I dont understand how poundage is rated for fuel injectors and how fuel pressure changes it. I have a pair of 90# injectors on my desk right now. How are they different from the 55 and 65 pounders in my car? Do they inject the same ammount of fuel at lower RPMs and just have higher capability at higher RPMs? Or do they inject more across the board? Are the pulses longer, or is there more fuel per pulse, or what? I dont understand how these work and what factors play a role in what makes the different from each other.

 

Flow is rated in either a volume or mass of fuel over a time base. Injectors are usually rated in pounds of fuel delivered per hour, grams of fuel delivered per second, or CCs per minute. P/H and CC/M are the most common ratings.

This peak fuel flow is affected by fuel pressure. Higher pressure will force more fuel though the valve and tip than lower pressure. A common pressure standard used to rate injector flows is 3 BAR, or 43.5 PSIG.

What makes the peak flow ratings differ are the injector tip and valve pintle sizes. A larger valve and larger set of holes in the tip will flow more fuel at a given pressure.

Any electronic injection system varies the fuel flow at different engine loads and RPMs by varying the amount of time the injectors are turned on, or open. This is the pulse width (or duration) of the injector signal.

Most injectors are rated for peak flow at an 80% duty cycle, or a pulse width which has them open 80% of the time and closed 20% of the time. This is considered the maximum safe sustained duration for injectors to remain energized without overheating and damage. Operating injectors in static mode (open 100% of the time) can create eventual overheating and solenoid winding damage within the injector.

The 90 pound per hour injectors sitting on your desk will flow 90 pounds of fuel at 43.5 PSIG if pulsed at an 80% duty cycle. The 55 or 65 PPH injectors in your TB will flow only 55 or 65 pounds under the same conditions.

You can increase the flow of almost any injector by increasing fuel pressure - to a point. The rate of variation is almost linear, or in other words, doubling fuel pressure would double flow - Almost. There are internal restrictions and physical limitations affecting flow, so the rate of change is not quite linear. Also, increased fuel pressure increases the effort the injector solenoid must exert to open the valve pintle against the pressure, thereby increasing the time delay and current required to open the injectors every cycle. This will reduce the fuel delivery somewhat, so that increasing pressure does not have a completely direct and proportional increase of injector flows under all conditions.

Further, there are two common types of injectors. There are "saturated" injectors and "peak and hold" injectors.

Saturated injectors are a simple solenoid coil energized by the control system directly, and have a moderately high current requirement while the solenoid core is being pulled from the closed to open position. These injectors get the same voltage to the coil once opened, and therefore must have adequate coil resistance to prevent excessive current and overheating once they are opened. This solenoid resistance also creates some delay in the operation of the core, so there is a brief time lag between the injector being powered and the valve tip actually opening (propagation delay). These are a common type used in older batch fire systems and relatively low flow applications.

Peak and Hold injectors have a much lower resistance solenoid coil. This helps the valve pintle open faster (lower propagation delay) for more precise fuel metering. This would also tend to cause rapid heating of the injector, except that the control system uses a different driver circuit for the injectors which allows a brief high current to open the injectors then reduces the current once the injector is opened to prevent overheating, thus the term "Peak and Hold." Larger flow injectors, those operated at higher pressures, and high RPM applications can benefit from this scheme, but the two types are not directly interchangeable. The control system must be designed to operate the injectors connected.

 

So that 90# rating is just a peak? They are not "always" flowing at 90#?

Is there any difference at all between a 90# injector and a 55# injector when the engine is just idling, the 90# just having a capability to go higher?

So the way I understand it there are 3 factors. Size of the valve, pulse duration (duty cycle), and the fuel pressure. Is there an equation that factors these all into play to determine how much fuel is entering the intake?

So the only thing that varies between injectors is the valve size? Theretically if I upped the valve size and lowered the fuel pressure but kept the duty cycle the same, there would be no difference? So I should be able to install the 90 pounders and lower fuel pressure for a nuetral effect on the engine?

Also, somebody made the FPR on my TBI adjustable. Which way is lower pressure and which way is higher pressure?

Another thing. I dont see how modding an engine can have an effect on the ammount of fuel that must be injected. There is an optimum air-fuel ratio for X RPM because the engine is filling its cylinders X ammount of times. How could ANYTHING, cam, heads, bolt ons, prom tuning, possibly change the required ammount of fuel? If I can reach redline with the setup I currently have, I dont see how bigger injectors can do anything except allow the engine to spin even higher.

 

if the injector was held open for 1 hour it would flow 90# of fuel. the rate of fuel doesn't change while the injector is open; the amount of time the injector is open determines amount delivered.

yes there is, the 90# injector will deliver about 40% more fuel each time it is opened by the ECM.


yes, but I don't know it off the top of my head.

why would you even want to do that?

more pressure on the spring = more FP

what any mod is trying to do is get more air into the engine, by improving it's efficiency. no normally aspirated street engine can pull in 100% of the volume of it's cylinders per 2 revolutions. any improvement you make towards that 100% mark, the more fuel you're going to need to burn.