Help me understand fuel pressure

[Coontail]

I am having a difficult time understanding how fuel pressure effects the amount an injector puts out on an LS. I am sure there a pages on this but some of it gets so in depth my head starts spinning.

My theory is that an injector only requires a minimum fuel pressure and that anything higher than that shouldn't matter as a, say a 38# injector, puts out 38 pounds no matter what. As far as i know the injector is not just a gate for fuel to flow through but it is pushing fuel out at a fixed amount. What I cant understand is that if the injector is a gate then why bother getting bigger injectors when you can just up the fuel pressure.

CHANGE MY MIND

pls

 

[Bent77]

Two things going on

Pressure and flow rate (pump rate). You can have either one or both. With out both, the injector rate is off

 

[Fastereddie]

A 20 lb. injector will flow 20 lb’s of fuel in an hour at 100% duty cycle. (Wide open) at 43.5 lbs of fuel pressure. If you increase the fuel pressure above the rated 43.5, you will squeeze more fuel out of it.

 

[bp71k5]

Like Bent and Eddie mention, the fuel injector is not a regulator. It’s a very fast valve that opens for a very short time. Higher pressure when the valve opens = more fuel coming out of the valve.

Just increasing pressure on a “small” injector may not deliver enough fuel for the engine. The pressure will be high, but the volume won’t be. The computer can turn the “valve” on longer to get additional volume, but you have to stay within a safe duty cycle or the injector just stays open or fails earlier than designed.

 

[Coontail]

Like Bent and Eddie mention, the fuel injector is not a regulator. It’s a very fast valve that opens for a very short time. Higher pressure when the valve opens = more fuel coming out of the valve.

Just increasing pressure on a “small” injector may not deliver enough fuel for the engine. The pressure will be high, but the volume won’t be. The computer can turn the “valve” on longer to get additional volume, but you have to stay within a safe duty cycle or the injector just stays open or fails earlier than designed.

Ah hah, thank you for clearing that up. I'm new so I have plenty of stupid questions to go around lol.

I bought an already mildy built LS and got an a750 pump along with the Aeromotive regulator but I'm just now realizing I may not need that regulator because the stock fuel rails have a regulator built in. But thats a whole other deal...

 

[Bent77]

You probably don’t need it, and I’d certainly advise against having two in the same system, that does create flow issues

 

[Blue85]

Within the range of "normal" fuel pressures, it's kind of linear relationship between flow and pressure, but not 1:1. In this example doubling the pressure only gives 30% more fuel. So changing the pressure is kind of a tweak. The real flow changes come from swapping the injectors.

So that's the answer to your initial question. The injector works correctly only within a range of pressures. A 100lb injector may have a terrible spray pattern at the 15psi you'd need to run it. A 19 lb injector might wear out or spray terribly at the 90psi you'd need.

But there's more to the story. The chart above is basically at 100%. As you vary the duty ratio from 0% to 100%, it is non-linear towards the end. This is because it takes a certain amount of time to open and close the injector, during which some fuel is flowing. That's why they don't just put 100lb injectors in every car. When you need to idle at very low duty like 2%, it's non linear and you may not have enough resolution to keep everything stable.

Keep in mind that "fuel pressure", as it applies to the injector, is the pressure across the injector and not simply the pressure in the rail. The injector sees rail pressure at its inlet and manifold pressure at it's outlet. That's why many pressure regulators have a vacuum line on them. They raise/lower the rail pressure corresponding to the manifold pressure. This keeps the pressure across the injector in its linear range (so tuning is easier) and also gives some more "dynamic" range to the injector (difference between the least and greatest amounts of flow available).

 

[Keitha]

Another thing: The Lb rating of an injector is not fuel pressure. An injector rated at 40lb means that at a given fuel pressure, the injector can flow 40 lbs of fuel in an hour. Just like a water hose/nozzel, if you turn up the pressure, more fuel will come out. Fuel pressure is rated at psi - pounds per square inch.