cadillac 1956 compression test , what do you think or suggest ?

[DBDB]

I need a motor for my 56 coupe De ville as the Original motor was lost .I bought a complete parts car last month , i heard it run and ran it good , today i did a compression test and when engine was ice cold i did get reading between 105 to 125 Psi , then after it ran one piston went from 105 to 125 , then i did squirt some oil in the engine and did get from 125 to 140 and one was 165 psi .After that i let it run again and it runs good and even and can hold a low rpm at idle .i thought to maybe get new piston rings or leave it as it is but did not decide anything yet .I have been reading someplace that anything over 100PSI for these old engines is OK but what do you think ? Thanks

[Dan LeBlanc]

I think you may have answered your own question

If, after oiling the cylinder you see an increase in compression, you have worn piston rings.  If the engine is being transplanted from the parts car, it would be an ideal time to do a re-ring at minimum.  Why bother transplanting to have to do a removal and reinstallation later?

[J. Gomez]

Daniel,

The normal compression while cranking and throttle open is 165-185 (per the Service Manual page 8-33).

Your reading indicates a tired engine and to add to Dan's comment above it may need a bit more work.

You have to draw your own conclusions on which steps to take..! 

Good luck

[fishnjim]

Another good indicator is the mileage on the parts car/motor.   Even if meticulously maintained, the normal wear for motors of this era is not as good as today.   If it has over 50K miles, or was rebuilt over 10-15 years ago, it's not worth the risk of delay unless you're going to flip it and let someone else determine but that will affect value.   

[TJ Hopland]

Has this new engine been sitting a lot?   Is the parts car derivable?  Maybe some fresh oil and some driving will help it if its been sitting?    Do you consider pulling an engine on this vintage car a lot of work?    If not maybe throw it in and get it driving and see what happens? 

[DBDB]

The car has been sitting for a rather long time and if it would not run so nice at low RPM i would be more concerned about the condition of the engine .i am in Belgium and a rebuild down here costs MUCH more than in the US because every part must be imported and also it is uncommon for shops to do these big engines .It has a little underpressure but not dramatic .I think i will open one side to see how much the cylinder walls are worn .Car did 60.000 mils since new as far as i can see .Thanks for the comments

[D.Yaros]

Did it not used to be a rule of thumb or maxim that a difference of 20 lbs compression p.s.i. between cylinders mean it was time for an engine rebuild?

While DBDB did not provide figures on individual cylinders, it seems the difference between 105 and 165 is pretty significant, as is 125 and 165?

[C Meskouris]

Would you be interested in selling any parts

[Caddy Wizard]

56 engine should be in the 175 psi range.  A cylinder down at 105 means the engine needs work -- at a minimum the heads will need to come off, might need a total rebuild.  Sorry, but a 10:1 compression ratio engine should develop a lot more than 105psi...

[wearymicrobe]

Need to pull the heads at least to take a look at the valve seats. Also check that the springs in the head are not super tired.

Might be as simple as the car was run without leaded gas for a period of time. That is a quick and easy job and the parts would fit in a UPS envelope and cost relative pennies to a rebuild.

Personally though I would pull a head completely and check the bore and look for ring wear. Rings and a quick hone can get the job done in a lot of cases again for not much money.

[Dan LeBlanc]

I've gone through a three tired 390's so far.  One thing is for sure, the materials Cadillac used in their engines was very durable.  In all 3 cases, I've got by with new rings and bearings and no machine work to date.  Freshen up the heads and good to go.  Now I'm looking down the barrels of a 331 that I haven't dissected yet, and may be in for a surprise having said that.

Murphy's law.

[DBDB]

Maybe i could sell parts from my parts car , let me know what you look for .Depends on what you are looking for .