Compression test desired psi ‘75 500

Started by benji808, October 10, 2021, 04:00:22 PM

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What should the psi range be? I rented an OEM tools compression kit from Autozone and I'm getting right at 60psi on the cylinders I've tested so far....that seems way to low, but the car starts and runs well, so I'm wondering if the kit is messed up holds the pressure as long as I let it sit so doesn't seem to be just a loose connection/silly mistake.

I've unplugged distributer and fuel line, and am cranking for 5-10 seconds


60 psi would be awfully low. Are all the plugs out  and the choke and throttle completely open while cranking the motor?  I would think that 125 - 185 psi would be more to the norm depending on the condition of the engine.
Enjoy The Ride,
David Thomas CLC #14765
1963 Coupe deVille


I'm only doing one plug at a time to keep plug wires straight.. Do I need to take them all out at once? choke and throttle need to be open? What would be the best way to facilitate that with the carburetor?

"Cadillac Kid" Greg Surfas 15364

Pull all the plugs, disconnect the hot wire to the HEI, and manually open the carburetor fully by pulling the cable back. With the secondary throttles open wide, take a long screwdriver and gently insert it into one of the secondary barrels.
Now taking one cylinder at a time, turn the motor over with a fully charged battery for a full 5 seconds. Keep track of the cylinders by number and by readings.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-



'75 is back in the lower compression era.   The spec is 8.5:1 compression ratio.
So 8.5 * 14.7 = 125 in the bottom of the range.   Compressing gases/vapors increases temperature so it gets higher in the cylinder.   More important that they are pretty even than the absolute value.   
There's a follow up "wet" test that you should also do, if low.
Easy to search the procedure.
You'll still have to determine what's wrong if one or more is low.  ring, valve, crack, gasket, etc.


colored electrical tape is useful for keeping tabs of which is which.

You can get a set with a few different colors for about $5
1972 Eldorado convertible,  1997 Eldorado ETC (now awaiting parts swap from '95 donor), 1993 Fleetwood but no 1926 (yet)

Daryl Chesterman

If you have some masking tape, just loop a 2" piece over each wire, putting the two sticky sides together to make a writing surface, and write the cylinder number on it.  Cheap, and easy to remove when done.

Daryl Chesterman