'51 Super- compression readings question

[Kestrel]

I ran cylinder compression tests today, first time since purchasing the car last November. These are the cylinder readings in psi. Engine is a stock 263 cu in, straight eight . Car has 27,500 miles with unknown mechanical history.

 

#1 = 135,  #2 = 125,  #3 = 135,  #4 = 125,  #5 = 130,  #6 = 145,  #7 = 125,  #8 = 130

 

The engine specs. in the manual show only 118 psi per cylinder.  I'm wondering what this disparity means, if anything. Calibration or accuracy of gauges ? The book allows for 10% variation in the bank. I like the higher numbers but would like to hear what others have found in their engines. Could my engine be getting stronger with age ? 😜 Thank you !

 

 

[Pete Phillips]

I've had variations like that, too. Could be due to a lot of things--how tightly you screwed it into the hole; how many revolutions the engine turned over for each reading; oil on the cylinder walls increasing the compression; accuracy of the gauge. At least you don't have any that are badly low, and if the engine idles smoothly, I doubt if there is really as much difference as these numbers show.

Pete Phillips, BCA #7338

[Bloo]

If it runs good, don't worry about it. Absolute numbers don't matter at all. Two mechanics will never come up with the same numbers. What matters is balance. I know all the textbooks say 10%, but you hardly ever see that in real life. If its within 25% I wouldn't worry. Manufacturing tolerances on old engines are pretty loose. How far the pistons are down the holes may vary. Most engines have combustion chambers that are rough cast. They vary. The valve job (how deep the valves are after grinding) varies as well.

 

If there are burned valves, then the bad cylinder(s) will be WAY low, and that is a problem. In that case the engine brings it to your attention by running horrible.

Edited February 6, 2019 by Bloo (see edit history)

[hchris]

Looks pretty good to me, variations could also include carbon build up on the high ones, a little valve leak on the low ones. With those numbers I certainly wouldn't be touching anything. 

[Kestrel]

Appreciate the replies. Thank you.

 

The test was done after I re-filled the crankcase after being drained and empty for 2 months. I actually did 2 tests yesterday, one right after the first. The 2nd test showed slightly higher numbers, probably due to better oil distribution.  

 

Yes, I'll leave it alone and count my blessings ! 😁

[avgwarhawk]

28 minutes ago, Kestrel said:

 

 

Yes, I'll leave it alone and count my blessings ! 😁

 

Yes sir. No sense losing sleep when it is not necessary!