Another great retro-looking...
Another great retro-looking EFI option is the Powerjection I EFI system from Professional Products. The system can really fool the die-hard car guys at the local cruise-in due to its standard carb look and ability to be used in a returnless-style fuel system with just an inlet feedline. The Powerjection bolts to any carb-based manifold as well for a simple upgrade. A port-style Powerjection II system is also available.
Size Matters
Fuel injectors are really nothing more than electronically controlled valves. When they're open, the fuel flows into the cylinder. If you don't supply enough fuel to the engine, it can't make the power that it's otherwise capable of. When sizing a fuel injector, the duty cycle must be considered. This is the ratio of the amount of time the injector is open during a given period. To maintain good control over the fuel delivery, the duty cycle shouldn't exceed 90 percent.
To get to the point where you can figure out what injector size will support what horsepower level, you'll have to either understand or blindly accept the idea of brake-specific fuel consumption (BSFC). This is a number that represents the amount of fuel needed to generate 1 hp at the flywheel over a one-hour period at WOT. The actual number can vary widely, according to the type of engine and any power-adders that are being used. For most production engines, the accepted number for BSFC is 0.50. Extreme examples of normally aspirated, pure-race engines can show numbers as low as 0.40, while supercharged engines are usually in the 0.60 range and turbocharged engines can run as high as 0.65.
Modular engines are Ford's...
Modular engines are Ford's bread and butter these days and the performance engine of the future. With factory installs ranging from 210 hp to 540 hp and crate and custom-build engines available upward of double those numbers, it's easy to see why these engines are being found in everything from Fox-body Mustangs to street rods. Ford Racing Performance Parts is currently working on an OE-style wiring solution to get one of these mod motors in your next project. Stay tuned for more info when it becomes available, and hang on for an aftermarket EFI installation in an upcoming issue.
With these two ideas firmly under our belts, we can move ahead. Let's say that your engine has been putting out 300 hp at the flywheel in carbureted form. To support that power level using fuel injectors, you need to know what size of fuel injector to use. Start by multiplying the flywheel horsepower by the BSFC. In this case, the engine is naturally aspirated, so we'll use a BSFC of 0.50. Then multiply by 300 hp. We'll need to supply 150 lb/hr of gasoline to the engine (300x0.50). Since this is a 5.0L engine, there are eight injectors, so each one must handle 18.8 lb/hr per cylinder (150/8). We also said that a 90 percent duty cycle is the maximum to use for reliable operation, so we have to add in this extra capacity.
Taking the current flow requirement per injector and dividing it by the duty cycle, we arrive at 21.0 lb/hr (18.8/0.90). The closest actual injector is 24 lb/hr, so we're not far off. You should never use an injector smaller than what you have calculated. Remember, if you short-change yourself on the numbers here, you're just placing an artificial limit on the maximum output your engine can have. Table 1 (p. 44) shows what maximum flywheel output levels can be supported by common injector sizes, both for normally aspirated engines (BSFC=0.50) and booster engines (BSFC=0.65).