Words and Photos: Richard Holdener
Losing control is not usually a good thing (okay, maybe once in a while!), but not having full control of the tune on your performance motor is definitely bad. The advent of self-tuning EFI systems has simplified converting a carbureted performance motor to modern fuel injection. These systems, like the EZ-EFI offered by FAST, rely on the oxygen sensor to provide information to the ECU to determine the air/fuel mixture. The ECU then compares the current air/fuel reading to the desired reading, and automatically adjusts accordingly. Assuming you have input the proper desired air/fuel mixture for the conditions, these self-tuning EFI systems can greatly improve things like drivability and fuel mileage over a conventional carburetor. The problem that often occurs with carburetion is, there is usually some sort of compromise. Very rarely can you get the exact desired air/fuel at idle, cruise and WOT with a carburetor. Since a carburetor cannot be tuned for specific rpm and load conditions, it is often necessary to settle for the best overall (compromised) combination.
To illustrate the potential gains offered by upgrading from carburetion to fuel injection, we took FAST up on their offer of testing an XFI Street system. Having previously run a small-block Ford with the self-tuning, EZ-EFI management system, we wanted to explore the possibility of tuning the combination ourselves. Luckily, FAST offered just such an upgrade in the form of the XFI Street management system. Designed as an upgrade to the EZ-EFI system, the XFI Street utilized the same wiring harness and throttle body employed on the self-tuning EZ-EFI system. The XFI Street was also available as a complete system with the throttle body and harness. According to FAST, the XFI Street was designed as a cost-effective alternative to enthusiasts wanting complete control of their normally aspirated combinations without the need to step all the way up to the complete, high-end stand-alone systems. The XFI Street can be used with the four-hole, EZ-EFI throttle body or, with a different harness designed for multi-port injection. We liked that the XFI Street also featured data logging when connected to a laptop computer.
To put this system to the test, we employed it on one of our favorite Ford test motors, the BPE 306. Originally supplied as a crate short block, the BPW test motor included a late-model, 5.0L hydraulic-roller block (bored .040 over) machined to accept a cast crank, 5.155-inch rods and hypereutectic pistons. Also present was a healthy hydraulic roller cam (.543/.554, 218/226 112 lsa), lifters and a front cover. The short block also featured an oiling system that included a production Mustang oil pan, pick up and high-vol oil pump. To complete the 306 Ford, we installed a 50-oz damper, a set of BPE as-cast, 190-cc aluminum heads and dual-plane, Eliminator intake from Speedmaster. Additional touches included a 650-cfm, four-barrel carb, billet distributor and 1 ¾-inch, long-tube headers. After adjusting the air/fuel (with jetting) and timing, the carbureted BPE 306 produced peak numbers of 371 hp at 6,100 rpm and 367 lb-ft of torque at 4,000 rpm. This was a healthy, carbureted small-block Ford ready for action, but we were not done yet. Now it was time to take it to the streets!
The EZ-EFI throttle body installed just like a carburetor, with four mounting bolts. The throttle body featured four injectors, a throttle position sensor (TPS) and inlet air temp sensor (IAT). The throttle body was supplied with a throttle ball to allow easy hook up on a typical carbureted throttle linkage. The throttle body also featured two fuel rails equipped with AN fittings, an idle air control (IAC) valve and various vacuum fittings for things like brake boosters, PCV and vacuum-advance distributors. It should be pointed out that the Street XFI was only used to control air/fuel and we relied on mechanical adjustments to dial in the timing curve. The software was easy to use, even for a first timer and we had the small block up and running in no time. Using the FAST XFI system, we were able to dial in the air/fuel curves precisely at all load points, including idle, part and full throttle. There was no need to compromise peak-power air/fuel because of a lean spot elsewhere in the curve. Dialing in the air/fuel resulted in some extra power, as the injected 5.0L Ford now produced 383 hp at 6,200 rpm and 371 lb-ft of torque at 4,300 rpm. The injection not only improved peak power, but offered gains through most of the curve thanks to an optimize air/fuel mixture. Improved idle and drivability were just icing on the Mean-Street cake!
Graph: FAST Street XFI 306 Ford
Sometimes you just want a little control. In this case, we wanted the ability to precisely dial in the air/fuel curve rather than settle for a compromise. Run first with the conventional 4-barrel carburetor, the 306 produced 371 hp at 6,100 rpm and 367 lb-ft at 4,000 rpm. After installation of the Street XFI, we were able to tune a little more power out of the combination. Once dialed in, the injected 306 produced 383 hp and 371 lb-ft of torque.