raising cranking compression

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Jan 25, 2012 | 08:43 PM

3rdCoastPowerSports

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From: Loony Bin

raising cranking compression

i have a LS6 with a 234/238 custom spec cam in it (11* of overlap). New head gaskets and heads have been freshened up. warm cranking compression is around 140 psi across the board with no coolant or compression leaks. car is a dog unitll 3500 RPM's then its fucken awesome. I want to raise my compression so the car performs better at the bottom end. I would like to see my cranking compression around 170-180 psi. how much should i mill the heads to accomplish this? heads are 66cc 241 heads with stock thickness gaskets. I have enough PTV clearance to mill .020" without any issues. will this get me there? or should i mill .035" or so and fly cut pistons?

Jan 26, 2012 | 12:13 AM

bww3588

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From: Chillicothe/Lima, Ohio

.020 should give you right around a 63 cc chamber, good for about 10.75:1 maybe more. You Will need to keep an eye on your dynamic compression ratio as well. I would take 10 off the head and run a .040 gasket to get your quench area down around .035

Jan 26, 2012 | 05:57 AM

mark21742

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I agree, but I think I'd go ahead and knock .020" off the heads with the .040" gaskets

Jan 26, 2012 | 07:31 AM

bww3588

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He can't...he only has enough ptv to go .020. If he takes 20 off the head and goes 11 thinner on the gasket he will start crashing valves.

Jan 26, 2012 | 11:47 AM

3rdCoastPowerSports

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From: Loony Bin

Quote:

Originally Posted by bww3588

He can't...he only has enough ptv to go .020. If he takes 20 off the head and goes 11 thinner on the gasket he will start crashing valves.

I could always flycut the pistons but wouldn't the valve reliefs lower my compression = a wash.

Jan 26, 2012 | 12:47 PM

bww3588

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From: Chillicothe/Lima, Ohio

Quote:

Originally Posted by 3rdCoastPowerSports

I could always flycut the pistons but wouldn't the valve reliefs lower my compression = a wash.

if your going to flycut, make it worth it. flycutting just to take an extra .010 off the head will not be worth it. .010 is about 1.5CC's off the chamber. which translates into about a .1 increase in compression. but when you flycut, your adding roughly .75CC's back to the volume, so in reality, that .010 extra you flycut for gained you about .75CC's less in the chamber resulting in about a .07 increase in compression, which=a wash.

now this is all off the top of my head math from inputting compression ratio calculations in all week at work while i'm bored, so take it for what its worth.

Jan 26, 2012 | 01:21 PM

mark21742

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Makes sense BMW3588, I did not know the ptv was that close on his setup....I didn't read his first post as well as I should have....so I change my statement and agree with you