Blowing Thru A Holley TBI

[Walter Sherwin]

>
>> 3. If a fangled system is out for me, due to any reason, can the
aftermarket Holley TBI
>> systems be easily made to work with a blow through turbo deal?
>
>Yes, absolutely.  From a fabrication standpoint pretend the holley TPI
>is a carbeurator, make a plenum box above it, attach turbo hoses, and
>crank away.  Might need a blow-off valve to relieve pressure if you are
>at WOT and let off the gas suddenly, but that need would be on any
>system.
>







You have to be really careful when blowing thru a Holley TBI unit equipped
with their "B2" style injectors (which covers almost all of their TBI units,
except for their one barrel OE replacement units which use Rochester TBI
injectors).  The Holley B2 injectors are very intolerant of even minor fuel
pressure increases, and will completely shut-down (ie: no fuel at all)  at
just over Holley's rated fuel system pressures.  Basically, the magnetic
forces and components within their injectors do not have enough "oomf" to
open the ball/seat combo against the increased internal hydraulic forces
created by increased fuel pressure levels.  Here's a real world example (as
tested on my wet flow bench) of why this is important to
know................

A Holley 900 4bbl TBI is factory equipped with four B2-80 injectors mounted
in a fuel injector "pod".  These injectors are designed to be driven via
four individual 4/1 amp peak & hold current drivers, at a max fuel system
differential pressure of 15psig.  Under such conditions, this setup will
deliver about 310 #/Hr of max fuel through the pod casting's inlet port.  In
a naturally aspirated draw-thru configuration this would easily satisfy the
requirements of say a 600'ish HP engine, which is well within the confines
of Holley's conservative 500'ish HP rating for this unit.

However, crank the fuel pressure to 18psig and you will note that the
injector response/linearity  begins to seriously degrade.  Crank the fuel
pressure to 20psig, and you will note that the injectors begin to visibly
and intermittently cut-out and refuse to open.  Crank the fuel pressure to
22psig and you will note zero fuel flow.  This situation is worsened
(happens sooner) by both hot operating environments, and reduced current
levels.   Reduced current levels?...............If you attempt to operate
two B2-80 injectors in an electrically parallel configuration, attached to a
common 4/1 amp driver (ie: typical two channel GM TBI boxes, and others),
then each injector will effectively experience 2/.5 amps of driving current,
which will seriously degrade both the response characteristics and the fuel
pressure tolerance of each injector.

Now, in an artificially aspirated blow-thru configuration, you must remember
that the fuel pressure differential across the injector has to be
"referenced" to the local air pressure (boost) at the injector tip (just
upstream of the throttle blades) in order to at least maintain a somewhat
consistent rate of fuel flow for a given injector duty cycle.   If your
particular combination of boost, regulator gain, injector duty cycle
requirements, and net fuel flow requirements necessitates fuel injector
pressures above 18psig then your setup   WILL NOT WORK!!!!

Also, you should know that the Holley B2-80 injector is the largest flowing
injector on the face of this planet that will readily fit within the Holley
injector pod.  If you absolutely must have more flow and/or pressure then
you will have to machine your own injector "pod" from a chunk of aluminum in
order to house alternative TBI injectors.  This is no small task, and is
fraught with other spray pattern problems, as most of the alternative TBI
injectors (ie: Bosch, Rochester, etc...) have much wider spray angles and
wind up spraying a large portion of their fuel charge very "high" on the
bore walls.  Mind you, the prevailing blow-thru boost/heat/flow usually
remedies this little quirk under most driving conditions.

For reference, most of the Rochester TBI's that I have tested can be run
continuously at 50psig with a 4/1 amp current driver, and for bursts at
70psig.

Sorry to ramble.........




Bye;
Walt.