View Full Version : Fuel pressure vs. Injector flow rate

06-21-2005, 10:18 PM
I have been reading that some guys are going with bigger than 42lb. injectors for high HP setups cause the 42s aren't keeping a safe margin of air/fuel at high RPM/boost. There is a way to make 42 lb. injectors flow like 60 lb/hr injectors. By turning up the fuel pressure. The formula is:

(Fn/Fo)= square root of (Pn/Po)

Fo = origional fuel injector flow rate
Fn = new fuel injector flow rate
Po = stock fuel pressure
Pn = new fuel pressure

So: if stock fuel pressure is 39 psi, then 80 psi of fuel pressure would make them flow 60 lb./hr.. 80 psi is probably too much and the injectors wouldn't have the right pattern. But with a 255lph forced induction pump, you could probably run 50 psi with good results. 50 psi would make for a 47.6 lb/hr flow rate.

If you have a tuner, or tunable chip, you can just program the new injector flow rate into your program and it should be right.

I know a guy who was running a fuel injected 302 on the stock 19lb/hr Mustang injectors and he was running low 11s in the quarter with no nitrous. He was running monstrous fuel pressure to make up for it and it took some tuning to get it right. Obviously it would be best if you want more injector flow to just get higher flow injectors, but there are options for those of us who can afford a set of big injectors.

06-21-2005, 10:25 PM
You can use a supplemental fuel injector controller and use some junk yard injectors pretty cheap. Carroll Supercharginf makes a good one and also makes the PAxtamap controller used on some Paxton kits. There are others out there as well.

Running extremly high fuel pressure doesn't pose a pattern problem you will reach a point where the injector can't open. Usually around 80-100 psi on SC injectors.

XR7 Dave
06-21-2005, 10:47 PM
I would question the wisdom of doing that if you are running out of 42lb injectors. By increasing fuel pressure you are increasing the load on the pump significantly. Pump output starts to drop quickly when pressure exceeds 60lbs particularly if battery voltage is not maintained (which it won't be with the lousy stock SC fuel pump wiring).

I have also observed that as an injector approaches saturation it begins to perform unpredictably. As the injector transitions from a pulse width to wide open it does some funny things with AFR. Not to mention running an injector at that high of a duty cycle is hell on it's life span (so I'm told).

I would rather say that if you are pushing the envelope to the point that you need more than 42 lb injectors you should pony up the extra $150 for some 60's. It's very cheap insurance and your injectors will be much better off at 80% duty cycle than 98%.