Another strange injector question

[Walter Sherwin]

The '747 TBI box (as with many other V8 2-channel TBI boxes),  when in
"normal" synchronous 2bbl TBI mode will fire one of the TBI injector
channels (think of a channel as either a single TBI injector, or a gang of
"N" port injectors) once per every other distributor REF pulse.  Or in other
words, twice per crank revolution.  The "other" TBI channel will fire in a
similar fashion, but will be delayed by one REF event with respect to its
mate.  Or in other words, delayed by 90 crank degrees.  Hence the name of
this mode is "Dual Channel Delayed Synchronous Alternating TBI Fire".  This
mode is intended to chop-up the fuel delivery in order to account for
certain wet manifold phenomena.

When the asynchronous TBI mode is triggered, both TBI channels will fire at
the same time, at time based intervals spaced 12.5ms apart.  Hence the name
of this mode is "Dual Channel Asynchronous Simultaneous Fire".

These firing schemes can be made to successfully control gangs of "port"
injectors in a port fuel injected application, if the injectors have
sufficient response and flow.  Edelbrock's retro MPFI kits are good evidence
of this.  P&H port injectors do not have quite the response characteristic
of a good TBI injector, but will be better than most saturated port
injectors.

Sounds like your Lucas injectors (@ 2.8 ohms each) are of the P&H flavour,
and are being crutched by the ballast resistor for use with common/cheap
saturated OE switch drivers.  This kinda makes sense, as 2.8ohms+6.0ohms
would result in approximately 1.4-1.6 amps of prevailing current per
injector, depending upon battery voltage.  Not an uncommon figure for a
saturated driver unit.

To gang your P&H Lucas injectors to the two channels of a TBI V8 box (hey,
you own the injectors already!)  in a 4+4 arrangement, you'll need some sort
of electrical driver box in between, which will trigger off the box channel
drivers and yet independently drive 2 groups of 4 P&H injectors.  You can
easily make a homemade driver box from scratch components for this purpose,
or there are a number of other DIY & Kit  options floating around on the
three lists.  One more commercially available solution is the "Variable
Injector Controller"  (VIC) from Accel.  It does exactly what you want to
do, and additionally by its design provides for individual injector
pulsewidth adjustability from a lowly batch fire port fuel setup.  The VIC
is also very cost effective.


Walt.






> I'm still looking at using the stock Rover (Lucas/Bosch) manifold, fuel
> rail, TPS, temp sensor, and, possibly, injectors with a '747 ECU.  I think
> it can be done and for the TR world would be a very nice setup.
>
> When I asked before there was a lot of discussion regarding saturation vs
> P&H injectors.  I'm trying to determine which type the Rover unit
> uses.  Electrically they have a resistance of 2.8 ohms and are wired with
a
> 6 ohm current limiting "ballast" resistor in series with each one.  Each
> injector has its own drive transistor (notorious for failure in these
> systems).   To me this looks it might be set up to use P&H injectors in a
> saturation mode.  The Lucas/Bosch ECU is a very primitive analog unit that
> uses a vane type air flow meter.  Lucas specs the normal pulse width range
> to be 1.5 ms to 10 ms.  The stock system operates as a dual bank with each
> bank firing once per crank revolution (this amounts to a fuel pulse every
> other ignition event).  Cold fuel enrichment is via a 9th injector
operated
> like a central port setup.  My attempt will be to have the injectors fire
> every ignition event a la '747 if they are indeed the fast P&H type.  With
> a little tweaking of BPW and async timing this should work and will
> effectively almost double the fuel delivery capacity of the injectors.
>
> Any comments?
>
> Also, is it true that on the '747 V8 TBI the injectors fire alternately on
> every ignition event except in async mode?
>
> Thanks,
>
> Scott
>
> PS - thanks to all who have responded to my other strange injector
questions.


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