Planning Your Fuel System

What you need to know to do it once and do it right

Everyone knows the recipe for power – getting more air and fuel into the engine. You can accomplish this through volumetric efficiency mods – like forced-induction, bigger cams, headwork, etc. – but without enough fuel, it’s impossible to take full advantage of those mods.

Stock fuel systems are great for stock or mildly-modded rides, but when the time comes to start making some serious power, it’s time to take total control of the amount of fuel that gets into the engine – this is where aftermarket engine management comes into play.

Even with the best aftermarket fuel management, if you are making much more power than stock, you risk of maxing out of injector. What happens in this situation is that the injectors will deliver as much fuel as they physically can, but still won’t be able to provide enough for the engine. This will result in a lean condition, which can cause detonation and a blown engine. This is why it’s important to have the correct sized injectors for your modified car.

While electronic boxes and trick fuel-pressure regulators can control how much fuel the injectors actually squirt into the engine, they can’t magically make the stock injectors any bigger. If the time has come to step up to some bigger injectors -congratulations. But take notice: putting on bigger injectors without addressing the rest of the fuel system is asking for trouble. When the injectors are changed, it’s important to make sure the other stuff is up to the job; namely the fuel rail, lines, regulator and fuel pump. If you’re considering beefing up your fuel system, we’ll assume that you already have some sort of adjustable fuel control on the car (like a standalone ECU) to control the upgraded injectors and fuel system.

Remember, you can’t just slap bigger injectors on a car without some sort of fuel management… it’ll just run pig-rich all the time… if it runs at all. The same goes for increasing the base fuel pressure. So sort out your fuel management first.

Here are some examples of the different types of injectors. From left to right, the first four injectors are of the Top-fed variety. The two on the right are side-fed. As you can see, the side-feed injectors take the fuel in from the middle of the injector, while the top-feeds are just that-top-fed.

Picking the correct sized fuel injectors isn’t rocket science. We’ve included a simple formula to determine optimal injector size that takes into account things like engine displacement, volumetric efficiency, maximum engine speed, air/fuel ratio, etc. This formula gives a good indication on the size of injectors needed for any specific application.

But contrary to popular belief, there is more to picking the right injector that getting one with the correct flow rate. Obviously, if the injector doesn’t physically fit, it won’t work… that’s a no-brainer. But there are less-obvious things like the injector’s internal electrical resistance might mean that an injector that physically fits still won’t work properly. To help you sort though the jargon and figure out what’s best for you, we’ve broken down a few things you should know about injectors before you throw down your hard-earned cash.

Flow Rating: The flow rate is a measurement of how much fuel a single injector can flow at a static pressure (usually 3bar or 43.5psi), measured in cubic-centimeters per minute (cc/min). For example, a single 550cc/min fuel injector will squirt out 550cc’s of fuel in one minute, which is 591ml of fuel. Multiply that times four (for a four cylinder engine with four injectors) and that set of 550’s can flow just more than 2 liters of fuel in one minute.

Pressure rating: Fuel pressure is just as important as injector flow rates when it comes to how much fuel the injectors expel. It’s a simple concept really. The higher the pressure at the injectors, the more fuel they will flow-even though the injector open time and injector size are the same. In theory, an injector that flow 440cc/min at 40psi or 2.1bar fuel pressure can flow 660cc/min at 60psi or 4.1bar fuel pressure.

Using this formula, you can find the approximate injector size for your modded engine. If your car is boosted, use 0.60 for the BSFC value and 0.50 if it’s naturally aspirated. B.S.F.C is brake specific fuel consumption-how much fuel you are using per horsepower per hour. We’ve included a formula to convert lbs/hr to cc/min, the latter of which is the standard for imports.

Impedance: Like all electrical devices, injectors have an internal resistance to the flow of electricity. The amount of resistance is measured in Ohms, or impedance. The higher the number, the more the resistance. Injectors are either high impedance or low impedance. It’s crucial to get the proper type. While high and low impedance injectors may look identical on the outside and fit the same, if they are the wrong type they can fry your ECU, injector resistor pack, or other expensive parts. High impedance injectors are sometimes called “peak and hold” injectors, while the low impedance injectors are often called “saturated injectors.” Make sure you get the right type. If you’re not sure what you’ve got, you can check the resistance of your injector with a DMM (digital multi meter) set to measure Ohms. Saturated injectors have a resistance between 10-18 ohms and peak-hold injectors have a resistance around 2-6 ohms.

Top Feed/Side Feed: Most injectors are either top-fed or side-fed. They both work the same way, but as the names imply, top-fed injectors take fuel in from the top of the injector, while side-fed injectors have their inlets on the side of the injector. Each will only work with fuel rails designed for that type, so make sure you know which type you have before buying injectors. If you have a side-fed rail and injectors and are upgrading your system, consider converting to a top-fed rail-there is a bigger selection of aftermarket top-fed injectors than side-fed.

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 flow enough fuel for the stock fuel injectors. If the upgraded injectors flow 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 kilowatts to these cars by providing more equalized pressure to all the injectors. And on high powered 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.

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 petrol tank.

Flow Rating: Most aftermarket fuel pumps are rated for how much fuel they can flow 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 flow 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 flow is dependent on the system’s total pressure. As pressure is increased, the amount the pump can flow decreases. For example, at 45psi, a given pump may be able to flow 255lph. That same pump in the same car might only be able to flow 200LPH at 90psi.

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 flow 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 won’t flow 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 of 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 is 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.

Fuel Filters: 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-powered cars. Don’t overlook the filter.