Compression test

[gilesdive]

Hi folks
Just want to pick your brains for a minute. On a 2500 engine (mine is a PI unit in a TR6, CR,  125 bhp version) roughly what should I expect if I put a compression tester on it?

Just cranking it over, I am getting between 70 and 90 psi. There is a bit of a spread of readings there, but do you think they are generally too low? I was advised by one chap they should be well into 3 figures - is this so? If that's the case, head off.

Thanks

[Nick Jones]

Sounds a bit low to me - I would have thought 140 - 180 ish/  Ideally to be equal within 10psi.

Are you doing this with throttle open or closed?  This can make a difference - should be open.

Putting around a table-spoonful of oil into each bore will temporarily improve the ring seal and help determine whether this is a valve or bore issue.

If the engine is an ex-US detox thing they have very low compression ratio so your readings may not be too far off what they should be.

Nick

[gilesdive]

Ah, did it with throttle closed.
It is not an ex-USA car, it is to UK spec.
Fairly sure it's not piston rings/bores as harly any smoke when it was last running.

Bad news is I have taken back compression tester (only borrowed it) - but not sounding good is it? :'(

Thanks for the quick answer though. :)

[Deleted User]

post removed on request of author

[gilesdive]

Thanks for those details. I have had to shell out for a guage as the one I borrowed is no longer available - will be able to check it with throttles open next weekend, at least I know what it should be.

[Richard B]

Good luck, also do a dry test followed by a wet test (add a drop of engine oil). I there is a significant improvement suspect Piston Rings. 

[Freebird]

Before I rebuilt my 2.5, the compression on number 1 was 7 bar - about 105 PSI with the throttle wide open, all the others were about 140 - 150 PSI. It had a CR of 8.5-1

Both top piston rings on No1 were stuck in their grooves, breaking up on pushing them past the big step in the bore (or broken already). A 20 thou rebore just about cleaned up the wear which was much the same in all bores - that's how bad it was. It didn,t actually smoke much even when cold, the oil control rings were all intact.

As Dave says 70-90 really doesn't sound right.

Glen.

[JohnD]

Compression testers are NOT precision instruments!
So the absolute pressure shown by each tester may not compare, other than 'ball-park' .
Far more important than absolute pressure is that all cylinders should be the same.
Suspect any that aren't.
If the oil test doesn't improve pressure, then suspect the valves.

John

[gilesdive]

Been a slight delay in getting this done (illness / work) but finally did compression check with throttles fully open - yes, a big difference.
Lowest one was 165 psi and the highest about 175-180, that was without any oil in the bores.
Good news, this suggests valves and piston rings ok, don't need to pull the head off.
All I need now is to actually get it going!

[Nick Jones]

That sounds pretty good, especially if done cold.  Normally improve a bit when warm.

Nick