G-Tech meter

[Dan Llewellyn]

I bought a G-Tech meter and it pretty much works.  You have to be on a
flat, level road for the thing to work correctly, though.  Since it uses
an accelerometer for all of its calculations, it can get thrown off
pretty quickly.  For example, if you are going up a hill, the
accelerometer will think you are going faster than you really are.  You
could even park your car on a hill, and G-Tech will think you are
accelerating continuously.  The device is also only accurate for HP,
Speed and ET the first 15 seconds or so and then the errors start
building up to the point where the info becomes meaningless.  Then you
have to stop the car on a level surface and start all over.  You can
have it continuously measure the G-force and that stays pretty accurate.

I made my own G-Tech type meter also using a 2-axis accelerometer
(available through a few sources for $100 or so) with temperature
compensation and hooked it into an inexpensive A/D converter ($80) that
plugs into a laptop parallel port.  The programming from that point is
pretty simple to calculate HP, Speed, ET and such as well as storing the
data to create your own HP or torque curves.  Still has the problem with
the hills.  To be accurate on the hills (even small ones), you really
need some sort of device that can sense the actual load on the engine.  
I have seen some torque sensors that you fit on the driveshaft but they
are very expensive and tough to install.  Too bad nobody has figured out
how to make an inexpensive stress sensor that can sense the driveshaft
deflection or motor mount compression or something to better calculate
HP on the fly.  Using an accelerometer, torque sensor, RPM sensor, and
speed sensor you could get a whole bunch of data.

There is a device called the On-Board dyno that uses and acclerometer
and speed sensor that works pretty well.  I think the unit runs about
$1600 with a few hundred more for the data logging output capability. 
Has a cool analog gauge on it that shows its horsepower calculations.

Dan