Computing mileage on an EFI engine is as simple as totalizing the
injector ON time, totalizing distance traveled, multiply in a
conversion factor and display. Hardware needed is a Parallax BASIC
Stamp (about $50) and an LCD display board (about $90.) We use the
PWM input across an injector to grab the pulse widths. We use a bit
line to read the vehicle's VSS to get distance. We write a little
BASIC code to wrap it all together and viola! Mileage/trip meter.
There are several methods of calibrating the fuel injector flow.
The most accurate is to build my fuel injector flow bench and
actually calibrate the injectors.
(**broken link removed**)
The next best way to do this is to hook up a pulse generator (or
even better a PC using my FIT software at the above URL) to the
injector, fire off the fuel pump, discharge 20,000 pulses or so and
catch the fuel in a graduated cylinder. The pulse width should be in
the 2 ms range. This must be done instead of just yanking the
injector open to measure the flow to account for the opening and
closing delays in the injector.
The crudest but still satisfactory method is to hook the Stamp up
with some code that will display the raw injector and VSS totals,
put an accurate amount of gas in the tank, drive until it's used up
(or fill the tank, drive, fill it again and record the amount), then
compute the volume per injector pulse by dividing the fuel used by
the total number of injector firings. -John De Armond