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Old 07-22-2007, 02:52 PM
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190 psi cylinder pressure

Just did a compression test on my 355 sbc. All cylinders were 185 - 195, mostly being 190 and above. This seems pretty high doesn't it? I think I just found my detonation problem!!!!!!! 268H cam, 461 heads, flat top pistons----guy must have shaved more off of the heads than he was supposed to. Are these numbers high or is it just me.

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Old 07-22-2007, 04:04 PM
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The number is high, However, we did a similiar build using 2 valve relief flat top pistons and vortec heads and reached the same numbers you did with a 215 duration roller cam. Even so the quench was set at .040, it still rattled, so we installed a adjustable vacuum advance and it solved the problem. With the advance connected to a full time vacuum source, unplugged and capped, we set the base advance at 12 btdc, then connected the advance and adjusted it until the rattle came out.
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Old 07-22-2007, 04:34 PM
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Maybe I'm doing the test wrong. I turned it over for 4 compression strokes before I read the guage, not just 4 crank revolutions. Am I doing this right?
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Old 07-22-2007, 04:36 PM
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Yeah, you're doing it right. Crank the motor until the gauge needle stops going any higher. Be sure you have the carb throttle blades wired open and the hot wire to the coil disconnected.

Last edited by techinspector1; 07-22-2007 at 05:20 PM.
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Old 07-22-2007, 05:45 PM
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one thing you could do(maybe two) is install a BIGGER cam (yea baby now you talkin) that would lower the static compression and may raise the top end power or install thicker head gaskets. that is a little too high for a street motor but that is what they make tetra ethyl lead additive for, right???
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Old 07-22-2007, 06:00 PM
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Do Not Install Thicker Head Gaskets. It will widen the squish and contribute to detonation.
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Old 07-22-2007, 06:48 PM
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I want to put new heads on it. The old double humps are only 160cc runners and four of my 8 plugs are wet with oil. I burn about a quart every 250 miles. Compression check proved its not rings so I'm left with valve guides or stem seals. What is a good runner size for my cam. Engine is in a 83 4X4 Silverado with 3.08 gears(will be switching to 3.73 shortly)
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Old 07-22-2007, 07:56 PM
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180 or 190cc intake runner volume would probably work pretty good with that cam and rpm range, 72cc chambers would be perfect. Alluminum heads would also help.
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Old 07-23-2007, 05:19 AM
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Quote:
Originally Posted by baddbob
180 or 190cc intake runner volume would probably work pretty good with that cam and rpm range, 72cc chambers would be perfect. Alluminum heads would also help.

A set of aluminum heads with 72cc chambers and 180cc runners would be just the ticket with the 268HE cam. With a quench distance of .046" your Static Compression Ratio will be about 9.8 and Dynamic Compression Ratio will be about 7.7. Not only will it make good power over a broad range but should easily do it on 87 Octane with no fear of detonation.
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Old 07-23-2007, 05:53 AM
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You mentioned four of eight plugs are wet with oil, are you certain it doesn`t have a intake leak? Yank the carb and look in the plenum, if you see oil you have a leak.
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Old 07-23-2007, 06:49 AM
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edlebrock makes a 70cc combustion chamber, 170cc intake port, aluminum head for 1000.00. A very good flowing head with a moderately sized port which is just what a 350 needs in a heavy truck.

In cool climates, get the regular edlebrock performer head (not rpm) because it has the exhaust pass over port.

The 3.08 with heavy truck is making the detonation issue worse. A gear change may fix it.
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Old 07-25-2007, 07:47 PM
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Checked the intake plenum already and it's high and dry, also put a new PCV on it just to rule that out. I know the 3.08's are killing me in acceleration, I can do 65 in first gear in this beast. Want 3.73's or 3.90's but in the end it still smokes like a freight train.
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Old 07-25-2007, 07:57 PM
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The oil in the combustion chamber is a major factor.
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