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Old 03-28-2012, 05:14 PM
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305/350 Compression Psi

Hey guys,

I have a 76' 350 4-bolt main with 416 305 heads on it. The 58cc 416's are freshly rebuilt & ported but have the stock 1.84 valves. The engine is not bored & the pistons are flat top's with the 4 valve reliefs. Bottom end was rebuilt in 1995 with maybe a few thousand miles on it since then. Head gasket is .039 thickness. Valve springs have 136psi seat pressure. Carb is a Holley 600cfm on a dual plane intake. Cam specs are 224/224 dur. @ .50, .450 / .461 lift. Now for the question! What should I be seeing as far as cylinder pressure? I'm reading 130-135 on 7 of the cylinders, 150 on the 8th. This seems too low. Any opinions before I bury my head in the sand?

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Old 03-28-2012, 07:09 PM
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Yeah, how did you test it? When doing a compression test you should pull all the plugs, pull the hot wire off the ignition so it doesn't start and block the throttle wide open so it can get air. If you tested it with the throttle closed it don't get much air. Next is the cam you have is fairly healthy and the loss during the overlap period causes the low psi reading but it's overcome in the upper RPM band where you can't test it.
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Old 03-29-2012, 03:26 PM
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I had a feeling the LSA was too much. All of the other motors I've heard with this swap have a more choppy idle. But this is what my engine guy said work. The engine feels a little stronger than it did with the 882 heads on it but not much. The pistons are rebuilder pistons and the block has not been decked. When I told my engine guy what the compression was, he said it was too low. But I put oil in a couple of the cylinders & the reading didnt change, so I think I'm good there. Still not sure why the one cylinder hit 150psi??

The heads have been fully ported. Pocket ported, as well as the runners. I will look into that Isky cam. I like the sound of it as well! I appreciate all the help with this guys! p.s. F-bird, what do you mean when you say C/L?
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Old 03-29-2012, 04:02 PM
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p.s. F-bird, what do you mean when you say C/L?
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Old 03-29-2012, 07:31 PM
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I have 2 compression gages (same make and model) and one reads 20 to 30 psi lower than the other.

what you want to see if is equal pressure over all 8 cylinders.
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Old 03-29-2012, 08:05 PM
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Yea, I used the same gauge for the whole job. All cylnders were the same, except for one which was 150psi.
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Old 03-29-2012, 08:07 PM
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Quote:
Originally Posted by 454C10
I have 2 compression gages (same make and model) and one reads 20 to 30 psi lower than the other.

what you want to see if is equal pressure over all 8 cylinders.
True that!

Forget trying to determine anything from the reading you get from a pressure gauge, I have three calibrated pressure gauges...one mechanical, one digital and the other is a Heise meter.

All of them give different peak pressure readings on the same motor, all that matters is the variance between cylinders...it's a diagnostic tool to determine cylinder sealing equality.
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Old 03-30-2012, 04:29 PM
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Its on a 76' GMC K25 pickup 4x4 with 4.11 gears 4 sp. manual trans. Not a rocket ship! haha. I don't really want to have to lift the heads off, but if I need to install a thinner gasket then so be it. Would I just advance the cam on the timing chain? Is there another cam that would work thats cheaper than the Isky?
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Old 03-31-2012, 11:18 AM
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*update* So I did another compression test this morning & propped the throttle open. Compression shot up to 150-155psi very quick so I think I avoided a rebuild. These are rebuilder pistons that are in it so they do sit lower below the deck. I'm thinkin that combined with a thicker head gasket might still get me lower than desired compression. My new goal will be the cam swap over to a 106-108 LSA. I'll keep you guys updated.
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