E30 Zone

Hi all, i think my 2.5 may be not quite 100% so i'm going to be doing a compression test soon. so does anyone have any results that i could be aiming for? it's a later lower compression variant by the way.
cheers
Kev

the actual result will vary from engine to engine and by test procedure, but what you should be looking for is an even number within 5% across all cylinders.

ok cool thanks.
Kev

what sort of ballpark numbers for a late/low comp M20?

did mine a year and a bit ago after the car had been parked up for like 2 days without being started/run and was getting in the region of 125psi or so across all 6 pots if memmory serves.

been meaning to check it again for fun and giggles so it would be nice to have an idea of what numbers a healthy low comp M20 should be seeing.

Assuming 1bar barometric pressure, then a healthy 8.8:1 CR engine should reach a compression pressure of 8.8bar or 128psi. This assumes that you have a compression tester (like mine) whose guage reads from 1bar up. In other words, it is crudely attempting to measure "absolute" cylinder pressure (under the assumption that zero compression = 1bar absolute pressure). In reality, however, as your barometric pressure falls (dependent upon weather and/or altitude) so will your measured compression pressure.

As HairyScreech said above, it's consistency in the values that is important. You can put any minor deviation from the above value down to variation in barometric pressure and inaccuracy in the guage. Consistently bad values would likely indicate a consistently buggered engine

if your struggling to get even values when turning it on the starter then either disconnect the icv or jack the throttle open a little.

also you will only see the full ~128psi with the throttle wide open.

i got 145 psi across all 6 cylinders when i tested it afterwards. mine is a low comp m20 b25 with the throttle held wide open.
cheers
Kev

These are the results of tests done on my old facelift 325

Cylinder)

1) 145 150
2) 150 150
3) 140 150
4) 130 130 130
5) 145 145
6) 150 150

Did two tests to check accuracy and a third test on cylinder number four. Car ran very well.

A leakdown test will tell you more if compression is off. I would post up why you think it's not 100% at the moment and see if there are any pointers you may get.

Make sure the battery is fully charged and hold the throttle fully open while testing. I also pulled the fuse (fuse 11) for the fuel pump to avoid bore wash while testing.