compression ???

[GARY F]

My shop manuals are short on info. 37 Olds straight 8.. What should I see when I do a compression test. If you can tell me what it was supposed to be when new I can judge from there. Thanks  Gary

[hwellens]

This my help

 

[Bloo]

I would caution you against going for a specific number. Buick published numbers at least for some cars, and it causes a lot of confusion about what a compression test is and what it isn't.

 

What it is: A pass/fail test for the tunability of the engine. Condition of the compression rings (but not the oil rings) can also be estimated by doing a second compression test with a little oil.

 

What it isnt: A quest to find a specific number. If you try to do that you are going to have a bad time, for a whole bunch of reasons. If the engine fails the test, you follow with a leakdown test. The leakdown test is the REAL test that shows you where compression is leaking, and some very strong clues what is wrong.

 

The absolute number almost never matters and wont match the book anyway. What you're looking for is variation.

 

As for the absolute number, I could only offer the following hints on what to expect:

 

When I was about 12 and first learning about engines, and trying to rebuild single cylinder lawnmower engines for my go-cart,  a local lawnmower repairman taught me some things. One of those things was simply "If it is lower than 50 pounds, it won't run". Keep in mind for now that those engines were NORMALLY quite a bit more than 50. Maybe more like 80.

 

Some really old cars (1920s back) run down close to 50 pounds normally maybe even below due to extremely low compression ratios by design. In that case the rule wouldn't apply.

 

Typical 1960s 2-barrel regular gas V8s run about 130 pounds, 70s smoggers less maybe 110. In high-compression high-performance engines the sky is the limit. Over 200 pounds wouldn't be impossible.

 

So, on something like a 1937 model, I would expect it to be in-between a 20s car and a 70s smogger. Maybe 80-110 pounds.

 

I am an old driveability tech, and have never yet seen two guys get the same numbers from the same car. Gauges vary, and methods vary. Some guys open the throttle and some don't. Some take the second puff and some take the last. I do both. Some guys always do a wet test. I often don't bother unless I am looking for something specific. Cranking speed makes a difference. The battery probably gets weaker as you go along. Typical cars usually have variations in the combustion chamber volume. The list goes on. What will be the same among different mechanics is the variation, and the variation is what matters.

 

25% is typical variation seen on engines in regular use with higher mileage that run just fine. It should be less but it often isn't. Some shop manuals say less. Some say a lot less. Some guy posted the other day that his manual said 5%. 5% is probably a pipe dream. I'll bet the car wouldn't do that brand new. It didn't matter because he had a huge variation, and something wrong.

 

If you find one (or more) cylinders that are way off the others, you have found a problem, and that problem is most likely burned valve(s). Probably exhaust valves.

 

Example1: 90 - 95 - 105 - 110 - 105 - 115 - 90 - 90 - 100  >> nothing wrong, tune it, drive it, check the oil consumption after you put some miles on. Do a wet compression test if you want to estimate the condition of the rings. Don't put too much stock in it because the oil rings could still be stuck or shot.

 

Example2: 100 - 60 - 35 - 100 - 120 - 105 - 105 - 115  >>  #2 and #3 are bad. 90% chance these are burned exhaust valves. Do a leakdown test to find where the compression is escaping to. It could be broken or holed pistons, or a head gasket, but probably isn't. DO the leakdown test to confirm. This engine will jump around, stink, and just generally run like hell. No amount of tuning will help.

 

Example3: 65 - 65 - 75 - 80 - 70 - 65 - 65 - 75   >> Lower than expected but even. This COULD indicate jumped cam timing. If that were the case it would be difficult if not impossible to get it to run well enough to even drive. If the engine runs relatively ok, it could be that the compression rings are worn out. A wet compression test would be a clue. The most likely thing is it's just the method, or the calibration of the gauge, and nothing at all is wrong.

 

One last thing, since we are talking about antique cars. If the engine has been sitting around take the results of ANY of these tests with a huge grain of salt. When an engine sits, probably at least 2 valves are up off of their seats, getting rusty, etc. The rings may stick. The cylinderwalls may rust a little. Thats why engines long dormant start to run better after some driving. It could appear you have a serious problem when you don't. Drive it and see.

 

 

Edited November 3, 2019 by Bloo (see edit history)

[trini]

According to my Operation manual there is mention of 4 Cylinder 1928 D B the compression is stated as 75 lbs. I also read an article in the AACA antique magazine , I think by Pat Forster,  not quite sure, that all the pistons came in a bin  for a particular model engine and the pistons were of different sizes by a few thou. The assemblers did not care because  the assembly line had to keep moving. The compression from the factory  most likely varied from cylinder to cylinder

[chistech]

A newly rebuilt engine should be around 80’s and a good running engine can still be i the 60’s. 

[trini]

chisteck, you are on the ball.

Cheers.

[PFitz]

12 hours ago, trini said:

According to my Operation manual there is mention of 4 Cylinder 1928 D B the compression is stated as 75 lbs. I also read an article in the AACA antique magazine , I think by Pat Forster,  not quite sure, that all the pistons came in a bin  for a particular model engine and the pistons were of different sizes by a few thou. The assemblers did not care because  the assembly line had to keep moving. The compression from the factory  most likely varied from cylinder to cylinder

 Cast combustion chamber volumes can vary a bit, too.

 

And if there was a valve job done in the past, how much variation each valve face and seat gets ground can vary the combustion chamber volume and therefore  the compression from cylinder to cylinder.  One of the items for blue printing race engines is to insure combustion chamber volumes  match as closely as possible by physically measuring each's volume and making all the same.  

 

Paul 

[JACK M]

21 hours ago, hwellens said:

This my help

 

 

This is interesting. Obviously 37 and newer are not looking at cranking compression  (the Cr foot note). I suspect pulling one plug at a time and running faster than idle would be a hassle.

I've never seen this before, but who reads the book? LOL I always look for the cranking numbers to be similar.

In the boat business they taught us that the lowest should be within 15% of the highest.

Two stroke rope pulls depend on how fast you can pull the rope.

[GARY F]

I want to thank you all for the long replies and info. I should get to doing the test wed. or thur.