m20 cylinder compression test

[CrazyB]

I just did a compression test on my engine as I am trying to decide if to keep the motor or get a used one from bavengine.

I am pulling the engine for some other work so I need to decide soon.
the compression test showed 150-160 on all cylinders bav engine said all their used motors are 180 minimum.

So I was wondering what should a stock motor in good condition show?

[jlevie]

The numbers should be a little bit higher than that. But to determine the cause you need to do a wet compression test (oiled cylinders) and a leak down test. You could have a good block and just need to rebuild the head.

[whodwho]

Those are not far off from the norm, a little on the low side.

For a compression test you are looking for even numbers, a leakdown test would be a better to tell how healthy it is. Compression testers can vary, cranking speeds can effect the results, if you have the throttle open, etc.

Did you have all the plugs out and the throttle open when cranking it over and do you have a strong battery?

[CrazyB]

so if I do the wet test and results are similar does it mean the rings are good and the head is likely the cause but if wet has higher numbers odes it point at the rings?

[jlevie]

Correct. Significantly higher numbers in a wet test point to worn rings, but if the numbers only rise slightly the valves are worn.

[digger]

142-156 is what Bentley says

[felix]

According to the Bosch Automotive Handbook the compression pressure you should see is

CP = (CR-1)**X*ambient pressure

CR is your engine's compression ratio, X is normally in the range of 1.1 to 1.2 and ambient pressure is 14.7 psi at sea level and declining as your altitude increases.

So if this is an "i" engine your static CR is 8.8 and if we assume you're at sea level you should see 140 to 172 psi. An even 150-160 isn't bad and within Bentley's spec as well. Try the wet test and/or a leakdown test before you send Bav Auto any money.

[whodwho]

Your formula looks odd??

Here is another one to play with:

AP=Atmospheric Pressure - 14.7 sea-level
CR=Compression Ratio
VE=Volumetric Efficiency - avg 1.15
CP=Compression Pressure

Cranking Compression Formula:

AP x CR x VE = CP

Using your numbers 14.7 X 8.8 X 1.1/1.2 = 142.296 / 155.232 and is right in range of Digger's Bentley numbers (142-156)

But I wonder how much cams must play a factor on static/dynamic compression numbers I get 200-205(Snap-On compression gauge) cranking PSI which by this formula puts me at like 12:1 :crazy:

I am running a stock 2.5 with a SETA cam and a skim to clean up the head. I was not thrilled when I asked how much they had to take off to clean it up and they didn't remember...I won't be going there again

[SpecM]

^ and why are you running a s-eta cam in an b25?

[whodwho]

^ and why are you running a s-eta cam in an b25?

Goes nicely with the supercharger :)