Richard Holdener
October 1, 2012

The problem with this type of rising-rate fuel management system (FMU) is that it made life much harder on the fuel pump. Not only were we asking the fuel pump to support the additional power potential of the force-fed Ford, but also to do so at a greatly increased fuel pressure. Not surprisingly, many early attempts were met with less-than-stellar results. The cure was to increase the flow potential of the fuel pump.

Naturally, it was possible to install a better-flowing pump in place of the stock 88-lph 5.0L pump. The aftermarket soon supplied fuel pump upgrades ranging from 110 to 255 lph, but now in-tank pumps flow as much as 340 lph. It was also possible to install an inline fuel pump to work in conjunction with the stock (or upgraded) pump.

Aeromotive, Aerospace Components, Holley, Weldon, and a few others have designed high-flow inline (and in-tank) pumps to meet a variety of performance levels. The benefit of the inline pump is that it can be installed without dropping the gas tank, but there are other issues (like noise) associated with external pumps.

High-flow inline pumps help improve the fuel flow of the feed pump (stock or otherwise) by reducing the system pressure between the two pumps. Since the inline pump outflows many stock-type in-tank pumps (though many aftermarket pumps are now available to solve this), the pressure between the two pumps decreases. The reduced pressure helps increase the flow potential of the feed pump, thus improving the overall system. It is important to note that the limiting factor of the inline system will still be the pump with the lowest flow rate.

Another popular method, made famous by Kenne Bell, is to increase the supply voltage to the fuel pump. The now famous Boost-A-Pump improves the flow rate of an electric fuel pump by increasing the supply voltage. Most fuel pumps are (flow) rated at 12-13 volts. If you have a fuel pump rated at 255 lph at 13 volts, know that the flow rate will dramatically increase if the voltage supply is increased to 17 (or even 21) volts.

The supplied Flow Chart illustrates the increase in flow rate offered by the Kenne Bell Boost-A-Pump at various voltages and pressures. The Kenne Bell unit will always maintain a minimum of 13 volts, and can be dialed in to produce the desired (increased) voltage, and activated under boost or zero vacuum (wide open throttle). The benefit to desired activation of the Boost-A-Pump (especially on an early return-style fuel system) is that the pumps are not running full speed all the time.

Despite Internet rumors that the increased voltage will somehow diminish the life of the fuel pump, Kenne Bell has never had a single pump failure related to the proper installation of a Boost-A-Pump. Ease of installation, the ability to control onset and supplied voltage, and to increase the flow rate of nearly any electric fuel pump by 30-150 percent makes the Boost-A-Pump an attractive option, and all but mandatory on high-horsepower ('11-up) Mustangs.

Photo Gallery

View Photo Gallery

BSFC & Delta Pressure

While most enthusiasts understand the basic math that illustrates how much fuel is required to produce a given amount of power, things get complicated when talk turns to forced induction. Simply put, a supercharged engine producing 600 hp will require more fuel flow than a normally aspirated engine producing the same output. Furthermore, a supercharged application will be more taxing on the fuel pump. Why? The reasons are twofold, including a change in brake specific fuel consumption (BSFC), and a change in the (delta) fuel pressure (and attending drop in flow from the fuel pump).

The primary change in BSFC comes from a richer mixture. Turbo and blower motors require a rich mixture for cooling and to help suppress detonation. More fuel flow for a given amount of power equals a higher BSFC. Blower and turbo motors always require a drop-in ignition timing to ward off detonation, making power production safer, but slightly less efficient.

Another area of concern is the type of supercharger being used. According to Kenne Bell, a less efficient Roots blower will require more fuel than the twin-screw (or turbo) because of the increased parasitic loses associated with driving the blower. This is similar to to the rwhp difference on a chassis dyno between a manual and automatic transmission on the same engine.

On a blower or turbo engine, it is necessary to increase the fuel pressure at a 1:1 rate with boost. The reason is that the fuel flow out of the injectors and into the engine is a function of what is referred to as delta pressure across the injector. On a normally aspirated engine with a fuel pressure of 50 psi, the delta pressure is 50 psi, since there is no boost present. On a forced-induction application running 10 psi of boost and 50 psi of fuel pressure, the delta pressure will be 40 psi. Thus, only 40 psi of fuel pressure will be supplied to the engine since the 50 psi of fuel pressure must work against the 10 psi of boost.

If we supplied the system with a 1:1 regulator (or transducer on a return-less system) that increased fuel pressure in relation to boost, we would have 60 psi of fuel pressure working against 10 psi of boost pressure for a delta pressure of 50 psi. The problem is that the fuel pump must now work against 60 psi of fuel pressure to provide just 50 psi worth of fuel. The fuel pump flows measurably less (6-7 percent) at 60 psi than it does at 50 psi, so increasing the delta pressure has a positive effect on the fuel flow only if the fuel pump can keep up.

Photo Gallery

View Photo Gallery