Compression test CSi

[BMW3.0CSi]

Hi all,

I did a compression test today with my CSi. According to the blue books the compression ratio should be 9.5:1. Would be 9:5x14.7=139.65PSI as a reference if I’m correct.

The value I measured today were between 115-125PSI. Prior before testing i let the car run in idle mode from cold start for 10min. After that I took all 6 spark plugs out and tested them one by one, with full throttle open.
Values look pretty consequent for all 6, but are quit a bit of the reference value if I’m correct?
Would like to hear your opinion on this, thx!

 

[Gransin]

Most of the compression testers aren't very reliable when it comes to the numbers, all you want from these is consistent numbers, which you got
I wouldn't worry.

 

[sfdon]

9.5 compression should be around 180

You can see the factory spec for 8.0 in the pic

 

[wilies13]

Hi,
by adding the measurement equipment the volume (especially the compressed) changes / gets bigger.
This is according to:

Avogadro's law - Wikipedia

en.wikipedia.org

I made in the past a rough calc for a Diesel engine and ended with about 20% influence on compression.
So the added volume by the measurement equipment should be as small as possible ....

 

[Dick Steinkamp]

This from the BMW Workshop Manual...

 

[Dick Steinkamp]

Hi,
by adding the measurement equipment the volume (especially the compressed) changes / gets bigger.
This is according to:

Avogadro's law - Wikipedia

en.wikipedia.org

I made in the past a rough calc for a Diesel engine and ended with about 20% influence on compression.
So the added volume by the measurement equipment should be as small as possible ....

Interesting. I never considered that. Attaching the test equipment is the same as adding volume to the combustion chamber. I wonder if the makers of the test equipment take that into account when putting the scale on the gauge?

 

[HB Chris]

The blue book doesn’t show US 8.0:1 though, those motors are 9.0:1.

 

[wilies13]

I started this estimation after the my rebuilded Diesel engine was running on the one side and on the other side the measured compression was to bad.
I was surprised about the influence, too.
Now it´s clear, why the better instruments have short adapters...

The compressed volume of different types of engines varies; size from 1 to 7 liter engines...
...the volume of the adoptable equipment varies... so it can´t be considered easily as a kind of a fixed factor...at least with classic instruments without an microprocessor not really solveaable...

 

[sfdon]

Csi compression pic.....

btw - I have 3 csi’s in the shop.
unless you have a cam in your car (highly unlikely) your numbers are very low.
first try a different gauge or at least a new o ring

 

[HB Chris]

Why does the blue book show 156? (9.0:1)

 

[sfdon]

Use a multiplier of aprox. 15-20 and you get your correct answer.




May 30, 2008 — As in the Wikipedia link in an earlier post, and only as a rule of thumb PSI is generally 15 to 20 times the compression ratio . Every engine will ...

 

[HB Chris]

Multiply by 1.15-1.2 I assume

 

[sfdon]

7 : 1 times 20 equals 140 psi
8 : 1 times 20 equals 160 psi
9 : 1 times 20 ..... and so on

take 10 #s off each and you will get actual gauge reading.

 

[sfdon]

Add a bit for carbon on the pistons!

 

[Dick Steinkamp]

I would think a big variable would be how aggressive the cam is (overlap). In other words, the more high performance the cam, the lower the measured PSI will be on the same otherwise identical engines with the same computed CR.

 

[sfdon]

That compression pic above is the csi on BAT today...

 

[sfdon]

A high duration cam is a mess on a csi- the MPS can’t handle the low idle manifold pressure.
goes into second stage thinking it is at cruise.

 

[Norm!]

7 : 1 times 20 equals 140 psi
8 : 1 times 20 equals 160 psi
9 : 1 times 20 ..... and so on

take 10 #s off each and you will get actual gauge reading.

My stock US 3.0 with 8:1 was measured between 153 and 161 by OCCoupe

 

[sfdon]

Perfect!

 

[Stevehose]

Interesting. About how much drop in compression reading results from going from stock to a 284 for example?

unless you have a cam in your car (highly unlikely) your numbers are very low.