The venerable carburetor has soldiered on because it's simple and performs extremely well. Some people have argued that switching to EFI can be worth a few extra horsepower, but even David is quick to debunk that myth. "If you have a carburetor that's properly dialed in, you may not get any more peak power with EFI. In fact, because the fuel is atomized higher up in the intake manifold, sometimes a carb will make more power," he says. "On the other hand, EFI will make more torque because you can optimize the air/fuel ratio much more precisely at low to midrange rpms. Plus, fuel economy, driveability, and cold-start performance will be improved dramatically, and emissions output will be reduced as well. And let's face it, some people just want the cool factor of having a muscle car with high-tech hardware like EFI."
Unlike carburetors, which continuously discharge fuel whenever the throttle cracks open, injectors squirt fuel into the combustion chamber in short pulses. Although it's rarely an issue in moderate-horsepower, low-rpm applications, proper injector timing is critical in more radical combos where the time required to fill the cylinders with air and fuel is dramatically reduced. "Injector events can't be too long or too short. They have to be just right," David says. "To get a complete charge of fuel into the chamber, the injector pulse should be optimized in relation to the opening of the intake valve. Injector sizing also plays a role, and bigger isn't always better. If your injectors are too big, then their pulse widths will be too short for any given volume of fuel, leading to poor atomization. Our XFI system gives tuners the ability to adjust the injector timing."
Cars powered by GM Gen II, III, and IV small-blocks can have their stock computers tuned with a laptop using software systems offered by several aftermarket manufacturers. Some even have the ability to tune in real time, and use two- and three-bar MAP sensors for forced-induction applications. While the limits of these tuning solutions are extremely high, there's still a point where stepping up to a stand-alone system is necessary. "I'm always amazed at the level of power guys can make with a stock PCM, but stand-alone EFI software is much more flexible and easier to use," David says. "For power-adder applications, you can change the timing for boost or nitrous on fly with a stand-alone setup. Also, the injector drivers are much more robust in a stand-alone system, which makes it possible to run low-impedance injectors or even dual injectors per cylinder. Integrated data logging and traction control capabilities are a huge plus for hard-core racers as well."
Pump and Injector Sizing
Since EFI operates at much higher fuel pressure than a carbureted induction system, upgrading to fuel injection requires greater fueling demands. For naturally aspirated combinations, David recommends selecting a fuel pump that flows 1/2 lb/hr of fuel for every horsepower. In other words, a 1,000hp naturally aspirated gasoline combination needs a pump that can flow 500 lb/hr at working pressure of fuel plus a 10-20 percent safety margin. For forced-induction applications, David prefers fuel pump flow rates of 0.60-0.65 lb/hp plus 10-20 percent. Proper injector sizing can also be calculated in a similar fashion. "Injectors are all rated at a certain fuel pressure, and increasing fuel pressure can bump up the flow rate of an injector. Typically, 42 lb/hr injectors are enough to support 600 hp, because 42 lb/hr multiplied by eight injectors gives 336 lb/hr of flow. Multiply that by 2 to get 672 hp minus 10 percent for safety margin comes to right at 600," David says.