Warning, long post ahead
If you just want updated proper scalings to plug and play, skip to the tl;dr at the end!
First, I would not use the sensor scaling to "limit" boost (you should use the other appropriate tables for this). You should use the sensor scaling to reflect the max that your given sensor can have, scaled to the max that your rom version / trans type can allow.
EDIT: For AT, 6MT, and DCT can set boost ceiling to 1.5 bar without issue now.
The initial N20 scaling (which was very slightly off since it was based on the numbers Cobb published for the N20 sensor, and not the sensor spec itself) was decided upon to report stock up to 10psi, and then scale from there up to 35psi as it gave a nice scaled boost of (X - 24.7)/2 + 24.7. 24.7 here is 10psi absolute, at sea level. If trying a scaling for yourself you should also use absolute numbers at sea level. 10psi was chosen as the middle breakpoint as it gave the nicer scaling equation, but you can go a few psi lower (I wouldn't go any lower than say 5-6psi), and certainly higher as well but then you lose scaled resolution.
Now comes setting the first two cells, and math for the third. You want to pick your first cell to be generally what the sensor spec publishes as it's min. For N20 it is 0.5V @ 7.30809 psi. For the common 4 bar, it is 0.4V @ 7.25189V. Do not worry about if this will set you under the voltage plausibility check for your altitude, as that should be disabled if need be the proper way with it's own byte. You DO NOT want to have scaled values in the first two cells, as it can effect idle, cruise, etc and hurt drivability.
For the second cell, you want to pick the psi you want to report stock up until, and add 14.7 to it. So we chose 10psi, which is then 24.7. You compute the voltage your sensor will have at this psi using y = mx + b. You can figure out variable M and B by using the equation for two points on a line from the sensor spec. For the N20 sensor, this is 2.15876V and for the 4 bar it is 1.861V.
Finally for the third cell. Setting the absolute psi in this cell is easier (and will never change!), so we'll start there. This value should be the max the DME will allow, in absolute psi. So this will be 2.56 bar absolute (hex max supported by DME, also seen in some places as 2559hPa). This comes out to 37.1x psi. However, to build a tiny margin in, we set it to 37.2psi. This absolute psi value will be the same no matter what sensor you are using, N20, 4bar, 5bar, etc.
Lastly, we need to figure out the voltage to go with our last psi values from above, 37.2. If you don't care about having a nice scaling divisor like mentioned above in our (X - 24.7)/2 + 24.7 example (the scaling divisor here is "2"), you can simply enter the upper voltage from the sensor data sheet. However, that can make rescaling all your load axis trickier and does not give any headroom at the top end of the sensor before it can start reading non-linearly. For the N20 sensor however and our scaling divisor of "2", here's how we picked that:
Stock sensor max: 22.5psi, scaling start: 10psi; N20 sensor max (that we want to scale to): 35psi, scaling start: 10psi;
stock: 22.5 - 10 = 12.5psi; N20: 35 - 10 = 25psi; 25 / 12.5 =
2;
Now to find your third cell voltage based on this, I setup two columns in excel and found first the real psi of the N20 sensor would give 37.2 scaled, using our formula of (X - 24.7) / DIVISOR + 24.7, here X is the real psi of the sensor, and 2 is our divisor. In this case it turns out to be 49.7psi gives a scaled 37.2. Now using y = mx + b again, we can get the voltage at 49.7psi, which is 4.543146V. And that would be our last voltage breakpoint!
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tl;dr:
Proper N20 scaling for all cars:
0.5V @ 7.30809 psi
2.15876V @ 24.7psi
4.543146V @ 37.2psi
Hope this helps. Again, follow the same process for other sensors like 4bar / 5bar etc. I will work on getting the ambient pressure to voltage check toggle added to the XDFs so you high altitude folks don't run into that issue when scaling a larger sensor.
JY - edited on 5/9/22 to remove separate DCT scaling which is no longer needed
