Injectors: P&H and SAT - Part 4

[Garfield Willis]

This is Part 4 for just the GMECM guys:

Now that we've opined one mustn't mix P&H & SAT drivers and injectors
willy-nilly, some astute pilgrim is bound to observe:

"But wait, GM built at least one ECM that can drive either 4 SAT
injectors per wire/driver, OR two P&H TPI (or one TBI) injectors on one
wire/driver. Both BATCH-mode scenarios."

So how do THEY do that? Altho I said you don't want to mix injector and
driver types, that doesn't mean electronics can't be designed to drive
either type, IF you can tell it which type it's driving. Here's what I
mean:

You CAN build robust, nay swarthy (heh) drivers that can pull open 2, 3,
or 4 P&H injectors OR drive a bunch of (say 4) whimpy SAT injectors in
parallel. GM did it for either 2 P&H or 4 SATs. The peak current
requirements for the former (that is, 4 P&H) are much higher, but the
thermal requirements for the latter are higher once you've got to keep
them on for long periods. So what's a mutha ta do? Just design them for
BOTH, but you have to KNOW (and telling the drivers would also be nice
:) which mode you want them to run in.

The problem is that this is perfectly economical if you're just dealing
with two wires you're going to drive in either case. Two BIG drivers
needed for either case, not a problem.

So, OK, yes GM or anyone can make a single driver to batch drive
either/both type of injectors, and quite a few at that if they wanted.
They just didn't happen to want to spend the bucks to do that in *all*
their batch-mode ECMs. Shhhhh, that's a good thing for moi; otherwise
you wouldn't NEED any interface modules. Heh.

But what about the SEFI case? You need there as many drivers as you 
have injectors. But if you're running BATCH you only need at most two
burly drivers. So combine the two? To make a module that would drive
BOTH, you'd have to have as many SEPARATE drivers, as you have injectors
(to cover the SEFI case). That means lets say 8 drivers. That's OK all
by itself; there are some really nice cost-effective integrated driver
devices available for individual injectors.  But TWO of these drivers at
least (for batch support), would require the ability to have current
limits setable for 2, 3 or 4 P&H devices in parallel (this to support
those guys upgrading from batch saturated to batch P&H for 4, 6, & 8
cyl). So on those, no integrated drivers. OTOH, these two potential
batch drivers need to be electrically and thermally extra beefy to the
point of covering any batch-drive and current-sense requirements of
multiple P&H injectors in parallel. A workable minimum requirement would
be 4 TPI P&H in parallel on each of these two special drivers.

So what do you end up with? A box that is BOTH sets of hardware, with
very little overlap. If you use it for Batch, you paid for 6 SEFI
drivers, their support devices and connector space, which you don't
need. If you use it for SEFI, you paid for two pretty husky more
complicated but versatile switchable drivers you didn't really need.
Either way, you essentially paid for BOTH. That's OK if you wanna pay
for both.

Oh well, I guess we're back to "mixing it up doesn't pay". I just knew
that was gonna happen.

Gar


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