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'56 cadillac Coupe Deville cylinder compression average psi

Started by Wacco65, July 04, 2018, 09:45:12 AM

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Wacco65

1956 cadiiiac coupdeviie when doing a comp test what should the average psi reading be

Dave Shepherd

Fresh engine, old engine?  Remember this must be done all plugs out and the throttle held wide open.

Bobby B

Quote from: Wacco65 on July 04, 2018, 09:45:12 AM
1956 cadiiiac coupdeviie when doing a comp test what should the average psi reading be

Just as important as a decent reading would be the difference(s) between cylinders....If you're having problems, try dry testing first, then add a little oil to each cylinder to see if it changes. Leak down test is a better indication if you detect a discrepancy with pressures. Good Luck!
                                                                                                                                      Bobby
1947 Cadillac Series 62 Convertible Coupe
1968 Mustang Convertible
1973 Mustang Convertible
1969 Jaguar E-Type Roadster
1971 Datsun 240Z
1979 H-D FLH

James Landi

another factor:  a stone cold engine that has not run in months,may only develop 60 psi per cylinder.  Run it, get it hot, as it gets sufficiently cool to unscrew the plugs, then  run the test (as stated above, with the throttle wide open), magically, you have 140 psi.    Adding light oil, as stated above, can provide an important insight.  Also, get it going, and attach a vacuum gauge to the intake manifold.... the various readings you get from the gauge can tell you so much about the condition of the engine...   Happy day,   James

Dr. John T. Welch

A compression test  reveals the ability of a cylinder to build pressure.  A leak down test reveals the ability of a cylinder to maintain pressure during the expansion of gasses at combustion.

Compression tests for older higher mileage engines are more significant for variation in readings among  cylinders rather than peak absolute cylinder values. An engine will operate smoothly with uniform low readings but will be reduced in power. A compression test can assist in locating a problematic cylinder or cylinders. A cylinder leak down test will aid in determining the reason(s) for  reduced compression by revealing the path(s) of compression loss.

As far as rules of thumb go, I defer to the legendary engine builder, the late Joe Mondello, “Dr. Oldsmobile”:

“If  you have 8% leak down anywhere on a fresh build, you don’t have an engine  You have an outhouse”!     

 
John T. Welch
CLC   24277

John Washburn CLC 1067 Sadly deceased.

Wayne,

Provide me with the altitude where your car lives and I can calculate that for you.

The Johnny
John Washburn
CLC #1067
1937 LaSalle Coupe
1938 6519F Series Imperial Sedan
1949 62 Series 4 Door
1949 60 Special Fleetwood
1953 Coupe DeVille
1956 Coupe DeVille
1992 Eldorado Touring Coupe America Cup Series

Caddy Wizard

The 56 engine is 10.25:1 compression as I recall.  I would expect compression on a freshly rebuilt motor in the  160-175psi range.
Art Gardner


1955 S60 Fleetwood sedan (now under resto -- has been in paint shop since June 2022!)
1955 S62 Coupe (future show car? 2/3 done)
1958 Eldo Seville (2/3 done)

Wacco65

Quote from: John Washburn CLC 1067 on July 06, 2018, 01:39:17 AM
Wayne,

Provide me with the altitude where your car lives and I can calculate that for you.

The Johnny
hi john we live at 5 metres above sea level

gatech1956

According to page 8-33 of the 1956 shop manual the engine compression ratio is 9.75 to 1 and the pressure at cranking speed should be 165 to 185, at 1000 RPM 212 to 230.

John Washburn CLC 1067 Sadly deceased.

I did not realize that you basically live at sea level so the requirement of 165 to 185 is correct for a new engine.


The Johnny

John Washburn
CLC #1067
1937 LaSalle Coupe
1938 6519F Series Imperial Sedan
1949 62 Series 4 Door
1949 60 Special Fleetwood
1953 Coupe DeVille
1956 Coupe DeVille
1992 Eldorado Touring Coupe America Cup Series