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MG MGB Technical - Compression readings good or bad?

Hi all,

last weekend I had a go at checking my engine, which is no longer running nicely. It sounds bad (tappets, thats on the todo-list) und runs very rough.
When I checked the compression I got a reading of 13 – 14 – 13 – 14,5 (bar) on cyl. 1 to 4.

I cant find a reference to a numerical compression ratio (like 8,8:1 or so), does this sound good to you or bad?

BTW: its a 1930cc-engine with 83,5mm pistons.

Thanks in advance

GHN3L 42173

It may be a tuned engine if it has been bored out. I make 14 bar over 200 psi, which is high, they do vary more than desirable as well. more than 20 psi between lowest and highest. Tappets can affect this so maybe adjusting via the rule of 8 and try again? Have you checked the spark and balanced the carbs (SUs?)
Stan Best


yes, the engine is a bit "optimised: stage2-head with BIG valves, freeflow manifold, HS6-carbs on ST-manifold, oselli B271 cam.
Tappets are adjusted but may still be wrong because the rockershaft is worn out.

You may have a pushrod that come off the rocker on one cylinder. Your readings: 195, 210, 195, and 217 are quite high compared to a high compression engine that runs at about 165 psi. RAY
rjm RAY

I suspect that Joern-M's pressures may be "absolute" whereas RAY is giving "gauge" pressures (the latter will be 15psi higher).

But perhaps they're not.
Geoff Everitt

Im using a simple gauge like this one in the pic, and the reading were taken, when the gauge didn´t go higher after turning over the engine.


"rule of 8"
Rule of nine!

Same gauge as mine, make sure you are testing with all plugs out and the throttle wedged wide open.

Stated pressure for a factory engine is 160psi which is 11 bar. It's usually said that cylinder pressures should be within 10% highest to lowest, yours are only just outside that so I can't see it being the cause of "very rough" running.

Are you sure you have the plug leads in the right order? *anti*-clockwise? Plugs, leads, car, rotor all good? Carbs balanced for air flow and mixture?

If you know the tappets are out of adjustment, or there are other defects, then you should be correcting those before coming here!
PaulH Solihull

With compression figures like that you should be running on diesel!
Art Pearse

there we are: I had the plugs out but didnt open the throttle. So its off to another test session :-)

And to be honest: I am not seeking any advice about a potential engine problem, I just wanted to see if the figures from the gauge are reasonable, because I didnt find a reference in my manuals.

The engine is booked in for a winter rebuild anyway, I just cant tell what has to be done while its not dismantled!

And the car still revs to 5.400 in fifth gear (I have an LT77 fitted), it only is shaking like hell and sounds like its blowing apart doing this....

If your rocker shaft is worn out, you may not be getting the same valve lift or duration on all cylinders, which would make it run uneven, esp. with that CR. The numbers will probably be higher with the throttle open. Can you read to 0.1 bar?
Art Pearse

Paul you are so picky
I was within 10%, Alistair Darling would like to say that with the PSBR
Stan Best

More seriously possibly the engine is horribly coked up inside. After using octane booster/lead replacement for some years on mine I couldnt get it to tune nicely. When I took the head off it was a mess full of sticky black deposits. Cleaned up retuned and run on the highest octane you can buy when you pitch up at any garage no problem since, Re throttle closed, that should make the reading lower as the engins cant pump air in. Of course the gauge could be out of calibration. Can you compare it with another one? I dont think it can be an exhaust problem, (blocked, air cant get out)you couldnt pull thos revs in 5th if it was.
Stan Best

I'm sure Darling and Brown were getting 8 and 9 (billion) mixed up, but it'll make a jolly sight bigger difference doing it with valves. It's why I tend to describe it as "when the front valve goes down adjust the back valve, when the 3rd from the back goes down adjust the third from the front and so on". Helps the arithmetically challenged ;o)

What octane booster/lead replacement are/were you using? I've been using Castrol standard ever since leaded went with no cause for concern. I did try the boosted version once but it didn't make any difference to pinking over the basic when using standard unleaded so like you say I use the highest I can get preferably Shell.
PaulH Solihull

This thread was discussed between 23/09/2010 and 25/09/2010

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