Pump Pressure

[Daniel Ciobota]

Sorry for the long trailing quotes folks, but my memory she ain't what
she used to be...

To answer the question, I don't know exactly how low a pressure the ford
injectors (I believe they're bosch, not sure) can tolerate before the
spray pattern suffers, but I've informally tested mine down to 30psi (by
reading the plugs).  BTW, that pressure was tried on both the stock
19lb/hr injectors and a set of 24lb/hr; aftermarket wise, you can buy
injectors as large as 82lb/hr that will probably work at that low
pressure.  Keep in mind, though, these injectors are flow rated at 40psi
pressure.

Here's my totally unqualified advice.  The best way to set up your fuel
system is to calculate your fuel consumption both at idle and at max
load/rpm; then, choose the largest injector which can deliver the idle
fuel at about 30psi and ~4% duty cycle (try to avoid using the absolute
min and max duty cycles) and calculate the fuel pressure required to
flow at the max rpm/load with a 95-96% duty cycle.  If you manage to
stay under 100psi, you've found the right injectors for your motor.

It has also been suggested by someone to vary the pressure at the fuel
pump.  If your fuel system is the recirculating type, with a fuel return
back to the tank (most modern port injections systems are), you can vary
the pressure at the return line with a simple diaphragm restrictor;
that's what fmu's do, and they're vacuum (actually boost) operated.
Since most injector systems also have a in-line pressure regulator
(which are usually vacuum operated), you can use those as well, although
they tend to be a little more limited in pressure range.

Daniel


Jon wrote:

> How low a pressure can Ford 5 liter injectors accept and still spray
> properly?  TVR Jon
>
> Daniel Ciobota wrote:
> >
> > Wayne,
> >
> > What you describe is pretty much the way an FMU (fuel management
> unit)
> > works for blown cars.  Those cars use the stock fuel pressure and
> fuel
> > timing curves until boost occurs.  The FMU increases the fuel
> pressure
> > in relation to boost (still at the stock fuel pulse timing),
> effectively
> > delivering more fuel to the chamber.  Be aware, though, that
> dropping
> > fuel pressure significantly from factory stock pressure (39psi on a
> > mustang) may also affect fuel spray pattern at the injector; those
> units
> > are calibrated with orifices which require a set minimum pressure to
>
> > give a good spray pattern.  On the other hand, increasing the
> pressure
> > (within sane limits) doesn't seem to affect the spray pattern at
> all.
> > There are some people running up to 100psi and still using stock
> > injectors with no problems.
> >
> > Hope this helps,
> >
> > Daniel
> >
> > Wayne.MacDonald at zurich.com.au wrote:
> >
> > > Does each injector require a set pump pressure to work correctly
> or
> > > will an
> > > injector still put out a good spray through a range of pump
> > > pressures?.
> > > I ask this because I was wondering if it would be possible to vary
> the
> > >
> > > injector pulse from long at low revs to short at high revs and
> control
> > > the
> > > fuel passed by the injector by regulating the pump pressure.
> > >
> > >      Wayne.