Cadillac & LaSalle Club Discussion Forum

Cadillac & LaSalle Club Forums => General Discussion => Topic started by: benji808 on October 10, 2021, 04:00:22 PM

Title: Compression test desired psi ‘75 500
Post by: benji808 on October 10, 2021, 04:00:22 PM
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 somehow...it 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
Title: Re: Compression test desired psi ‘75 500
Post by: dadscad on October 10, 2021, 04:23:34 PM
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.
Title: Re: Compression test desired psi ‘75 500
Post by: benji808 on October 10, 2021, 05:05:32 PM
I'm only doing one plug at a time to keep plug wires straight.. Do I need to take them all out at once?

Hmmm...so choke and throttle need to be open? What would be the best way to facilitate that with the carburetor?
Title: Re: Compression test desired psi ‘75 500
Post by: "Cadillac Kid" Greg Surfas 15364 on October 10, 2021, 05:24:04 PM
Benjamin,
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
Title: Re: Compression test desired psi ‘75 500
Post by: benji808 on October 10, 2021, 06:07:44 PM
Very helpful on procedure- thanks, Greg!
Title: Re: Compression test desired psi ‘75 500
Post by: fishnjim on October 10, 2021, 07:12:48 PM
'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.
Title: Re: Compression test desired psi ‘75 500
Post by: dochawk on October 16, 2021, 02:55:35 PM
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
Title: Re: Compression test desired psi ‘75 500
Post by: Daryl Chesterman on October 17, 2021, 02:04:54 AM
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