Compressions for 1098 engine

[Bawdy2]

Advice sought please from you knowledgeable lot. I have a 1968 minor traveller and the engine is an unknown history but is the original and I think unmodified. I checked the compressions for the 4 cylinders in a dry state, cold engine all plugs removed, and got very consistent readings of 107.5-108 psi. Is that low or fairly average? The car seems to run ok and pull ok.
Any guidance would be most welcome. Thank you.

[Bawdy2]

Sorry guys I’m new to this and posted in the wrong area. I think the technical section would have been more appropriate. Also since finding that section there are a few posts which include compression, soI’m now fairly convinced mine are very low so investigations will be necessary.

[svenedin]

It is advisable to do this test with a warm engine, all the spark plugs removed and the throttle must be wide open. In order to prevent sparks disconnect the power to the ignition coil.

Around 150 psi or above for a 1098cc HC (high compression) engine.

Stephen

[Bill_qaz]

It would be unusual to have an engine issue with all 4 cylinders so consistent, usually within 10% variance is acceptable.
Did you have the throttle fully open?
Did you turn over equal number of compression strokes per cylinder or until gauge stops rising?
Is you gauge accurate and good fit?
Is it a standard motor?
Repeat test with a few drops of oil put in the cylinder, does it give a significant increase?
What is the reason for doing the test? Running issue, oil consumption, lack of power or just curiosity?

[Bawdy2]

Hi guys and thanks for your posts. I have repeated the compression tests on the engine when fully warmed up. First just test all cylinders and still they come out consistently low around 110. I then placed a squirt of 20/50 and tested again with only a very slight change of about 10 psi so consistently 120. I’m thinking valves deterioration due to not being modified with hardened valve seats in the past. The engine number plate is showing it to be a high compression.
Any thoughts guys?

[svenedin]

I'm not an engine expert or expert on anything really but here's my tuppence....

There is more than one reason why an engine might lack compression. A "leak down test" might help to determine what the problem is. In this test the cylinder is filled with compressed air and the pressure gauge is then monitored to determine how fast compression is lost and also where the compression is lost. For instance, air could be heard leaking into the exhaust if an exhaust valve is leaking. However, your problem is not isolated to one cylinder.

A general loss of compression could be due to a failing head gasket but there are usually other symptoms as well (e.g oil in the cooling water, water in the oil or either water or oil leaking at the union of head and block). Also the head gasket usually fails between 2 cylinders but all of your cylinder compression reading are low.

If there is excessive wear in the block you would expect symptoms of crankcase compression. With this, gasses from the combustion leak past the piston rings and cause pressure in the bottom of the engine which overwhelms the engine breathing system causing oil leaks from all the usual places and sometimes in extreme cases from less usual places too. There is always some blow-by even in a healthy engine.

So I would suggest you try to narrow down where the problem lies and go from there. Of course, there could be problems in multiple areas at once or an engine that is just generally worn.

Have you checked and set your valve clearances?

Stephen

[liammonty]

I'd be careful before you start stripping the engine for a problem that likely doesn't exist, as you said the car runs and pulls OK. There's not a great difference between 'wet' and 'dry' readings, suggesting the rings aren't bad, and as someone else pointed out, the readings are extremely consistent between cylinders, which is a good indication, i.e. there are no outliers. Plus, there's quite a chance that different compression testers would give different values, as they are unlikely to be accurately calibrated.

If you're really keen to investigate further, I would suggest a leakdown test (as already suggested above) to give an indication of valve sealing etc., but if it was my car and was running well, I would enjoy it and leave well alone if running well!

[Bill_qaz]

As I said previously have you tested your compression tester before doing any engine work? Or borrow another and repeat test
In 50 years plus of working on engines, with such little variance there's nothing wrong.
I've never seen a defect or wear that's identical on all 4 cylinders. Some manufacturers don't give a compression figure just a maximum variance.The only time I've come across it is deliberate by using thicker head gasket or even two to lower the compression usually to allow use of poor quality fuel overseas.
Unless you want to strip your motor just for fun

[svenedin]

As I said previously have you tested your compression tester before doing any engine work? Or borrow another and repeat test
In 50 years plus of working on engines, with such little variance there's nothing wrong.
I've never seen a defect or wear that's identical on all 4 cylinders. Some manufacturers don't give a compression figure just a maximum variance.The only time I've come across it is deliberate by using thicker head gasket or even two to lower the compression usually to allow use of poor quality fuel overseas.
Unless you want to strip your motor just for fun

Yes the compression readings are so consistent across all 4 cylinders. It is hard to see a problem

[Bawdy2]

As I said previously have you tested your compression tester before doing any engine work? Or borrow another and repeat test
In 50 years plus of working on engines, with such little variance there's nothing wrong.
I've never seen a defect or wear that's identical on all 4 cylinders. Some manufacturers don't give a compression figure just a maximum variance.The only time I've come across it is deliberate by using thicker head gasket or even two to lower the compression usually to allow use of poor quality fuel overseas.
Unless you want to strip your motor just for fun

Thanks for all your advice. I am now minded to leave it be unless something more substantial occurs! The reason I did the tests was because of curiosity because the engine was largely an unknown quantity with recent 15 -20 years of history only. Most of its 100,000 ish miles were done before this. It’s a bit clattery even having checked the valve clearances, so I’ll double check them again. I guess I need to listen to a few more minors to compare.

[svenedin]

Clatery is the timing chain. That’s A-series through and through