M50 compression test

[mashtun]

Hi all. I thought I'd test the compression on the M50TUB25 engine I'm fitting to my Cabrio before starting to strip the donor - on the basis it might indicate any problems that could be investigated while the engine's on a stand. The numbers are consistent but seem ridiculously high. To the nearest 5psi:

#1 : 225
#2 : 225
#3 : 230
#4 : 220
#5 : 225
#6 : 220

Max values were reached after about 6 strokes in each case. Bentley gives a minimum value range of 142-156psi.

Does this just indicate the tester I bought is poorly calibrated (or crap...)? If so then I suppose the fact the figures don't vary too much is the main thing?

I noticed a little oil had run down into the plug recesses on nos. 3, 4 & 5 cylinders - not enough to form a pool, but certainly enough to need wiping off the bottom of the plug socket. Could this be anything more serious than the cylinder head cover gasket needing replacing?

Mark.

[Patrick]

Looks fine, it's the comparison between the 6 that's more important, if you have one way lower than the others it would be an indication of a problem. Numbers seem familiar to the one on my M20 although its been a while now

Does the M50 have those round rubber gaskets around the bolts that hold the rocker on? If so those can be a source if the rocker gasket look's OK. I'd replace the gasket anyway while you're working on it

[mashtun]

Thanks for the reassurance, Patrick. Looks like the M50 gasket's in three parts:



I'll take a closer look when the engine's out but I guess there's poor sealing around the tubes for cylinders 3, 4 & 5. I also just read on wikipedia (yeah, yeah...) that cylinder pressure in psi can be roughly estimated at between 15 and 20 times compression ratio. 20 x 10.5 = 210 so my figures seem reasonable after all.