Injector Fuel Flow

[William Boulton]

Hey! Does anyone want to know how to estimate fuel flow requirements?
Please note; I said ESTIMATE. What follows is a formulae for making an
initial injector selection.

                     BHP * BSFC
     FlowRate = -----------------------
                 Inj.Count x DutyCycle

     where:    BSFC = Brake Specific Fuel Consumption
                    = 0.5 lbs/hp.hour   (US)
                    = 227 gms/hp.hour   (SI)

You'll notice that the formulae uses MASS and not volume. Gasoline (on
average) has an sg of 0.73gms/cc. I'll leave it to someone else to do
the conversion for US units.

If anyone has access to the Bosch micro-fische on injector specs, the
flow rates are in GRAMs/MINUTE and not CCs/MINUTE. As you can see, there
is a big difference.

In the one example of injector specs I was able to obtain (in German),
the continuous (static) flow rate was quoted in gms/min and the dynamic
in gms/1000. Nominal fuel pressure is 2.5 bar in all (?) cases. The
specs also nominate the pulse width used in dynamic sizing (2.0 or
2.5ms). Just to clarify, heres a couple of examples of EV2 injectors
(Ford CFI). They are the little stubby flow through type.

     Bosch PN     Static   Dynamic  ti.ms       Used in
     --------------------------------------------------
    0 280 150 601  513.5     21.5    2.5    (Ford USA & Holley?)
    0 280 150 608  441.3     12.82   2.0    (Ford Australia)

In the past, I've tried to get fuel flow specs from people who have had
the micro-fische and was told "they don't mean anything". Well they
won't if you don't do the conversion, will they?

If there are any engineers out there who would like to confirm, deny, or
otherwise massacre the above, please feel free. I'll listen to anyone
who has knowledge based on a solid foundation. The formulae came from an
engineer at AC Delco and was confirmed by empirical evidence, so I have
no reason to doubt its validity.

If anyone wants to select injectors by working back from a BHP figure of
a vehicle, heres a little trap to be avoided. I estimated the flow rates
of 2 Porsche 911s, 1 NA and 1 turbo. The nominal flow rate of the turbo
units was actually less because it ran at 3.5 bar fuel pressure and the
NA used 2.5 bar. Tricky. One other thing, the injectors I listed above
normally don't run at the 2.5 bar used for flow rate testing. I think it
is usually 1.5 bar or there abouts.

BTW. The BSFC figure is a sort of average. Newer engine types may give a
little better and older ones a little worse. It's a starting point with
the final configuration set by adjusting the fuel pressure.

That's it for now

Bill Boulton