What compression to expect?

[Axolotl]

I've just tried out my shiny new compression tester, and got the follwowing results. Should I be worried?

1: 165 psi
2: 135 psi
3: 142 psi
4: 155 psi

There are no symptoms, apart from the jangly noise I posted about elsewhere. Just interested in other people's reaction to those results. What would you do?

[Packedup]

2 is rather low, 3 isn't great, and 4 is just about OK compared to 1.

Have you done the wet test (drop of oil down teh plug holes to seal leaky rigns)? If so, what're the results? Unless this was a wet test, in which case how does it compare to dry?

[Axolotl]

It was a dry test.

How big a drop of oil for the wet test?

I'll try that on Monday.

[bmcecosse]

Not good, but not desperate either - would point to needing the head off and the valves all ground in. A good blast with an oil can - several squirts in fact - down each spark plug hole. Then try again.

[Axolotl]

O.K.,

These were the original "dry" tests:

1: 165 psi
2: 135 psi
3: 142 psi
4: 155 psi

I did the wet test this morning and got the following:

1: 181 psi (+16 )
2: 153 psi (+18 )
3: 172 psi (+30 )
4: 191 psi (+36 )

I think I got a bit over-enthusiastic with the oil on number 4 . Which could account for the big difference there.

In combination with the earlier dry tests., this suggests to me slightly worn bores throughout, possibly worse on nos. 3 and 4 with leaky valves on nos. 2 and 3. Do you agree?

I notice the book gives 160 psi as the normal compression figure, so numbers 1 and 4 were basically sound on the first dry test.

I'm not in a rush to get the head off, unless you think a little effort now will save a bigger job in the long run?

[bmcecosse]

Stitch in time - and all that. Yes - it does look like 2 and 3 could do with some valve attention. the longer you leave it - the worse it will get. Why not source a spare head - get it ready, and then the swap is very easy and quick.

[Axolotl]

Spare head. Good plan, I hadn't thought of that, that would take the time pressure off having an undriveable car while I worked on it.

thanks.

[RogerRust]

I wouldn't worry about the exact figures. I would doubt the calibration on any domestic equipment. However the important thing is not the absolute value but how the pots compare.
My comp tester is an old cheap Gunson and never reads better that 140 even a really good engine, but I ignore the numbers and just look at how they compare. Another thing to try is taking the readings on seperate days the ratios will still be the same but often the actual readings are different!

I haven't bought a new one because I know I'd worry about whether my old 130psi was better than my new 165psi!!

[bmcecosse]

Exactly - the actual readings are not all that important - but the variation from cylinder to cylinder is the sign all is not as it should be.

[AndrewSkinner]

i have just run a dry test on my 59 morris 1000 (998cc) - got these results

1 -130psi
2 - 135psi
3 - 135psi
4 - 135psi

apparently the book says 160psi - shud i be worried?

[paulk]

I'd say not.
As previously mentioned most guages can be a little adrift.

Two posibilities spring to mind here

1) your comps are very good and even and your guage is reading a little low.

or
2) Your engine has worn exactly evenly across 4 seperate cylinders and your comp guage is accuratly reading the presures.

I know what my moneys on.


More to the point how is the engine running?
Is oil consumption high? Is the exhaust smoky? does it puff smoke on the over run?

If the engine is basically running fine I'd leave it untill something suggests something is up, then start comp testing and worrying about the results.

[bmcecosse]

Try again - make sure engine is warm, battery fully charged, all spark plugs out - and throttle held open during the test.