Everyone knows the recipe for horsepower-getting more air and fuel into the engine. We've spent lots of time preaching to you about how to 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...
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 move at a static pressure (usually 43.5psi), measured in cubic-centimeters per minute (cc/min). For example, a single 550cc/min fuel injector will squirt out 555cc's of fuel in one minute, which is enough to fill up half of that empty 40 oz. bottle of Steel Reserve you've got chillin' in the garage. Multiply that times four (for a four cylinder engine with four injectors) and that set of 550's can move almost 80 ounces (0.63 gallons) 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 flows 440cc/min at 40psi fuel pressure can move 660cc/min at 60psi fuel pressure.
Using this formula, you can...
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.