SGII Horsepower gauge & related gauges
 

I followed the instructions in the manual for creating a Horsepower gauge in my 1996 Subaru Legacy. The output of said gauge is about 30% higher than what would be plausible. It was showing something like 30BHP cruising around at 49mph. At redline and 80% throttle, it was showing 165BHP, while my engine is only rated for 135.

So, how does the SG calculate HP? It's obviously doing it wrong. Can I get it to multiply by a constant calibration factor?

Also, if a working HP gauge could be made, you'd probably want a few related gauges:
BSFC gauge = BHP/GPH, showing how many milligrams of fuel per second it takes you to make a horsepower.
Torque gauge = BHP/RPM, because that would be cool.

Or does anyone at least know how (or whether) I can take Gauge X and divide by Gauge Y and display the output as Gauge Z?

Check the coding manual ScanGaugeII - Add-A-Gauge

Ah, so I see the answer is no, that can't be done.

The MTH field doesn't let you multiply or divide by a gauge or a data stream, just by a constant. Although the stock mpg and cpm gauges involve division of one data stream by another, I don't see any way to add such gauges to the SGII, so I suppose the BSFC gauge is never going to happen. :-(

I see the stock Horsepower gauge has a multiplication by 4.35 built in. I could correct this value to make it accurate at at least one point on the torque vs. RPM curve, but I don't know what method the SG uses to calculate HP, and I'm skeptical about its accuracy.