Compression numbers

I seem to recall that 175 is excellent for an engine that hasn't been rebuilt, and anything over 145 acceptable. Our mutual friend clocked mine at 155 during PPI and said that was good for a car that hadn't been running much with the previous owner; "probably" would increase a bit with more driving, he said.

 

Too many variables to be looking for a particular #. More important is consistency between cylinders. It is very improbable that they will decline or increase (carbon) at the same rate. So if you have high variation you likely have a problem.

 

I posted these compression numbers about 2.5 years ago for my Euro 84 QV:

Compression results, on a cold engine that hasn't been run for 2 months:
Cylinder: PSI
1: 142
2: 150
3: 145
4: 150
5: 155
6: 152
7: 155
8: 160

I was pretty happy about those numbers. #1 seemed low, but it was the first cylinder tested, so I suspect the ring sealing wasn't great after not being turned over for 2 months! I should have re-tested it after finishing the others, but didn't think of it until all the new plugs were in. Also of note, Calgary is about 4,000 ft above sea level, so atmospheric pressure is 10% lower than sea level, and compression numbers here tend to run 10% below what would be achieved at sea level as well.

 

My shop mentioned the vacuum on the brakes was weak and perhaps the engine needed to be rebuilt. She is an '83QV & has 97K on the odo

I did the test after she was sitting a week & cold with a Sears gauge with maybe 5 cranks on a reading. I can only get the adapter finger tight, so she may have been leaking some.

1-137 5-138
2-139 6-146
3-135 7-136
4-136 8-138

a bit low, but consistent. So if the gauge was off on the readings, I was consistent
Maybe another day I'll take her for a run & re-run the compression readings on the rear bank. The front is no fun.

 

I prefer the leak down test. It takes about the same amount of time and tells you much more about the health of your engine or where the trouble lies.

 

One does not replace the other. Both are important and tell overlapping but somewhat different information.

 

175 PSI = 12 bar (excellent just rebuilt car or like new: almost impossible to find)
160 PSI = 11 bar (excellent conditions: hard to find)
145 PSI = 10 bar (an average good car)
130 PSI = 9 bar (can still work, but not for too many miles)
115 PSI = 8 bar (very old engine, rebuild soon please)

please pay attention: the value reported is for each cylinder, not "the average". Seven cylinder with 11 bar and two with 9 bar is not good: the engine won't turn smooth.

My blu sera drysump has:

10,5
10,5
10,5
10,8
9,8
9,8
9,2
9,5

It doesn't turn smooth due to the 9,XX bar cylinders and it's on rebuilding at Toni shop.
Difference from top to lowest should be no more than 0,5-0,7 bar (7,5-10 PSI). More or less, of course, but I have 1,6 bar (23 PSI) from top to lowest: it cannot turn smooth, of course.

My compression ratio clearly shows a badly rebuilt engine, done by the previous owner not too many miles ago.

ciao




Edit:

ClydeM engine:

1-137 5-138
2-139 6-146
3-135 7-136
4-136 8-138


this one is very old and cannot develop the official declared power (my one has much more than this), but it turns smooth much more than mine

SO: OVER 150 PSI AN WITHOUT BIG DIFFERENCES FROM EACH OTHER IS WHAT YOU SHOULD HAVE TO HAVE A GOOD ENGINE