If you are upgrading your injectors, it's important to make sure the fuel rail is up to the job. Stock fuel rails are designed to work with the stock injectors. The internal diameter of the rail, its design, location of supply and return lines were all engineered to move enough fuel for the stock fuel injectors. If the upgraded injectors move more fuel than the fuel rail can supply, fuel pressure in the rail will drop. If this happens, the pressure in the rail will be different at each injector and can cause lean conditions in one or more cylinders and possibly destroy the engine. When it comes to fuel rail design and selection, the key is to have equal distribution of fuel to each injector, which requires having equal fuel pressure across the entire length of the rail. Even mildly built engines can benefit from the even fuel distribution that a well-designed aftermarket rail can offer and have the potential to add a few horsepower to these cars by providing more equalized pressure to all the injectors. And on higher-horsepower cars, it can mean the difference between a perfectly tuned and running engine and one that blows up. So, don't overlook the fuel rail.
Single/Dual Feed: As mentioned earlier, ensuring equal pressure across the rail is key. In a single-fed fuel rail, the pressurized fuel comes in at one end of the rail and exits at the opposite end. This can lead to a drop in fuel pressure from one end of the rail to the other, which means each cylinder will get a different amount of fuel. Dual-fed rails have an inlet at both sides, which helps ensure equal fuel pressure throughout the length of the rail.
Bore size: Obviously, if the diameter of the fuel rail is too small, it can't supply enough fuel to all the injectors, even if the rail is a dual-feed type. On the other hand, a single-fed rail with a large enough bore can supply equal pressure to all injectors. Most aftermarket single-fed rails have large enough bores to accomplish this. Most OEM single-fed rails are not up to the job of handling larger injectors. That said; it is possible to modify a stock single-fed rail into a dual-feed. Just add a nipple at the opposite end. If you go this route, consult a professional, as fuel leaks and hot engines are a recipe for disaster. We've warned you.
Mechanical: It's rare to find a mechanical fuel pump on a fuel-injected car. Mechanical pumps are typically driven off the engine and operate at much lower pressures than electrical pumps made for fuel-injected cars. However, there are some ultra-high capacity mechanical fuel pumps that operate at high pressure. These pumps are typically found on full-blown 1000hp plus race cars.
Electrical: Electric fuel pumps are the norm for computer-controlled fuel injected cars. Most stock electric fuel pumps are good for up to 250hp, but some can support a bit more, or a lot less. Most of these pumps consist of a small electric motor connected to a set of gears. The gears turn at high speed and pressurize the fuel. A typical stock fuel pump can work at up to 65psi. Most fuel pumps are located in the gas tank.
Flow Rating: Most aftermarket fuel pumps are rated for how much fuel they can move and at what pressures they are designed to operate at. Most pumps are rated in LPH, or liters-per-hour. So... a 255lph pump should be able to move 255 liters in one hour. To put a number to that, a 255lph pump should be good for up to 450-500bhp (different from whp) on most cars (assuming the correct fuel pressure, injector size, etc.). While the ratings of these pumps are straightforward enough, their real-world performance isn't so simple. The amount of fuel a pump can move is dependent on the system's total pressure. As pressure is increased, the amount the pump can move decreases. For example, at 45psi, a given pump may be able to move 255lph. That same pump in the same car might only be able to move 200LPH at 90psi.
While it's technically not...
While it's technically not part of the fuel system, a wideband O2 sensor is a must-have when it comes to tuning the fuel system. A wideband sensor should be on your list of parts to get when putting together your fuel system.
High Capacity/High Pressure: Now that you know that the amount of fuel that a pump can move depends on the pressure, you can pick the right one. A general rule of thumb is to use a high capacity (or high flow) pump if you are using a standard fuel pressure regulator (1:1 rate) and to use a high-pressure pump if using a variable-rate fuel-pressure regulator. Typically, stock fuel pressures are around 40-45psi with the vacuum line to the regulator unplugged and the car running.
Additional Fuel Pump Info: Another thing to consider when upgrading the fuel pump is the power and ground for the pump. The pump gets its power from the fuel pump relay and a mess of other connectors and wires before it actually reaches the pump. All of these wires and connectors can decrease the voltage that actually gets to the pump, meaning it wont move as much fuel as it should. It's not uncommon to see voltage at the pump around 9-10 volts on some cars. An easy fix is to run heavy 10-gauge wires to and from the fuel pump relay and for the grounds. Another thing to remember is that the pump is in the tank and cooled by the fuel itself. If you regularly run the tank while low on fuel, it's possible to overheat the pump and drastically shorten its life expectancy. Lastly, when upgrading the pump, it's a good time to consider upgrading the fuel lines too. Most stock fuel lines are 5/16-inch in diameter, which is enough for around 400hp. 3/8-inch diameter lines should be good for most street applications. As a rule if thumb, for most street applications, the stock fuel lines are sufficient, assuming there're in good shape.
As we've said before, fuel pressure is just as important as injector size when it comes to how much fuel gets into the engine. There are two basic types of pressure regulators, and we'll break it down for you.
Standard Regulators: Fixed-rate fuel pressure regulators typically work on a 1:1 ratio. Most stock regulators are the fixed type. What this means is that every one psi increase in manifold pressure will result in a 1psi increase in fuel pressure. For example, say we have a naturally aspirated car with a fuel pressure around 32psi at idle (about -7psi manifold pressure). If we were to floor the car on the highway, the pressure in the manifold would increase to 0psi (remember it was negative at idle). This 7psi increase in manifold pressure would make a 1:1 fuel pressure regulator bump the pressure up by 7psi too; from 32 to 39psi. If the car was boosted and running 7psi, fuel pressure would increase by 7psi more, to 46psi.
Adjustable-Rate Regulators: These regulators are almost exclusively found on forced-induction vehicles. Just like the name implies, adjustable rate regulators have an adjustable gain ratio. For example, if the rate were set to 2:1, each one psi increase in the manifold would increase the fuel pressure by 2psi. The most appealing feature of these devices are that they will add extra fuel as a car goes into boost, eliminating the need for expensive fuel management. While they can do the job, they are generally not recommended to high-power applications because they cannot control the AFR throughout the rpm range. A gain of around 2.5 is typical.
Another often-overlooked part of the fuel system is also the most basic-the fuel filter. Obviously, the filter should be replaced on a regular basis on any car. And the need to have a clean fuel filter becomes even more important on a modded car. The reason for this is that as the filter gets older, it gets contaminated, which creates additional restriction and can reduce the fuel pressure at the injectors and can cause a lean condition, especially at wide-open-throttle when the car is most susceptible to engine damage. For high-power applications, a stock fuel filter might not be able to flow enough fuel to keep the fuel pressures at the proper level. If you reach this point, there are two options. One option is to use dual filters and the other option is to step up to a high-capacity aftermarket filter designed for high-horsepower cars. Don't overlook the filter.