Compression check
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Trailering
Joined: Jul 2002
Posts: 16
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From: Roseville, CA
Compression check
I just ran a compression check on my 460 with D3VE-A2A heads, and all cylinders are holding steady at 120 PSI. I know the compression ratio is 8 to 1, but what should I be at in PSI? I've searched this site for articles pertaining to compression measured in PSI, but have had no luck. Any help would be much appreciated. Thanks
Postmaster
Joined: Aug 2001
Posts: 2,882
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From: Nevada
Compression check
I think this is what you're looking for.
http://www.smokemup.com/auto_math/pressure_ratio.php
It'll give the ratio from your compression tester and you can compare that to the calculated c/r.
http://www.smokemup.com/auto_math/pressure_ratio.php
It'll give the ratio from your compression tester and you can compare that to the calculated c/r.
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Trailering
Joined: Jul 2002
Posts: 16
Likes: 0
From: Roseville, CA
Compression check
Thanks, that's a great site, but I don't have the barometric pressure. And even if I did, there's no telling that that reading would be normal for my engine. I'm wondering if anyone has done a compression test on a stock 460 and what pressure they came up with in "PSI". It seems to me that my reading of 120 is a little low for a 460, but I'm not sure.
New User
Joined: Jan 2008
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Compression check
When new they cranked up from between 150-175 psi.
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Postmaster
Joined: Aug 2001
Posts: 2,882
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From: Nevada
Compression check
Another thing to consider is that you need a hot engine (normal operating temp) to get an accurate compression reading. Cold engines will show higher numbers.
Wasn't the '73 an 8.5:1 engine? I thought 8.0:1 started in '74?
Wasn't the '73 an 8.5:1 engine? I thought 8.0:1 started in '74?
Senior User
Joined: Apr 2002
Posts: 482
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From: lake ariel usa
Compression check
my rebuilt less than 100 miles engine has 120 in all the cylinders also and it runs great. Thats all the compresion it had before i rebuilt it also. Personal Im not worried I do have the same heads with stock 30 over pistons
Elder User
Joined: Apr 2001
Posts: 997
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From: texarkana, tx
Compression check
Mike of generous stature - Harrison is running 150 pounds across all cylinders - the rings / .040 pistons are about 20,000 miles old...
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Laughing Gas
Joined: Apr 2002
Posts: 846
Likes: 2
From: Bakersfield Ca.
Compression check
120 PSI doesn't sound too bad. At least it's consistant. I wish mine was. But judging from the previous posts mine seems pretty good considering the mileage, except for one cylinder. My 460 has 125,564 miles on it now and my compression check with the D3VE A2A heads reads 150 psi in all cylinders except in one, number four which reads 120 psi. Yes it was checked with the engine warm and WOT. I have put some Marvel Mystery Oil in the crank case in and effort to hopefuly free up a possible sticky valve or lifter which might give me my a low reading in number four. I haven't done a compression check yet to see if it has done any good. Maybe at the next oil change. I have a set of D0VE heads on the back burnner that I hope to get put on after the summer. Then hopefully I can get a little better than 6 MPG. :P
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Postmaster
Joined: Aug 2001
Posts: 2,882
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From: Nevada
Compression check
I sent this to somebody, but guess I didn't post it.
14.7 is the sea level atmospheric conversion rate for calculated compression ratio to psi.
'74 + - 8.0:1 x 14.7 = 117.60psi
'72/73 - 8.5:1 x 14.7 = 124.95psi
'68-71 - 10.5:1 x 14.7 = 147.0psi
'68-71 - 11.0:1 x 14.7 = 161.70psi
Sea level atmospheric pressure is just that. If you test at a higher elevation or in weather conditions with fluctuating barometic pressures, it changes the calculation. The higher the elevation, the thinner the air. NHRA has a conversion chart for elevation. This is the reason serious racers have mobile weather stations, so they can adjust their fuel jetting based on barometric pressure and humidity, which affects combustion.
14.7 is the sea level atmospheric conversion rate for calculated compression ratio to psi.
'74 + - 8.0:1 x 14.7 = 117.60psi
'72/73 - 8.5:1 x 14.7 = 124.95psi
'68-71 - 10.5:1 x 14.7 = 147.0psi
'68-71 - 11.0:1 x 14.7 = 161.70psi
Sea level atmospheric pressure is just that. If you test at a higher elevation or in weather conditions with fluctuating barometic pressures, it changes the calculation. The higher the elevation, the thinner the air. NHRA has a conversion chart for elevation. This is the reason serious racers have mobile weather stations, so they can adjust their fuel jetting based on barometric pressure and humidity, which affects combustion.
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