I have read that most of the fuel injectors used for our cars are calibrated for a certain fuel pressure, i.e the #22 injectors should deliver 22 lb/hour at 43.5 PSI.

The question is, how much can the fuel pressure be varied without the need to change this injector constant?

Is there even a need to change it... Or have I missed something out?

------------------
ZaphodB a.k.a Carl Andersson - zaphodb@faxancruisers.org
'89 Camaro IROC-Z 5.7L TPI - Mods under construction :-)
Custom ARAP Chip Under Development/180 t-stat, Edelbrock TES Headers,
SLP Airfoil, K&N Filters, Accel 8mm wires, Relocated MAT, AFPR @ 45 PSI

 

I know this doesn't really answer you questions exactly but from what I've noticed:
Fuel pressure can make a big difference. The more fuel pressure the more fuel, I like to think of it as a supersoaker. If you didn't pump it up much or consistantly pump the pressure dropped and a lot less water got pressed out. Obviously liquid doesn't compress like gas so the actual hole size on the injector has more to do with it than pressure. If you're talking about TPI injectors, it doesn't make a huge difference in fuel flow but when you tune it right it's been proven to release some extra hp. I think there is a tech article about it.
As for TBI and low pressure systems, fuel pressure is more critical (unless you burn your own eproms). I know that right now I'm running more than 11psi on 65 injectors and my BPW level is set down to 109 just to get it to idle at 450mV (o2 readout). Stock BPW is 134 or 5 I forget but you can see that it's a big difference. I could talk about TBI all day but I don't think it matters in your case.
To answer you first question: You can adjust fuel pressure to make the car run "better" than stock but it isn't a variable pressure like what can be tuned in an eprom.
To give you an example of a mechanical fix vs. an eprom adjustment. Car with aftermarket bolt ons and making more power than stock and with a speed density setup. The car is going to run lean across the hole board if you haven't touched anything. If you increase fuel pressure you'll most likely end up with either a rich idle and near perfect WOT OR you'll have a normal idle and a lean WOT because of the engine efficency graph. Stock slope is not as steep. At low rpm the stock engine will probably suck in closer to the same amount of air as the one with bolt ons but uptop the bolt on engine will want more fuel. It's a nasty and PITA process to get it right.
I know with TBI you can just crank up the pressure on the GM injectors to flow as much as the next level or even more. The stock L03's were 40, the 350's had 55 and some had 65, the 454 TBI had 85 and I think some had 95 but I'm not sure. Since TBI injectors have larger openings it's probably why small increases in fuel pressure can yeild huge results in flow. For TPI etc I believe you'll only get +/- 2 lb/hr. If you find a source explaining fuel pressure and flow for injectors please post the link.

------------------
, Jon (350 TBI!)
91 Red My website

 

Well, actually the thing I was thinking of was that many people increase the fuel pressure in order to get a better spray pattern, and also a bit more fuel. But since the injectors are calibrated to deliver X lbs/h at Y psi, would it be wise to recalculate the injector flow constant in the prom to account for this even if the change is only from lets say 40 psi to 47 psi?

I haven't done any chips for TBI, since I only have a TPI car, but interesting to learn a bit about TBI as well

 

Right or wrong, I've been doing that myself recently. I want to run the 6E code and enable highway mode fuel, but have to get my block learns around 128, first.

In my case, I can get them close in cells 1-14, but not in 15; or vice-versa. On the highway, I'm in cell 15 the vast majority of the time, so I'm focusing on getting that cell right. With the stock, unaltered FPR, most values were running very high. So, I shimmed the FPR. This was an overcorrection. All values were then too low. I used a thinner shim. This got 1-14 pretty close, but 15 was down around 120-122.

My next step was to change the injector constant from 23 to 24 (it's much easier than fiddling with the FPR). Now, 1-14 are a little high (133-135), and 15 is really close (127).

Cells 1-11 only deal with the very lowest MAF values (a very small part of the tables). Cells 12-14 only cover 900-2000 rpm, so that leaves cell 15 to cover a very broad range above 34g/sec and 2000rpm. Therefore, I thought it would be easier to tune the MAF tables in the sub 34g/sec range. I used fuel pressure and injector constant to take care of cell 15. I look at the FPR as a "coarse" adjustment and the injector as a "medium" adjustment. MAF tables are then used for "fine" adjustments.

In hindsight, I think I didn't need to mess with the FPR. I could have achieved the same thing with the injector constant alone, but bumping the pressure, even a little, seems to be a good idea, anyway.

I'm not sure this is a good approach, but It's the only way I could come up with to manage my block learn values. I'm not where I want to be, yet, so I'm not yet running the 6E code (I have other problems on that code, but that's another story).

------------------
'87 IROC 5.7l TPI - original owner!

 

In general, an injector constant will globally add or reduce fuel. Altering FP generally will have its greatest effect at HI load conditions since the regulator is regulated by engine vacuum.
Jason