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MG MGB GT V8 Factory Originals Technical - V8 Compression

I have a Leyland P76 V8 in my car (bigger version of the Rover). The other week I had a leakdown test done. Most of the cylinders were OK (6-14% leakage) but two cylinders, numbers 3&4 showed 20&28% leakage respectively. The compression on the cylinders ranged from a high of 170psi to 155psi on cylinders 3&4.

I did a compression test at home with the engine 3/4 warm and found that by squirting some heavy oil down the cylinders bores, I could get the compression on the cylinders (including the 'bad' ones) to rise by about 10psi.

Can I draw any useful conclusions from this?

I'm wondering whether its worth giving the heads a valve job and possibly swapping in a fresh cam.

Any opinions welcome!

Thanks

Simon
Simon Sparrow

Doing a 'wet' test and comparing the results with the 'dry' is useful for seeing if low dry figures could be due to bad bores/rings (large increase from dry to wet) or valves/gasket (not much increase), and you should do it to all cylinders, otherwise you have nothing to compare your suspect ones to.

10psi isn't much, you will probably find that the others will rise a similar amount, in which case it is pointing at valves/gasket. Since it is adjacent cylinders it could be gasket. I know this is a not uncommon problem on the 4-cyl, but don't know how prevalent it is on the Rover V8.

PaulH.
Paul Hunt

Paul,
I thought the cylinder numbering was 'down the crank from the front' so the left bank is 1-3-5-7 and the other bank is 2-4-6-8, making 3 and 4 not adjacent ? correct me if I'm wrong please....
David Smith

Good point. Must be a really devious leak : )
Paul Hunt

Hi,

You are correct, cylinders 3&4 are opposite each other, so no gasket leak scenario there....

I guess the obvious thing to do is to pull the heads off and have a look.

As always I'm wondering if I can get away with a set of reconditioned heads and a cam swap, rather than going for a complete rebuild.

The engine runs well, the plugs are clean and it only leaks a bit of oil :-).

I figure for about NZ$1000 I can re-con the heads and put a new cam in. A rebuild would be about 3 to 4 times as much, because if you're going that far you may as well do a good job!

Simon Sparrow

I would normally regard a sound engine to be one that has a total pressure range of 20psi. Yours is just outside that at 25, which is very close.

The increase of just 10psi for the wet test indicates that the ring seal is good. (I trust that you were able to ensure that the oil for the wet test was able to be directed to the ring areas.) The inference from this is a slightly weeping valve.

If you do have the ehads off then consider having some minor mods included, such as the inclusion of a three angle valve seat for the head and a slight cut back on the backs of the valves to smooth the flow into the engine. Simple things like this and any other minor cleaning up of the ports, especially within an inch of the valve seats will have quite worthwile effects.

There are, or should be, a number of options for the cam choice. Normally I would advise caution on the profile, but with 4.4 litres you have the capacity to absorb a wild profile and still win a torque contest with the 3.5.

Rog
Roger Parker

This thread was discussed between 18/10/1999 and 24/10/1999

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