Local M50 engines secretly built for tarbo?

[fandango]

I just did a compression test on my engine:1. 175psi2. 170psi3. 170psi4. 180psi5. 180psi6. 170psithis is from a freshly top-overhauled unit. A search at bimmerforums showed M50B25s getting 195-210. :hmmmm:malaysian units shortchanged? or designed for boost? or designed for poorer quality fuels?

 

[paultantk]

I'm experiencing the same thing. All of us including our mechanic and my tuner are scratching our heads really, wondering why there's such a difference in compression test results as well as the dyno measured power output. Our much much lower. I wonder if using Japanese halfcut M50 engine will be any different?

 

[paultantk]

The used old S52B52 engines on Ebay for sale show compression test results of around 210 each cylinder. They're basically based on iron M50-ish blocks.

 

[paultantk]

BTW, perhaps it is just the American blocks have much higher compression? Try searching the European forums and look at what their M50/M52 block compression test results are.

Here's one:

1. http://www.bmwland.co.uk/talker/viewtopic.php?t=56633

He got about 160+ psi for all cylinders.

2. http://www.bimmerforums.co.uk/forum/f17/1996-e36-328i-compression-test-results-whatcha-fink-t21737/

This guy got around 170+ psi for all cylinders.

3. http://www.bimmerfest.com/forums/showthread.php?t=445547

The thread above says the Bentley manual says "142-156psi minimum with no more than 7 psi difference between cylinders". Interestingly the thread above is on an American forum.

But, as usual this American fella: http://www.unitedbimmer.com/kb-e36-spark-plug-replacement-and-compression-test.php

As usual, Americans always get way over 200 psi!

 

[fandango]

The pressure in a compression test can be calculated using the formula below:

Compression ratio X 14.7 psi

14.7 psi is the nominal atmospheric pressure at sea level and you multiply that times the number of times it's volume is compressed (compression ratio).

Example:

Compression ratio for the M52 engine is 10.2:1

Compression test pressure should nominally be 14.7 x 10.2 = 149.9 psi

For the M50 the compression ratio is 10.5:1 so:

14.7 x 10.5 = 154.4 psi

You'll get some variation readings due to engine tolerances, altitude, etc.

If you were in Denver at 5,000 ft altitude the atmospheric pressure is 12.23 psi so your compression reading would be:

12.23 x 10.5 = 128.4 psi

If you were really high in the mountains at 10,000 ft the pressure would be 10.1 psi so your compression reading would be:

10.1 x 10.5 = 106 psi

Now you know why your performance falls off at higher altitudes..............

(source: bimmerfest.com)

 

[paultantk]

Those calculations definitely sound like what we're getting so how on earth do they get compression test readings of over 200?

200 / 14.7 = 13.6:1 compression ratio?!

 

[climenta]

My last test on my old engine was recording over an average of 6 tests per cyl, all 6 gave me a reading of 13+ BAR but still within the difference within 7PSIs for all 6 cyls.

 

[lozo]

170 - 180 psi on cold engine or hot/warm engine ?

 

[seech]

I just did a compression test on my engine:

1. 175psi
2. 170psi
3. 170psi
4. 180psi
5. 180psi
6. 170psi

this is from a freshly top-overhauled unit.

A search at bimmerforums showed M50B25s getting 195-210. :hmmmm:

malaysian units shortchanged? or designed for boost? or designed for poorer quality fuels?

Yes, yes.... definitely designed for turbo in mind..... :rock:

 

[fandango]

170 - 180 psi on cold engine or hot/warm engine ?

piping hot.

 

[lozo]

maybe the forumer from bimmerforum has included the atmospheric pressure of 14.7 psi = 1 bar. he might used absolute pressure gauge

if you minus 14.7 psi from 195 psi = 180 psi. ok

maybe only

 

[paultantk]

look at this guy's results

http://www.unitedbimmer.com/forums/...rk-plug-replacement-compression-test-diy.html

230-230-230-223-224-232 !!!!

 

[paultantk]

http://forums.bimmerforums.com/forum/showthread.php?t=1559594