P&H Injector Interface - Wiring Approach

Tue May 30 00:04:46 GMT 2000

This is a  bit long winded, so anyone not interested in this topic, please
feel free to hit the DELETE key now

Whoa Gar! I think I see the problem here. We have vastly differing
expectations for the wiring approach to this box thingy. You appear to be
interested in driving the various injector clusters through whatever OE
harness wiring happens to exist, whereas, I feel it is important to
appropriately rewire each of the low impedance injectors (no matter the type
of P&H injector) to its own 4/1 P&H driver. Your approach may work in a
limited number of applications, but can be fraught with more trouble than
what it's worth. Also, I have to wonder why the OE wiring harness is such a
sacred issue? Allow me to expand.........

In conversions of this nature, where any number of controller types may be
connected to any number of clusters and types of injectors, I picture the
ECM and the bulk of the OE harness doing nothing more than triggering the
input side of an underhood external injector driver box. I picture the box
having 8 inputs, 8 discrete 4/1amp current drivers, 8 outputs, and enough
configurability on the input side so that a given ECM channel can
simultaneously trigger (sink) a user defined number of inputs at any given
time, which in turn would fire a corresponding number of individual
drivers/outputs. Each of the outputs would in turn be APPROPRIATELY and
individually hardwired to an individual injector.

Trying to drive "X" number of P&H injectors, through OE wiring which did not
originally encompass one circuit per injector, is a bad idea IMO. Without
going into detail, what about: 1)significant line losses, 2)current sharing
issues within a cluster of injectors, arising from impedance
shifts/variations that normally occur with injectors, 3)unknown OE wiring
quality and splice quality, 4)unknown terminal current capacity anyplace
along the path, 5)widely varying current rise times & characteristics, based
upon the RL time constant of whatever equivalent circuit results?  Both the
magnitude of the current at an injector, and the rise time of same, are
important issues.

There exists more than just a few variants of differing "P&H" low impedance
injectors, even within a given style family (ie: within the port family
port, or within the TBI family). Therefore, it is impossible to sufficiently
generalize clustered P&H injectors, and the equivalent circuits which
result. When you ponder the potential mixes, the above quickly become very
real concerns. The OE's have in the past ganged 2 and sometimes 3 P&H port
injectors in specific applications with a common current driver with good
results, but, that is not to say that such applications were not
problematic. For example, certain GM 4cyl ECM's had two 2.05 ohm 2/.5 amp
Rochester port injectors ganged together in parallel, and driven via a
common 4/1 amp driver in the ECM. However, as little as a .4ohm impedance
shift or mismatch between the injectors in a pair would cause one or the
other to not function properly. If you expand your approach to gangs of 3
injectors, or even 4 injectors, or even more, then the opportunity for
trouble increases.

My last general statement, before answering some of your specific questions
below, is this. Anyone who typically finds themselves requiring either more
injectors, or bigger/expensive P&H injectors, during a DIY project has
probably arrived there by previously performing a whole bunch of DIY
swap/tweak/rework/reprogram/redesign stuff. So you are assuming that these
same people for some reason either don't want to, or can't, add a wee bit of
wiring?

>
>>The most probable DIY-EFI injector “conversion” configurations that I can
>>think of would be: 1) SEFI MPFI SAT to P&H – assume a max of 8 inputs
and
>>a max of 8 injectors. 2) BATCH MPFI QUASI P&H (current sharing ganged
>>pairs) to individual P&H – assume up to 2 inputs and a max of 6 injectors.
>
>Whoops gotta go slow here. Why SIX? Or do you mean 6 total, a max of 3
>PFI injectors per driver/input?

This would be a max of 6 injectors (to cover certain V6 applications),
individually wired back to 6 outputs/drivers at the external driver box.

