Fuel pressure / Flow rate formula

[Daniel Houlton]

David A. Cooley wrote:
> 
> The problem lies not in pumping the pressure up, but the rated max
> operating pressure of the injector...
> You crank up the FP too far past the rated pressure (in your case 40psi)
> and 1 of 2 things happen... the injector gets held open by the fuel
> pressure, or it refuses to open because of the excessive pressure.


That's a pretty blatant statement isn't it?  Are you saying that if I go
over 40 psi it will fail, period?  Everything I've read suggests that an
injector will still work up to 120 psi, but that you should set as a maximum
around 90 psi.  At 70, it will still work just fine.  Anything over the rated
pressure will make the injector work harder and shorten it's life, but it's
*highly* dependant on how much higher you go and for how long.

In my case, I'm looking at fueling for a turbo.  It's not a race car, but
my daily driven truck.  Hitting boost will be an intermittant thing measured
in the order of seconds.  Longest period may be up to a minute or two when 
going through the mountains on some of the big, long climbs.


> The safest and most reliable route is find an injector that flows what you
> need in it's rated capacity.  Overpressure kills them.
> 

Yeah, I agree.  But this also requires a computer or something that can
manipulate my current computer/MAF signal so that it can control them.  Not
an option in my case as I don't have the several hundred $$ for these kinds
of things.

For my use, either bumping the pressure when needed, or using a couple
additional injectors using a rather simple controller is the best option.
FWIW, I'm leaning towards the latter as it'll give me more room for 
improvement in the future than bumping the fuel pressure can.


What about lowering the pressure?  How low can you go and still work/meter
reliably?  I've read that most computers can handle about a 20% change in
injector metering to allow for wear over time.  The computer learns what 
they actually are flowing by monitoring them during O2 feedback in closed
loop mode.  When going to open loop at WOT, it uses what it's learned to
adjust it's maps for the current state of the injectors.

So if this is the case, I can use slightly larger injectors (20%, so say 
24 lb/hr) and the computer will learn and adjust for them.  Stock, I run
around 40 psi.  What if I were to drop the pressure to around 30 psi.  This
would let me use even larger injectors.   Let's see, doing the calcs, a
28 lb injector (rated at 40 psi) would flow about the same at 30 psi as
a 24 lb injector (also rated at 40 psi) would at 40 psi.  So the computer
shouldn't have a problem with them.  

Now, I would basically have an injector flowing 24 lb at 30 psi so I have
another 10 psi to work with when bumping pressure.  Doing the calcs again
( this time from 30 to 70 psi instead of 40 to 70) would give me about 36.6
lbs/hr at 70 psi.

Now this is all theory and I'm kinda pulling it out of my a** as I'm think-
ing about it.  Would this really work OK?


--Dan