[200q20v] Re: bar: from .1 to 1.8...is this normal?

[Calvin & Diana Craig]

Actually, the specs can be found at
http://e-www.motorola.com/brdata/PDFDB/SENSORS/PRESSURE/MPX4200.pdf for the
stock sensor, and
http://e-www.motorola.com/brdata/PDFDB/SENSORS/PRESSURE/MPX4200.pdf for the
2.5 bar sensor.  Phil's comment about offset may further help explain a
little bit of a low reading on the low end.  The typical offset for the 2.0
bar is .302 volts with a rnage of .2 to .413 volts.  The typical full scale
output is 4.896.  Since most of the time these parts follow the typical
ratings, the normal "calibration settings" will be done to the typical
values.  So we will just look at the typicals for now.  The typical of the
2.0 bar sensor is .302 volts at 0 psi.  The typical of the 2.5 bar sensor at
0 psi is .204 volts.  The typical full scale readings are identical, 4.896
volts.  This gives a slope on the stock 2.0 bar sensor of 0.23 volts per
tenth of a bar.  Since the 2.5 bar sensor has a slightly lower (0.1 volts
less) of an offset, this would look like a difference of approximately 0.05
bar to the computer, which is possibly enough to make the tenths digit
change one on your display towards the low end.  So the 1.25x factor holds
very well towards the upper end (identical full scale output
characteristics), with maybe a 1 tick too low value on the low end.  This
even lends more credence to it's propensity to be a little lower towards the
low end of the spectrum.  This is assuming that the aftermarket chip
manufacturer's are using these motorola sensors, but I see no reason they
would deviate.  The descriptions even talk about it's use in turbocharged
engines.  Incidentally, this also makes a very easy Homemade boost gauge
with an analog voltmeter and the right amplification....maybe a meter with a
3 volt swing, use the right multiplication factor to yield an output of 1.0
volts = 1.0 bar, or something along that line.  I'm actually thinking about
making a small digital LED display that fits in some of the blank "switch"
spots towards the center of the dash on the later S4's that have no boost
gauge...but that project comes after my fuel injection controller for my '72
Firebird :)
--Calvin