish is, the more resistant the motor is to detonation.
Of course it is and I've known this for years. But when I type in a hurry I get brain farts!!
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Old 08-19-2007, 09:33 PM
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Quote:
Originally Posted by Greg T
Of course it is and I've known this for years. But when I type in a hurry I get brain farts!!

So that's what I smelled when I sat down here?
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Old 08-20-2007, 07:47 PM
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.025" might work on a Mopar, but on a Chevy, you're just begging the pistons to hit the heads.

GM makes .026" shim gaskets for less than $10 each, and .028" composition gaskets for a little more. I'd be more comfortable with one of those.
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