Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Compression Testing

I just came across a little piece on the internet - -
which says that the throttle should be held open whilst cranking to measure individual cylinder compressions.
Questions - as I have always just done the test with the throttle left alone, why the need to open it? Will it make any difference?
- also, what is "normal" for compression on a standard road engine, both pushrod and twin cam?
Grateful for any info on this.
Bruce Mayo

I assume the theory is that if the butterfly was closed you would be creating a vacuum and have no air to compress. With leaks around the butterfly at idle position I would think there is plenty of air at this speed to compress.
Mike Ellsmore

Just looked at the instructions on my compression tester and it says the throttle should be open. I have, of course, never read the instructions before.

There is a short list of typical readings. Its a bit modern for us and the nearest I can get is an 850cc Mini with 8.3:1 compression which is given as 150psi and the Marina 1.8 with 9.0:1 compression ratio and that is 180 psi.

Malcolm Asquith

You should always have the throttle wide open when you are checking the compression, if the throttle is closed the engine cant really draw enough air/fuel mixture into the combustion chambers to give an accurate reading.

A friend of mine ( a mechanic with many years experience) was replacing a burnt valve in a 2.00 litre MG Montego a few years ago and after refitting the head, when he checked the compressions, he found that they were all lower than they should be. He recommended that the engine should either be re-bored with new pistons or at least have new rings fitted.
The owner agreed to a complete rebore, new pistons and replacing the big end and main bearings etc.

When the compression was checked after the rebuild he was amazed to find almost no improvement!

We were talking about this in the pub later and I asked him if he had checked the compressions with the throttles open or closed. He looked at me as if a light had been switched on and admitted that he hadnt thought about the throttle being open or closed!

Next day he called me to say that with the throttles open the compressions were all fine and that all he now had to worry about was how to broach the delicate subject of an un necessary engine rebuild to the cars owner!


Colyn Firth

It would be interesting for someone to do the test - check compression on their A with throttle in normal idle position and fully open and report results.
Mike Ellsmore

But not interesting enough to go do it?
It's even better on a car with carbs out, such that it has one carb completely closed and the other open far enough to keep it idling - can sell a head gasket every time.
Published numbers are pretty useless, since battery condition, cable connections, oil viscosity, and a bunch of other stuff can make it vary. I've had cars that seemed fine show something like 170, 160, 150, 140. After thinking about it a while you get reversed and somewhat lower figures by taking the measurements backwards. That's a bad battery with slow recovery.

Fletcher R Millmore

Thanks everyone - food for thought! I shall test it properly shortly and let you all know how it looks.
Bruce Mayo

This thread was discussed between 09/05/2010 and 10/05/2010

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.