Richard Holdener
October 1, 2012

Pressure Down & Injector Up

In terms of the pump, fuel flow and pressure are inversely related. Raising the fuel pressure increases the fuel flow through the injectors, but decreases the flow capacity of the fuel pump. The ideal situation for elevated power levels is to decrease the fuel pressure, but increase the fuel flow. How is this possible? The answer is to run larger injectors.

An example works well here. If you are trying to make 600 hp with 36-lb/hr injectors (flow rated at 43 psi), it will require considerably more fuel pressure than 43 psi. Math (injector size x 16) tells us that 36-lb/hr injectors can support roughly 575 hp at 43 psi with a 0.5 BSFC. Raising fuel pressure to 50 psi, 60 psi, or even 70 psi, will increase fuel flow through the injector, but remember that we must subtract the boost pressure from the fuel pressure to reach the true delta pressure to the motor.

The problem with this scenario is that the elevated pressure will increase fuel flow, but only if the fuel pump is able to support the additional flow at the desired pressure. A review of the fuel flow chart illustrates that dramatic drop in fuel flow with increased pressure. The cure is simply to increase the size of the fuel injector to 42-, 50-, or even 65-lb/hr and drop the fuel pressure to make life easier on the fuel pump. This is especially true of the system run on the '11-up Mustang, in which the internal bypass of the pump limits fuel pressure at the injector to around 55 psi. More pressure drop is evident a higher boost (and horsepower) levels.

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The injector flow rating can be increased (or decreased) with a change in the fuel pressure. Most injectors are rated at 43 psi. The maximum power level is obtained by dividing the flow rating by the BSFC number then multiplying by the number of cylinders. In the case of the 19-lb/hr injectors, the formula looks like this:

Max power = 19/0.5 BSFC x 8, or 19/0.5 = 38 x 8 cylinders = 304 hp

It is possible to increase the power output of a given flow rating by improving the BSFC number. If we use the 19-lb/hr injector and run a (more efficient) engine with a 0.4 BSFC. We get the following:

Max power = 19/0.4 BSFC x 8 = 47.5 x 8 = 380 hp

The reverse is also true, where the power potential of the injector is decreased with an increase in the BSFC number (from 0.5 to 0.6). This is typical of forced-induction applications that run reduced timing and richer air/fuel mixtures. Once again running the 19-pound injectors, we see the following:

Max power = 19/0.6 BSFC x 8 = 31.66 x 8 = 253 hp

System Flow & Pressure Loss

Fuel pump flow is all well and good, but the fuel flow from the pump must reach the injectors before it can supply the engine. Unfortunately, there are a number of obstacles along the way that can diminish fuel flow and make life harder on the pump.

The first might be a dirty screen located before the pump. This filter (or sock) keeps debris in the tank (like rust or other particles). Next on the list are the fuel lines and fittings associated with the pump assembly. The size of the line(s), number of fittings, and number of turns between the pump and exit of the pump assembly (hat), all combine to diminish fuel flow. We see the diminished fuel flow in the form of a drop in fuel pressure.

Next up are the actual fuel line(s) used to run fuel from the rear- (tank-)mounted pump up to the fuel rails. Once again, the length of line, fittings, filters, and bends all contribute to drop the fuel flow and pressure. The rails are the final hurdle for the fuel system. Testing has demonstrated the losses associated not just with the entire system, but with the individual components.

The Kenne Bell fuel flowbench is a state-of-the art flow system that allowed us to test the flow rate of the various fuel pumps at different pressures and voltages, and also individual components of the systems, including hats, fittings, lines, and rails. Testing the complete fuel system (pump, hat, lines, and rail) on the flowbench illustrated a drop in fuel pressure of 7.5 psi (registered from the outlet of the pump to the rail).

Obviously, this would decrease the maximum fuel flow potential of the system on an '11-up Mustang GT, but don't be too quick to run out and purchase a whole new fuel system for your Stang. Despite the drop in fuel pressure and the internal bypass valve of the stock pump limiting maximum system pressure, it is still possible to exceed 800 rwhp with the stock fuel pump, lines, and rail.

Running with a Kenne Bell 3.6L supercharger on an otherwise-stock Boss 302, Kenne Bell was able to eclipse 800 rwhp using nothing more than larger 80-lb/hr injectors and a Boost-A-Pump. If larger (12-14 ohm) injectors were available, even more power would be possible with the stock system, but 800 hp covers a minimum of 95 percent of the street Mustangs. If you are looking to take your new GT beyond this power level, look for multiple pumps and a complete (and expensive) revision of your fuel system.

Another area of concern is the wiring, or more accurately, the voltage supply to the fuel pump. We all take for granted that having the fuel pump running means it has plenty of voltage, but testing has shown that even turning on lights and other electrical accessories can diminish the voltage to your fuel pump. While your supercharged engine might love the cool night air, the fuel pump might not keep up due to reduced voltage.

The same goes for the actual wiring, especially when switching over to larger or multiple pumps. Wiring (and associated fuses) must be sufficient to carry the required current to the pump. Speaker wires aren't going to get the job done for that Aeromotive A1000 or dual (or triple) in-tank pump assembly. Testing on a supercharged 5.0L illustrated a drop in air/fuel ratio of over 0.5 points from simply turning on the lights and stereo.

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