ECU pulse width calculations...?

[John Dammeyer]

>
>Date: Sat, 11 Dec 1999 13:38:23 -0800
>From: "Andrew Brownsword" <asword at telus.net>
>Subject: ECU pulse width calculations...?

[snip]
>scaling back the injector values by the ratio of the flow rates of the old
>and new injectors.
>
>Doing this, as I understand it, should have reduced the pulse widths
>controlling the injectors and therefore decreased the amount of fuel being
>injected.  We cut the numbers by fully half and measured the effect using a
>(high end) A/F meter.  We expected to see the engine lean out significantly
>at idle... instead it leaned out slightly.  From 11.5 up to 13.2 or so.  It
>would momentarily jump up to around 14.5 and then slowly drop back down to
>13.1 or so.
>
>Does anyone have theories, advice, hints, etc?


You haven't mentioned what the actual pulse widths were with the old injectors
and then after you scaled them.  The Open/close time of injectors remains
somewhat fixed and if the pulsewidth is made too short the injectors operate
erratically.  So if the pulse width with the old injectors was, say, 2.5ms and
you change it to 1.7ms by your scaling algo. then you just barely get the pintle
off the seat before it starts to close again.  This causes erratic mixture as
sometimes it opens more than other times.  Been there done that.

It's for this very reason that picking an injector for an engine is so touchy.
Big enough for gobs of power at high end and you dispense, with those injectors,
too much fuel at idle.  It's for this reason that the concept of staged
injection is used by some performance enthusiests.  Small injectors for idle and
midrange and large injectors for high end.  This is much like a two barrel
carburetor.

One alternative,  is to use a throttle body injector for idle and cold start
enhancement and use the multiple injectors for high end.  The throttle body
injector has to supply all cylinders and so has a much better chance of never
being asked to be open less than a minimum time.  The new problem introduced
with that is then you need carb heat and an ability to make a smooth transition
between the two injector set.

Cheers,

John