The only problem with this is that tweaking sensor inputs won't give you a
totally predictable change in injector duration, because the ECU calculates
the pulse width from so many variables. Changing just one parameter by a
certain amount won't always give the same change in output.

One potential problem I can see is measuring the pulse width.  Many injector
drivers apply a full 12 volts across the injector to open it quickly and
then pulse the injector drive to keep current consumption/heating to a
minimum.  This would have to be filtered if you were measuring directly from
the injector wiring, obviously not such a problem if you can get to the
injector drivers themselves.

The peak current that an injector draws is very high, you'll have to have
something pretty big to take the current without causing a voltage drop
which would delay the opening of the injectors.  Injectors are quite
inductive as well so you will need to snub the big spikes you get when you
turn them off.

Regards

Mike Rigby-Jones


PS Didn't the SuperChips ICON use a PIC to do something similar?

> Hi Jon,
> I think you are going the wrong way about the control of injectors.
> A better way to trim injector duty cycle is to modify the inputs
> the cars ECU gets, ie vary the signal level from the air flow
> meter + or - an amount.. Trick the ECU into delivering less fuel.
>
> Regards
> Peter Mcalpine
> petermca@ozemail.com.au - www.ozemail.com.au/~petermca
> mcalpine@gme.net.au - www.gme.net.au
> www.wrx.org.au
>
>
> ----- Original Message -----
> From: Jon Petty <Phxsys3@AOL.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Wednesday, 19 May 1999 9:56
> Subject: Limit fuel injector pulses
>
>
> > Hi everyone
> >
> > I want to use a PIC or even a stamp to manipulate fuel injector pulses,
> > mainly reduce the pulse width. I really don't want to utilize individual
> > injector drivers (for sequential injection systems) that will complicate
> the
> > project. As you are probably aware sequential systems usually use an
> > individual driver for each injector. One side of all the injector has a
> > common 12v source to all injectors and the individual drivers (sink)
> control
> > the ground.
> >
> > I was wondering if it is possible to modulate the 12v power line which
> is
> > common to all injectors. For example say the PIC measures the previous
> > injector pulse at 5 msec and I want the injector to pulse 2.5 msec on
> its
> > next fire. Could I allow the injector to fire normally and cut the power
> at
> > 2.5 msec (to shut off the injector) and turn power on again in time for
> the
> > next injector firing? I am not sure what effect the  rise and fall times
> of
> > the power supply will have on injector firing or operation.
> >
> > If this is possible, I would just need one power control circuit to trim
> 4-8
> > injectors. I could easily use a PIC to measure the pulse, allow the
> initial
> > injector firing to happen on schedule and trim each injector by
> controlling
> > the power source which is common to each injector.
> >
> > What do you think?
> >
> > Any comments or suggestions for hardware that could accomplish this?
> >
> > Thanks in advance
> >
> > Jon