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Old 06-27-2007, 05:35 PM
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need help to know c.r for my 327

i want to know my compression ratio on my 327 i have double hump 462 64cc head bowl ported ,L-79 domed gm piston 327 small journal 3.25 stroke and a thick federal mogul gasket head thanks you!!!

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Old 06-27-2007, 05:43 PM
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The factory ratio was 11:1 but that was with a .028 thickness head gasket, the stock fel pro replacements are .041, which likely dropped it to about 10.75:1. too high for a street engine running pump gas.
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Old 06-27-2007, 06:02 PM
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c.r

are you sure i cant run pump gas 92 octane with 10.75-1 my cam is sum-1105 282 and 292 adv duration
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Old 06-27-2007, 06:18 PM
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i thought you could run up to a 11-1 cr with a tight squish ?
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Old 06-27-2007, 06:25 PM
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Quote:
Originally Posted by 383silverado
i thought you could run up to a 11-1 cr with a tight squish ?
what you mean by tight squish
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Old 06-27-2007, 06:42 PM
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techinspector1 explained it the best like this


Squish is the clearance distance from the piston crown to the underside of the cylinder head with the piston at TDC. You set it this way:
1. Juggle the compression height of the pistons you buy.
2. Cut the block deck for less piston deck height (measurement from the piston crown to the cylinder block deck with the piston at TDC).
3. Juggle head gasket thicknesses.
It makes no difference if you have the piston down in the hole 0.024" and use a 0.016" shim gasket to get to 0.040" squish or you zero deck the block (piston crown even with the block deck with the piston at TDC) and use a 0.040" gasket. You arrive at the same 0.040" squish either way.
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Old 06-28-2007, 12:03 AM
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Yes a tight quench lowers the octane requirement, but if his block has not been decked then the pistons are .025 in the hole, if the gaskets are the stock .041 thickness, that makes the quench distance .066, and quench is non existance over .060. Besides that, my 4.3 V6 rattles like rip on 87 pump gas and it`s got 9.6:1 compression, I have to add octane boost to make it stop and that`s with the timing at the factory 0 setting, my engine has a quench distance of .047, which is real good, but high compression engines with iron heads don`t like low octane gas, and even if your quench was down below .040 which I don`t recommend, it would still detonate on 93 octane fuel. There`s more to it than just quench to make it stop detonating. You could add lead additive as it acts as a octane booster and anti detonation agent. Cool intake charge, cool as you get it helps, cooler operating temps around 170 or so but the most important is fuel mixture and spark timing curve.
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Old 06-28-2007, 12:11 AM
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Thanks for the additional points DoubleVision, right on. Plus the fact that most stock Chevy pistons don't have a good flat spot on the piston crown to make a squish band, just the narrow band around the o.d.
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Old 06-28-2007, 08:23 AM
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I run 11:1cr with a compcam 292H with 92 octane pump gas in my 350. 38 degrees of timing and no problems. However, this is in a light car (2500 lbs) with a 3.90 gear and 5 speed trans (light load).

If 11:1 cr is with a 0.028" gasket (6cc) then it will be 10.6:1 with a 0.041" (9cc). See cr calculations below.

(670cc +67cc)/67cc =11:1

670cc= one 327 cylinder volume
67cc= total all other volume (head gasket, piston, chamber, piston recess, ect)

Then add 3cc to all other volume to figure out new cr with thicker gasket
(670cc +70cc)/70cc =10.56:1
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