>
>>3) BATCH MPFI SAT to P&H – assume 2 inputs and a max of 8 injectors. 4)
>>Single Channel TBI to Dual Channel TBI – assume 1 input and 2 TBI
injectors.
>>5) Dual Channel TBI to 4BBL TBI – assume 2 inputs and 4 TBI injectors. 6)
>>Dual Channel TBI to twin 4BBL TBI – assume 2 inputs and 8 TBI injectors.
>
>Good Golly, are you SERIOUS? You mean to suggest 4 1.2ohm injectors per
>driver? for a total of 8!!? OK whose got such a beast wired up? That'd
>be 16A peak current on a single harness wire. I don't believe it!
>

No. Look at item #6 above for example. This would be a maximum of 8
individual TBI injectors, individually wired to 8 individual outputs of the
external driver box. The two ECM channels would be wired to two inputs at
the external box, and three more additional driver inputs would be sinked to
each of the original two inputs connected to the ECM.

This would be typical of a couple of Holley 900 TBI's perched atop either an
8-71 or 10-71 Roots blower, controlled by a dual channel GM truck TBI ECM.

>>Every one of the individual injectors listed above, can be safely driven
via
>>one individual 4/1 amp current driver per injector.
>
>Yes, close, but that's NOT even the problem. OHHHH if only it was, Walt.
>The problem is that nobody want's to rewire their harnesses to run an
>individual wire up and out to each injector. Neither the original TBI
>harnesses, nor the Batch PFI harnesses, have more than the two wires
>coming from the ECM and going out to the injector's loom.
>
>THAT is the crux of the problem. If we need to stick to the original
>harness, then the drivers have to be capable of driving multiple PFI's
>and TBI's *on a single wire* or two, namely the original harness
>wire(s), which is the way they're presently wired. In the batch
>harnesses, there are at most TWO wires going out of the ECM and up to
>the engine, where at some point they get split out to the individual
>injectors. But not at the ECM.
>

I am aware of the various OE wiring harness configurations, but again I ask,
why the hang-up on rewiring? Make the box robust enough to live
underhood, and run a few short lengths of wire from the injectors to the
outputs of the box. The wires would not even have to pass through the
firewall.

>There is also another problem with this scenario which I'll mention
>shortly.
>
>>Soooo, how about a basic driver box housing 8 individual 4/1 amp current
>>drivers? This would cover every probable conversion configuration.
>
>Nope, I don't think that dog will hunt. You can't just wire these
>drivers up in parallel if you need to drive more than one injector on a
>single wire. That's the problem. Your scenario must either assume that,
>or additional wires from the interface out to the individual injectors.
>The present pair of wires in a batch-fire harness IS able of carrying
>the current (not the ECM pins, but the wires are OK; we would splice
>into the two wires outside the ECM).

Nope, never said to parallel connect the drivers in the manner you mentioned
above. Individual wiring to the injectors is assumed.

>
>Yes, it *would* be absolutely trivial to create a module containing 8
>individual TBI drivers, or 8 individual PFI drivers, or whatever. And
>the former is what's needed for SEFI applications, but for batch-fire
>ecms...

Not the case if each injector has its own driver. In fact, almost all of the
available Port or TBI low impedance P&H style injectors can be reliably
driven by individual 4/1 amp drivers.  There are qualifications to this
rule, which we’ll look at in a minute.

I agree, that with your approach the capacity of the master current driver
would be an issue.

>
>The issue is, how do we practically drive multiple PFIs and TBIs on the
>SAME wire. Once again, that raises other questions about that VIC box.
>I'm beginning to wonder if that box actually ASSUMES a re-wire of the
>injector loom? If it does, it's not only whimpey and pricey, it's also
>unnecessarily intrusive. Geez, it was bad enough suggesting to guys that
>they break into at most two wires outside their ECM, but the idea of
>insisting on them wiring a bunch more individual wires out to their
>engine harness is WAY over the top, at least to me.
>

I can send you some snail-mail pertaining to the VIC box, if that helps. I
think however that you might best contact Accel for their latest info.  The
VIC box is a decent product IMO, but it does fall slightly short in the
grand  scheme of configurability and universiality.

