Fuel Injection System Selection

Making the most out of a fuel injection system begins with the system selection. Not all fuel injection systems are the same and there are many things to take into consideration before making a purchase. The most important part of any fuel injection system is the actual injector and its placement. It’s extremely important that the injector’s placement allows it to aim fuel directly at the intake valve. The fuel vaporizes when it makes contact with the hot valve, delivering the entire, finely atomized, fuel load into the cylinder. If the injector is not aligned properly or has an incorrect spray pattern, it will deliver fuel directly against the intake wall causing a poorly atomized fuel mixture, as well as inconsistent and poor combustion.

Correct Injector Angle	Incorrect Injector Angle

Contrary to popular belief, an injector's spray pattern and its actual atomization are two seperate entities. A wide spray pattern does not automatically lead to efficient atomization, nor does a narrow spray pattern. More importantly, injector design and fuel pressure determine the level of atomization in a fuel injection system. In very high RPM engines a narrow spray pattern provides better performance in the useable racing RPM range (8,000 to 15,000+RPM). Wide spray patterns provide better performance below 6,000RPM, well below the useable racing RPM range. Even on a track that uses the broadest RPM range, injectors with a narrow spray pattern will still be a better choice.

Another important aspect of the injector is its response time, or the time it takes to open once energized. At 15,000 RPM, the injector has only 4 milliseconds to deliver its entire fuel load while the intake valve is open. The stock fuel injector only takes 1 millisecond to open, while some of the older, larger design injectors with their heavy internal parts take up to 1.8 milliseconds to open. That takes up nearly half the time available to deliver the fuel load and much too long for optimum high RPM performance. Due to cost and availability, these larger, old-style injectors are widely used in most fuel injection conversion systems. It’s also important that the fuel flow rates are closely matched across all injectors in the system. This maintains even fueling across each cylinder allowing air/fuel ratios to remain constant while maximizing efficiency. It is also extremely important that the injector selected is appropriate for your fuel requirement. For example, if you use alcohol to fuel your racecar it is important that the fuel injectors selected are appropriate for use with alcohol. Finally, electrical compatibility is a must in order for your system to function properly. The stock fuel injection controllers are designed for “saturated” injectors. These injectors have a high coil resistance (12 to 18 ohms) and only draw about 1 amp. The other type injector is a peak/hold injector. These injectors have a low coil resistance (1 to 5 ohms) and require a special injector driver circuit for proper control. If a peak/hold injector is used in a system designed for a saturated injector, the injector driver circuit can be permanently damaged.

In conjunction with the proper injector selection is fuel rail selection. Each fuel rail must be custom designed to handle the increased fuel requirements and hold the injector in place securely. A large plenum fuel rail is also important for even fuel flow to each injector. The stock fuel rail will not be able to supply the additional fuel required in an alcohol system. The fuel rail must allow for proper injector alignment and hold the injector securely.