GM OEM TBI up to 420hp?

[Walter Sherwin]

>First off thanks for the detailed effort in your reply.
>>Hey Ross there are three TBI hardware approaches that you may want to
>>consider, if you want to feed a 420'ish HP V8 at reasonable [less than
>>6000 - beyond is quite impractical] RPM's.......
>Impractical as in just not needed with an appropriate power band setup
>below that? Motor will run to 6500 easily. In my setup I don't need it as
>shifts up their are unnerving for me:-( Don't like jumping sideways.



I meant "impractical" within the context of typical TBI injector firing
schedules versus climbing engine RPM.

Most dual channel TBI V8 boxes tended to fire the injectors once each per
every other distributor REF pulse (also out of phase with each other)  to
promote more even fuel distribution.  As engine RPM's climbed, the time
between pulses would shorten, and eventually a firing frequency would be
reached where it was no longer practical/reliable to inject.  As most of the
OE TBI applications were low RPM oriented, pulsewidth headroom was not a
huge concern for the OE's.  However, even some of the OE TBI packages had an
"escape hatch" which would allow you to revert to time based fueling at
fixed (say 12.5ms) firing intervals, based on high MAP and RPM thresholds.
Still, this only offered a minor extension in headroom over the previous
scheme.

In your case, with a healthy 6500 capable mill, you might want to consider
stepping right on up to port style fuel.

As a saw-off I guess you could try using a GM "port" ECM to run a TBI unit.
In contrast,  the port box will fire both injector channels in unison at
either once per every two crankshaft rotations or once per every one
crankshaft rotations.  You'd have a lot of calibration manipulation to jump
into, if you took this approach, and there are mixed reviews/opinions on
exactly what effect the reduced port firing frequency has on idle & low RPM
TBI fuel performance.



>> Oh, and you might as well buy a Holley 12-903 [or 12-903HP] gerotor
>in-tank EFI pump kit
>Can I use an out of tank pump? My car was EFI OEM so tank/outlet is baffled
>etc.



Sure!  In fact, you can use the Bosch GFP216 (aka 0 580 254 984) mentioned
earlier, as an external pump as well.   There are a number of suitable
external pump models available, from Bosch and others.   I like to put the
pump in the tank whenever I can, just because it avoids all the suction/lift
problems associated with the external setups and it makes things quieter.
If using any portion of your vehicle's OE fuel circuitry then keep an eye
open for restrictions, especially on the suction side of the pump.



>>conventional TBI intake manifolds [which you ***really*** will want with
>TBI fuel , with EGR if required]
>Now which TBI intake would I want? I've understood (correction welcome as
>usual) OE ones won't flow past 5500 tops. Mine has to flow to 6000 or a
>hair below. Fairly simple 'adaptor' I assume to bolt your 2bbl TB above to
>the intake?



Yes, what you've said is correct.  With a bit of porting and clean-up the OE
's can be made to flow quite well, but not to the 6500 type limits you're
talking of.  Even the aftermarket TBI specific manifolds won't typically do
that.  In your case you'll want an adapter plate between a TBI and a
conventional manifold of your choice.  If on the other hand you were only
spinning to say 5000'ish RPM, then I'd recommend keeping a TBI specific
manifold, if the vehicle was to be anything less than a full-on race car.

To give you an idea of what's possible I run a heavily ported OE 93 BBC TBI
water-jacketed manifold in one of my applications, and have spent a few days
abusing it on the engine dyno.  Excellent torque and response, pulls hard
way past 5000 (with healthy heads and valvetrain below), and all EGT's would
happily hover within a 70 F spread of each other at WOT at any RPM.  Plugs
almost all looked identical.  On the street it's quite a handful.  Is fed by
the 2x2" TBI mentioned earlier as option #1.



>2) My second choice........Is a Holley 4bbl TBI assembly [there were not
>any OE 4bbl GM TBI assemblies produced
>-now I do know of a holley 4D setup that's their 700cfm model barely used
>for $325US, seems high for the TB but I guess I'd get
>pump/injectors/regulator etc as well and I could drive it with GM ECU if I
>wanted??



That might be a good route to try first, if wanting to stick with TBI.
These Holley 4bbl's are fairly decent units, and can be easily adapted to
most carb manifolds.  You can run it with almost any GM box (remember notes
above), but make sure that you have an external driver arrangement between
the box and the injectors.  You want one 4/1 driver per each Holley
injector, and you also want to keep the fuel pressure at no more than
15-18psig otherwise the injectors will lock-up.  You can use the pumps and
filter mentioned earlier.  I've also seen the 900 CFM units floating around
on E-bay and such places.  Just make sure you get one with IAC, and make
sure you flow & check the injectors before you use it (hint-hint-hint.).



>[I have a hard time getting my mind around 2-2" blades feeding 400hp when I
>see same inlet size on prepped 240Z's. Obviously they're not making max use
>of that inlet as they're not running twice my rpm to ingest same air.]



It's all based on your individual application's breathing requirements.  The
2x2" flows 670 @3"Hg pressure drop.  You'll have to estimate your peak
airflow,  and compare that to the setpoint above.  A 2x2" would probably be
a notable restriction in your case.  The Holley above sounds like a better
deal, or better still go port.



>Now possibly from outside misinformation I have assumed I can't do this
>'custom/plug'n'play' setup with multiport? 747 ecu etc is all TBI is it or
>can it be used on multport? Costs of ECU and injectors were holding me
>back from multiport. I've seen 30pph SVO and bosch/takeoff injectors
>regularly on ebay etc for quite cheap ($50 and up) but have been told I
>should run 36pph? If a the GM ECU can be cheaply used for TPI as well I'd
>love to hear about it as I just realized I'd assumed from outside info this
>could not be done and seeing the plethora of info/experience on this list
>I'm hoping I might be told otherwise?
>Ideally I'd like MPI plumbed on my Vic. Jr. if I can find the system within
>my budget. TBI could be fun meantime if MPI is too pricey.



It would be quite easy to step up to a roll your own GM port system for
about the same cost as TBI, and honestly I think you'd be happier with the
finished results in the long run.

TBI  fueling can be pushed to higher than OE anticipated power levels, but
this is usually only done in cases where people either    1)see TBI as an
intermediate temporary step before going port, or who      2) "have" to
stick with TBI to maintain compliance to state/class laws.


>thanks again



Any time.  Good luck and have fun.  There are others here who know port fuel
far better than I, so I'll bow-out now.  By the way, what flavour of  Master
's of Engineering are you pursuing?  UBC?


Walt.



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