Think of the VIC as a box which would most appropriately be used to
convert a batch fire TPI GM L98 5.7 litre engine from 2 batched banks of
4 saturated injectors each, to 2 batched banks of 4 individually driven P&H
port injectors each.

The VIC is supplied with both an input side harness, and an output side
harness.  The ECM/OE harness connects to the input harness, to provide the
trigger signals, and the output harness connects the injectors to the
individual drivers within the box.  Additionally, the VIC facilitates BPW
corrections per injector, via lockable twist knobs.

Let’s explore your approach to this same injector conversion

. If  I
understand correctly, you would propose that one P&H current driver would
simultaneously drive 4 low impedance port style injectors per batched bank
through the existing OE harness.  Let’s also assume the use of Rochester low
impedance port injectors having a typical impedance of around 2.05 ohms and
3.2 mH and a design drive current requirement of 2/.5 amps.   Now model the
equivalent circuit, taking into account all sources of voltage drop, and you
will find that the net current will saturate at just over 10.5 amps at
around 4ms at a system voltage of 13.2 volts

I have a little homebrewed Excel model which I use for stuff like this,
which I can mail you if interested.

“IF “ everything  in your resultant circuit is up to snuff, and “IF” all
four injectors in a given bank are balanced, then your approach will work as
long as you construct a custom driver capable of such current.  But, as
things age that's a lot of "IF's" to worry about.

>Now, here's the other problem with your conjecture that "8 individual
>4/1 amp current drivers would cover every probable conversion
>configuration". You CANNOT safely drive a single PFI with a TBI driver.

I completely disagree with you here, Gar.

>The reason is that with the TBI driver with it's higher peak and hold
>current limit settings, (1) the PFI may not reach the peak TBI current
>and proceed to current foldback; remember a PFI has twice the resistance
>and inductance of a TBI, so it's intended peak current is 2A, not the 4A
>of a TBI driver.

Not all low impedance injectors are created equally.  I agree with you that
most of the TBI injectors will be around 1.2’ish ohms, and are typically
driven via an individual 4/1 amp current limiting driver.  However, low
impedance port injectors are more varied.  The Rochester’s will typically be
around 2.05 ohms, but the offerings from Bosch (and others) as an example
can be anywhere from 2.2 to 4.6 ohms.

Yes, the Rochester’s had a design drive current rating of 2/.5 amps, but
they will also operate quite happily (all day long) at 4/1 amps, using a
conventional off-the-shelf 4/1 amp driver.  When driven by a 4/1 amp driver,
at a system voltage of 13.2 volts, these injectors will typically attain 4
amps at around 2.5ms and then transition to 1 amp holding current.

How do some of the “other” low impedance port injectors fare at 13.2 system
volts?  For example, a Bosch ‘912 @ 4.6 ohms will saturate at around 2.3
amps and a Bosch ‘170 @ 2.8 ohms will saturate at around 3.5 amps.  As long
as your driver circuit contains a default current transition timer, then it
is possible to drive these injectors (and others which are similar) via a
conventional 4/1 driver.  Roughly 4 ms would be a typical timer value.

Simply put, you put a 2.4ohm injector on a driver that
>expects to get up to 4A before folding back to it's hold current, and it
>may not get there in time, but (2) suppose that poor PFI ends up drawing
>all the way up to 4A eventually, and tripping the driver into it's
>"hold" mode. BUT what's the hold current now? 1A, not the originally
>intended 0.5A of the PFI. You stressed it to peak, now you're stressing
>it in hold mode. Not good.

Not an issue.  Lots of examples of such configurations in use.

>
>I appreciate the suggestion, unfortunately it's not really a workable
>solution, AFAIK.
>
>Gar
>

Always a pleasure chatting with you.  Good luck on your project, whichever
way you choose to steer it.  Give a holler if you want some of the free
stuff I mentioned above, or have more questions.

Walt.

----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org