*Sorry if this is already posted somewhere but I didn't find it with search*

The following is cut and pasted from a post on another forum. I have done this to the SGIIs in my VW and CRD. I used the mean yearly atmospheric pressure for my county. In the VW, the SGII boost now reads within 0.1psi of my boost gauge. In the CRD the SGII boost seems accurate because it runs ~15psi below MAP. So I think it works. Here is the post with coding:

With help from CarlD and 2way on the cleanmpg forums I now have an XGauge that comes close to showing the gauge pressure of the MAP (turbo boost) instead of the absolute pressure (atmosphere plus boost). The coding is as follows:

TXD: 68 6A F1 01 0B
RXF: 04 41 85 0B 00 00
RXD: 28 08
MTH: 00 91 00 64 FF 71

This will get the absolute MAP pressure, convert it to PSI, and subtract a constant, which in my case is the average atmospheric pressure in Brandon. Subtracting the average atmospheric pressure gives a close idea of how much boost the turbo is making. If a drive has large changes in elevation or pressure the gauge can be out by a few PSI.

The last four characters in the MTH field are an addition. To subtract, you must add the 2's complement of the number you wish to subtract. In this case, the result is being divided by 10, so the number being subtracted must also be multiplied by 10. For example:
I want to subtract 14.3 PSI. 14.3 * 10 = 143 = 008F in hex. 2's complement of 008F is FF71. The last 4 characters in the MTH field are therefore FF71 in my case.

Of course, pressure varies with elevation so you may need to subtract a different value. Here are the 2's complement representations of common values for pressure:
12.0 -- FF88
12.1 -- FF87
12.2 -- FF86
12.3 -- FF85
12.4 -- FF84
12.5 -- FF83
12.6 -- FF82
12.7 -- FF81
12.8 -- FF80
12.9 -- FF7F
13.0 -- FF7E
13.1 -- FF7D
13.2 -- FF7C
13.3 -- FF7B
13.4 -- FF7A
13.5 -- FF79
13.6 -- FF78
13.7 -- FF77
13.8 -- FF76
13.9 -- FF75
14.0 -- FF74
14.1 -- FF73
14.2 -- FF72
14.3 -- FF71
14.4 -- FF70
14.5 -- FF6F
14.6 -- FF6E
14.7 -- FF6D
14.8 -- FF6C
14.9 -- FF6B
15.0 -- FF6A

source: http://forums.tdiclub.com/showpost.php? ... ostcount=1

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2005 CRD rolled August 2009

Thanks for posting this.

I guess that I have always been happy with absolute pressure displayed. I do a lot of elevation change, so I would rather have the actual pressure displayed rather than a erroneous, biased value.

Plus... the MAP gives me local atmospheric pressure when the engine is at idle ! and this could be used as an altimeter!

hey... we can write Xgauge code to display the altitude (only correct at idle)... ya, right.

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06 CRD LTD - Suncoast TC- Shift Kit - Spicer UJ - FRKNLIFT - F37 - Magnaflow - 22.0 City - 24@65MPH - Fumoto F-102 - AUX T Cooler - Tank Lift Pump