Injector supply voltage & positioning.

[David Cooley]

On Wed, 21 Dec 1994, Lou Faustini wrote:

> 
>     Aren't injectors a binary-type thing? ... Either they are on, or they 
> are off right?. I thought there was some mechanical hysteresys built in.
> I assume if you were to drive them at very low voltages you might find a 
> point where they are in a semi-open state, but I assume that that voltage
> would probly be around 5 or less volts. I know that Toyota corp actually
> puts a 2-ohm resistor in serise with their injectors. They are also 
> low-side swithced. 


   True, they are in a sense binary, but the pull-in time increases as 
voltage drops.  As pressure rises, pull in time increases also.

> 
>     Question.. What fuel pressure do you folks recomend running the
> injectors at. I heard that the Ford-type injectors, like the ones used
> on the Mustang V-8, run at about 40+psi. Is this true? That sounds like 
> a real problem for the hobyst who would like to use Trak-Auto fuel line.
> I know that my Holley setup runs 15psi. That is a little more reasonable
> to work/play with.
> 
> 
   Lou, The port FI injectors do run at a higher pressure.  Most often 
they run at 38-45psabove manifold pressure.  The regulator on a PFI 
system will hold this differential by referencing fuel pressure to 
manifold press.  Typically with approx 20" vacuum, fuel pressure is 30-35 
psi.  In a turbo or supercharged application, fuel pressure will increase 
by 1 psi for every psi boost pressure.  The reason for the higher PSI in 
a PFI application is this:  The injector is only open a short time and 
must deliver a highly precise amount ofd  fuel.  The throttle body 
systems have much larger orifices and are typicaly open longer than the 
PFI injectors.  The PFI system must have a good pressure to atomize the 
fuel as it doesn't have a lot of distance to travel to the cylinder... 
Anything still liquid just washes oil off the cylinder walls and is 
detrimental to performance and emissions.
Later,
Dave Cooley
cooldave at nando.net