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Fuel Fight: Injectors vs Pump

Words and Photos: Richard Holdener

When it comes to maximizing performance, there is no better time to be an automotive enthusiast than right now. Not long ago, if you were rolling with 500 hp, you had little to fear out on the mean streets. Time and technology have marched on at such an accelerated rate that 1,000 hp has quickly become the new 500 hp. With forced induction making it possible to more than double the power output of a naturally aspirated motor, junkyard combos can easily reach four-digit power levels, with dedicated build ups going well beyond that! While blowers or turbos provide the required airflow, the other critical element in power production is fuel. Unfortunately, the fuel flow needs an otherwise powerful build-up often gone overlooked or miscalculated. This is especially the case when blower or turbo combos rely on the use of E85. Let’s check out two of the key components in the fuel system of a successful boosted application.

The fuel system for any boosted application consists of many things, including regulators, fuel line and filters, but the two major components we covered here include the fuel pump(s) and fuel injectors. From a most basic standpoint, the injectors provide the fuel to the engine while the fuel pump supplies the fuel to the injectors. Fuel pumps come in a variety of different shapes, sizes and (most importantly) flow rates. There are both mechanical and electric fuel pumps available, as well as differences in mounting locations and drive mechanisms. For this discussion, we will be focusing solely on the flow rates and how these rates are affected by both boost and system pressure. Unfortunately, the flow rate of any electric fuel pump actually goes down with fuel pressure. The pump has to fight against the pressure and the flow rate drops substantially with any increase in system pressure. On a boosted application with 20 psi using a boost-referenced (1:1 rate) fuel pressure regulator, the fuel flow drop from the increase in system pressure can be 15% or more, depending on the pump.

Before we get deeper into the math, let’s get a basic understanding of the fuel system. The fuel system for a typical (port) injected V8 consists of a fuel pump (in tank or inline mounted) feeding fuel lines up to the injectors. Somewhere in the system is a fuel pressure regulator, fuel filter and often times a return line from the regulator back to the fuel tank. On a return-less system, some of these components are eliminated or positioned closer to the pump itself. Using a return-style EFI fuel system as an example, the fuel is supplied by the pump through a fuel filter (the pump itself usually incorporates a pre-filter as well) up to the fuel rail. The fuel rail will generally have a fuel pressure regulator (often times adjustable) that determines the system pressure. The regulator dumps the excess fuel back through a return line to the fuel tank to maintain the system pressure. Most regulators include a vacuum/boost reference fitting that allows the fuel pressure to change with vacuum or boost. Vacuum and boost both apply pressure to the diaphragm mechanism and plunger in the regulator, with boost increasing and vacuum decreasing overall system pressure. The regulator must be sized (with the pump) to be able to bypass the required excess fuel at lower demand levels. Big power and pumps require high-flow regulators.

The increasing system pressure caused by the boost-referenced fuel pressure regulator does make life harder on the fuel pump, all without actually increasing fuel flow. What do we mean? Well, as you increase the boost pressure, the fuel pressure will increase from the boost referenced regulator. Unfortunately, the fuel flow from the pump drops with increased pressure, meaning the power output (fuel flow) the pump will support at 80 psi is significantly less than at 40 psi or 60 psi. The other downside is that the presence of boost pressure offsets some of the fuel flow through the injector. If you increase the fuel pressure on an injector, you usually get an increase in flow. If that increase in fuel pressure behind (feeding) the injector is accompanied by a corresponding increase in boost pressure on the opposite side (in front of) the injector, there will be no increase in fuel flow. If you have 60 psi of fuel pressure feeding the injector and 20 psi of boost, you have a delta of 40 psi, meaning the injector will flow like it has just 40 psi of fuel pressure (and not 60 psi).

If you have a drop in fuel flow from the pump at higher system pressures associated with running boost, combined with no increase in fuel flow from the injector, what should you do? The answer that cures both of the problems is to simply run larger injectors combined with less static fuel pressure. This accomplishes two things, the first of which is increased fuel flow through the injector at any pressure. As a guideline, FAST offers a number of injectors for many popular applications, ranging from 33-85 lbs/hr. To put these flow rates into perspective, simply multiply the flow rating by 16 to get the amount of power they will support in normally aspirated trim. For example, the 33-pound injectors will feed a 528-hp (normally aspirated) motor, while the 85-pounders will supply enough fuel to support 1,360 hp. Of course, this assumes 100% duty cycle and a BSFC of .50. It is usually not recommended to run the injectors at 100% duty cycle, but most LS applications run much more efficient than the .5 BSFC number, so the estimates can still be fairly accurate. These numbers change when running under boost, as the motors must run richer, meaning a higher BSFC number of .55-.60. This drops the power potential of the 85-pound injectors to a tad over 1,130 hp (at .60 BSFC).

The second benefit of running 1,000-hp injectors on your 800-hp motor is that it will allow you to decrease the static fuel pressure. Since you don’t have to rely on elevated fuel pressure to reach the desired flow rate of the injector, life gets much easier on the fuel pump. If you drop the static (system) fuel pressure by 20 psi, the flow rate of the pump goes up by roughly 15%, meaning a pump that might struggle to support 1,000 hp can now support and extra 150 hp. This will also likely increase the life of the pump. In case we haven’t made it clear, the answer to your boosted fuel pressure woes is to combined BIG injectors with sufficient fuel pump flow (a stock one won’t do). Of course, it is also possible to increase the flow rate of the pump with a voltage amplifier that raises the fuel pump voltage (think Kenne Bell Boost a Pump). The flow rate of a fuel pump is a function of the voltage supply to the pump. Most automotive systems are listed as 12V, but thanks to the alternator, they can as high as 14 volts. The higher system voltage increases the flow rate of any fuel pump, but even with the extra voltage, big injectors are always a good idea.

FAST Injector Sizing, Applications, Plug Style and Horsepower Rating

Flow Rate   Application/Inj Style   Plug Style HP Rating (NA)  HP Rating Boost

39 Lbs            LS3                            USCAR     624 HP                520 HP

50 Lbs            LS3                            USCAR     800 HP                667 HP

65 Lbs          LS3                              USCAR   1040 HP                 867  HP

85 Lbs          LS3                              USCAR   1360 HP                1133 HP

33 Lbs          LS2                              USCAR     528 HP                 440 HP

46 Lbs          LS2                              USCAR     736 HP                613 HP

57 Lbs          LS2                              USCAR     912 HP                 760  HP

85 Lbs          LS2                              USCAR   1360 HP                1133 HP

36 Lbs          Univ                             Minitimer  576 HP                 480 HP

60 Lbs          Univ                             Minitimer  960 HP                  800 HP

220 Lbs        Univ                             Minitimer 3520 HP                2933 HP