RE: E30 M3 Compression Ratio

["Ron Buchalski" <rbuchals@xxxxxxxxxxx>]

Jose,

What you wanted to learn was your compression "ratio".  What your 
mechanic provided to you was the compression in each cylinder.  There is 
a big difference between the two measurements.  A compression ratio has 
no "units", but it's a ratio of the largest to the smallest volume that 
the combustion chamber can have.

The compression is a pressure measurement, and the units are pounds per 
square inch (psi) in the USA, or bar (in your part of the world).  It's 
a measure of how much pressure is developed in each cylinder during the 
compression stroke.

It's been awhile since I had to think about converting measurements like 
this (Physics class, 1980), but I believe that:

1 BAR = 1 atmosphere (1 atmosphere = 14.70000 lb/sq in)

So, for your car,

13.75 atmospheres = 202.12500 lb/sq in
13.25 atmospheres = 194.77500 lb/sq in
13.50 atmospheres = 198.45000 lb/sq in

(I'm no f**king genius.  These conversions provided by:
http://microimg.com/science/pressure_equiv.html)

That's good!  But it doesn't mean that your compression ratio is 
13.xx:1.

To determine the compression ratios, a very accurate measurement of each 
combustion chamber needs to take place.  I believe that this is done by 
using a liquid to fill the cylinder when the piston is at it's maximum 
downstroke, and again when it's at it's maximum top stroke.  The ratio 
between these two combustion chamber volume measurements will determine 
the _true_ compression ratio of each cylinder in your engine.  (If, for 
example, at the top of the compression stroke, the combustion chamber 
volume is 10.2 times smaller than the volume at maximum downstroke, the 
compression ratio is 10.2:1 for that cylinder).

Or, you can look it up in the owner's manual and be done with it! ;-)

- -rb
>------------------------------
>
>Date: Mon, 10 Aug 1998 22:05:28 +0100
>From: "Jose Santana" <mpower@domain.elided>
>Subject: E30 M3 Compression Ratio
>
>Hello,
>a few days ago, I went to the BMW Service for measuring the
>compression ratio on my m3  (just curiosity, I don't have any
>power problem). When the mechanic finished, he showed me
>a ticket with the following information:
>   Cylinder  1: 13.75  
>   Cylinder 2:  13.25 
>   Cylinder  3:  13.50 
>   Cylinder  4:   13.75 
>   Compression value in bar. 
>   MOTOMETER 
>As far as I know, the compression ratio for the E30 M3 (not evo)
>is 10.5 : 1 and that is what I told him, but the mechanic  had no idea
>of why that difference, he only said it was perfect because the values
>were quite close.  I assume that the machine was calibrated, so does 
>anybody know if those values are normal? Do I have to divide by any 
>number those values to obtain the compression ratio? Or should I 
>say that it's 13.x : 1 ? Very confused...
>
>Thanks in advance for any help
>Jose Santana <mpower@domain.elided>
>'83     316
>'88  ///M3
>
>------------------------------


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