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Old 03-20-2017, 09:08 AM   #21
55 Tony
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One reference I read said to undo the rockers but a couple of others don't mention that. The same reference also said to actually run the engine for a few seconds while others say to disable the ignition and just crank it over 5-10 times.

Essentially all you're doing is doing a compression test which usually involves just sticking a pressure checker in the spark plug hole and cranking the engine, so do it that way, then divide by the ambient air pressure.

Of course each of the cylinders may vary based on the condition of the valves and rings so you might want to do all cylinders and accept the highest reading as the correct one.

cc'ing the combustion chamber and computing the cr will give you a theoretical answer while measuring the actual compression should give you a real answer.

[To get a 12.85 comp ratio you'd need compression readings of 189???]
Yes, I do get 185 to as high as 200 sometimes. Before the rebuild I was getting in the 140 range with the old flat top pistons. My barometric pressure was 14.4 which gives me 13.89:1.
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Old 03-20-2017, 04:01 PM   #22
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Yes, I do get 185 to as high as 200 sometimes. Before the rebuild I was getting in the 140 range with the old flat top pistons. My barometric pressure was 14.4 which gives me 13.89:1.
I have no doubt that you know how to do a compression test and that you are reading the numbers correctly on your pressure gauge but I really must question whether the gauge you're using is accurate. A reading of 185-200 psi is very nearly in the diesel range or the effective pressure range of a 12 psi boosted engine.

Various factors change things but in general you can assume that temperature of a gas under compression is linear; doubling the pressure doubles the temperature, tripling the pressure triples the temperature, etc. So if your IAT is around 60 degrees, an increase in pressure of 13.89 times would result in a compressed air temperature of 833 degrees. Normal combustion temperatures of a gasoline-air mixture in a cylinder are in the 700-800 degree range.

I would recommend trying a 1) different gauge, 2) warming the engine to operating temperature and 3) disabling the exhaust valve by removing the rocker before re-doing the test.
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Old 03-20-2017, 10:12 PM   #23
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I have no doubt that you know how to do a compression test and that you are reading the numbers correctly on your pressure gauge but I really must question whether the gauge you're using is accurate. A reading of 185-200 psi is very nearly in the diesel range or the effective pressure range of a 12 psi boosted engine.

Various factors change things but in general you can assume that temperature of a gas under compression is linear; doubling the pressure doubles the temperature, tripling the pressure triples the temperature, etc. So if your IAT is around 60 degrees, an increase in pressure of 13.89 times would result in a compressed air temperature of 833 degrees. Normal combustion temperatures of a gasoline-air mixture in a cylinder are in the 700-800 degree range.

I would recommend trying a 1) different gauge, 2) warming the engine to operating temperature and 3) disabling the exhaust valve by removing the rocker before re-doing the test.
I doubted my compression gauge also, but after reading the above I went back out and checked it. At my little compressor that happened to have 90psi in it, read the same on the compression tester. Then to my other one where the pressure goes up to 175, but the regulator only goes to 130, again, the reading was close if not exact. Then for the hell of it I tried another cylinder but it was cold. I got 175 with an old tester with a rubber tip you have to hold against the spark plug hole, but it had more pressure, I just couldn't hold it tight enough and it would leak. If I disable the exhaust valve that would/could only raise the compression higher (depending on the cam), but never lower.

I don't know? I'm just reporting my findings. What in the world could I do wrong that would give me higher psi readings?
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Old 03-21-2017, 09:15 AM   #24
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I think your are correct with your readings Tony, I had a 396 that I built back in the 70's that had 190 lb compression warm and that thing was a beast...
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Old 03-21-2017, 11:13 AM   #25
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My only conclusion is that you're running at 13.5:1 cr. Are you using pump gas? I'd think you would need avgas or racing gas, 100-110 octane to keep detonation from destrowing the engine.
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Old 03-21-2017, 03:50 PM   #26
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My only conclusion is that you're running at 13.5:1 cr. Are you using pump gas? I'd think you would need avgas or racing gas, 100-110 octane to keep detonation from destrowing the engine.
I'm running 93 octane and the pistons looked like new when I had the heads off recently (except for the carbon from the leaking valve guides). And I can hear knocking or even pinging when it's there, but it's not. I forget how much I had to back off the initial timing so it wouldn't kick back when trying to start it. I'll check that again sometime.
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Old 03-21-2017, 03:55 PM   #27
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I think your are correct with your readings Tony, I had a 396 that I built back in the 70's that had 190 lb compression warm and that thing was a beast...
Did it run on pump gas? Although back then Sunoco had some pretty high octane gas I think?
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Old 03-21-2017, 06:04 PM   #28
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Somethings wrong here with all these calculations thrown about. A BB Chevy with iron heads is only safe at around 9 3/4 to 1 with today's pump premium. 12.5 or 13 AIN'T gunna cut it on nowadays regular fuel!

I've run 12.5's before on the street, but that was a long time ago with really high octane pump gas. Even then you HAD to be careful with timing.
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Old 03-21-2017, 07:01 PM   #29
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Did it run on pump gas? Although back then Sunoco had some pretty high octane gas I think?
At that time I used American white gas, don't remember the octane rating, but was 10 or 15 cents higher per gallon..
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Old 03-21-2017, 07:53 PM   #30
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In my opinion, I don't think you can really check actual compression by cranking compression.. Cam events play a big part in what the gauge will read also how fast the starter is spinning..Example; I had a 355 in a bracket car years ago, I played with the cam advancing and retarding it to see what it would run or if it effected how the engine would respond. It had JE flat top pistons, zero deck height, Brodix track1 heads ect. Measured compression was 10.8 to 1.. Cam installed straight up it cranked 185psi. Advancing cam I was able to get it to 210psi.. Just my opinon for whats it's worth..
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