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New Compression Numbers
#1243447 Wed Nov 15 2017 09:28 PM
Joined: Mar 2002
Posts: 9,035
Master Gabster
The 216 in my 51 Canadian GMC has served me well for over 40K miles. Initially I replaced the rings and pistons and installed a completely rebuilt head. The cylinders have significant wear at the top and one cylinder has been honed out some by a previous owner. That cylinder yields quite a bit of piston slap noise. The motor runs well and does not use a lot of oil. However I am planning to eventually do a rebuild and have the block bored. Several times in previous years I have done compression checks. The numbers ranged from 80 to 90 lbs. This year I converted to a 12 volt system keeping the 6 volt starter. So it turns over much quicker. I did a compression check the other day and all the numbers are between 110 and 120 lbs. The only conclusion I can come to is that the compression numbers can be greatly affected by the speed that the starter turns the engine over.


1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed


My Chevy Master 4 Door is on the Road!
Re: New Compression Numbers
truckernix #1243464 Thu Nov 16 2017 12:36 AM
Joined: Dec 2008
Posts: 1,490
P
Shop Shark
Correct, also ambient air temperature and oil viscosity.

Re: New Compression Numbers
truckernix #1243467 Thu Nov 16 2017 01:02 AM
Joined: Feb 2004
Posts: 21,240
H
Boltergeist
One of the principles of physics is that certain loads increase in a geometric fashion, not linear, as a function of speed. For instance (being VERY inexact) if the load on a surface is 10 lbs. when it is hit with a hammer at 10 ft/sec, that same surface will see 100 lbs of force when the hammer speed is doubled- - - - -the load squares! Now apply that to the pressure compression exerts on a piston ring- - - - -it pushes the ring out against the cylinder wall, and down against the top of the ring land below it to help the ring achieve a seal. As the speed of the piston increases, the force applied to the ring also increases, and the ring gets a better seal. Today's "low tension" rings rely almost entirely on cylinder pressure to get a seal- - - - -there's very little spring tension against the cylinder wall involved.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: New Compression Numbers
truckernix #1243479 Thu Nov 16 2017 03:09 AM
Joined: Mar 2002
Posts: 9,035
Master Gabster
I have read over and over that the differences between the cylinder readings is really the important part of the test. I guess this sort of points that out!


1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed


My Chevy Master 4 Door is on the Road!
Re: New Compression Numbers
Hotrod Lincoln #1243520 Thu Nov 16 2017 03:40 PM
Joined: Sep 2013
Posts: 1,062
M
Shop Shark
Originally Posted by Hotrod Lincoln
One of the principles of physics is that certain loads increase in a geometric fashion, not linear, as a function of speed. For instance (being VERY inexact) if the load on a surface is 10 lbs. when it is hit with a hammer at 10 ft/sec, that same surface will see 100 lbs of force when the hammer speed is doubled- - - - -the load squares! Now apply that to the pressure compression exerts on a piston ring- - - - -it pushes the ring out against the cylinder wall, and down against the top of the ring land below it to help the ring achieve a seal. As the speed of the piston increases, the force applied to the ring also increases, and the ring gets a better seal. Today's "low tension" rings rely almost entirely on cylinder pressure to get a seal- - - - -there's very little spring tension against the cylinder wall involved.
Jerry


Jerry, GREAT description Sir!


1951 3600 with Clark flatbed, T5, 4.10 rear
1970 340 Duster
1990 5.0 V8 Miata (1990 Mustang Gt Drivetrain)
1951 Farmall Super A



Re: New Compression Numbers
truckernix #1243987 Mon Nov 20 2017 02:20 AM
Joined: Jun 2008
Posts: 1,458
S
Shop Shark
Okay Jerry, I’ll really show my ignorance and ask a question...... with the key off do you crank the motor over just a couple of seconds and then read the pressure gauge or do you spin it over longer? Maybe you’re supposed to read the gauge with the engine running??? Is this a one or a two person job?


Chuck
1950 Chevy 1/2 ton (original)
1951 Chevy 1/2 ton (future streetrod)
1941 Chevy coupe
1938 Chevy coupe streetrod
Re: New Compression Numbers
truckernix #1244004 Mon Nov 20 2017 03:41 AM
Joined: Feb 2000
Posts: 4,192
J
Shop Shark
When I check compression, I count the revolutions, you will hear it hit the gauge each time it comes up on compression, I usually go five hits per reading.

Joe

Re: New Compression Numbers
truckernix #1244006 Mon Nov 20 2017 04:17 AM
Joined: Feb 2004
Posts: 21,240
H
Boltergeist
Ditto- - - -I usually count 4 or 5 compression strokes at cranking speed with all the spark plugs removed and the throttle blocked wide open. I look for cylinder pressures within 10% of each other, not any specific pressure reading. A 5% tolerance is better.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: New Compression Numbers
truckernix #1244061 Mon Nov 20 2017 05:15 PM
Joined: Jun 2008
Posts: 1,458
S
Shop Shark

“When I check compression, I count the revolutions, you will hear it hit the gauge each time it comes up on compression, I usually go five hits per reading.”

When I checked my compression last I didn’t hear anything “hit” the gauge. I was in the cab stomping on the starter button. Guess I was supposed to be under the hood and manipulating the starter linkage?? I’ll try that method this week.

Jerry, I didn’t know I was supposed to take out all the spark plugs. I only took out the one and replaced it with the screw in hose to the pressure gauge. I’ll remove the other five next time.


Chuck
1950 Chevy 1/2 ton (original)
1951 Chevy 1/2 ton (future streetrod)
1941 Chevy coupe
1938 Chevy coupe streetrod
Re: New Compression Numbers
truckernix #1244149 Tue Nov 21 2017 04:28 PM
Joined: Feb 2000
Posts: 4,192
J
Shop Shark
Once you get all the plugs out, you will hear the one cylinder hit. Be sure to wire the throttle wide open and have the choke wide open. Pulling the air breather also helps. You want the engine to pull air as easily as it can.


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