Another choice is that of bank versus sequential firing of the fuel injectors. In bank mode (also called batch-fire mode), a batch of four injectors—assuming an eight-cylinder engine—fire at the same time. Typically, each batch consists of the injectors for one cylinder bank. In sequential, each cylinder's injector fires once for each time the spark plug fires.
No matter which of these modes you select, the computer relies on feedback from a wide-band oxygen sensor mounted in the exhaust collector. This sensor allows for “closed loop” fuel-flow adjustment, whereby the computer makes fine alterations based on actual engine performance. While some systems use a narrow-band sensor that simply toggles between telling the computer “too high” or “too low,” a wide-band sensor assigns an actual numerical value, ranging between 9:1 and 16:1, to the air/fuel ratio.
An extension of this feature is the XFI's ability to run in Flex Fuel mode. Yes, with a little tuning and the addition of a Flex Fuel sensor, the XFI will adjust to accommodate different fuels, including E85, LP, compressed natural gas, and others, all of which function at a different ideal stoichiometric ratio than the 14.7:1 of pure gasoline. While I'm not wild about running a Corvette with any of these, in this world of ever-changing fuels, it sounds like cheap insurance to me.
Other tunable parameters include the way the computer offsets the injectors' opening based on fluctuating voltage from the battery (it used to have a table embedded in the computer's firmware; you can now adjust it on your own as well), after-start enrichment and decay (basically a choke-like effect), and rev limiting with both fuel and spark.
While the XFI can control all of these variables, doing so is not mandatory. The system offers three different levels of tuning—basic, intermediate, and advanced—so you can choose just how deep into the computer you want to go. No doubt, the roughly 200-page manual can seem a bit daunting, especially for those of us whose tuning experience consists of turning a screw on a Holley and listening until it “sounds right.” With that in mind, I left the initial tune to a professional, and if you're in the same boat I am, you'd probably be wise to do the same.
There are, however, other parts of the tuning process that I'll be tackling myself later on, such as configuring the outputs for the shift light and tach. (As a side note, while the tach signal from a standard LS computer is usually a four-cylinder output, the XFI input is eight-cylinder—something you'll need to know if you intend to use aftermarket gauges, as we'll be doing.) The most important part we'll tune, however, is the traction control, which is contained in the intermediate tuning mode.
The recurring question about putting this engine in my car is whether or not the end result will be controllable. (In the words of my girlfriend's father, "So…what car are you going to drive my daughter in?") Personally, the most fearsome thing I've driven is a five-speed-equipped '71 Stingray with a 550hp 502. The idea of adding about a hundred hp to that and then expecting to drive it with a manual transmission, on hills and in the rain, is more than a little intimidating. Hence my appreciation for the XFI's intelligent traction control, or ITC for short.
ITC measures (and subsequently limits) wheelspin by using a shaft-speed input, usually with a sensor that's attached to the driveshaft. While FAST offers a series of different sending units that use a pickup and a separate magnet mounted to the driveshaft, these are typically set up for a Ford 9-inch rear. In our case, we'll be using the digital speedometer output from our T56 Magnum transmission for this purpose. The Magnum comes with both mechanical and electronic outputs, and since we're keeping a cable-driven speedometer, we won't have to worry about splitting the signal between the speedo and traction control.
The computer reads the shaft-speed input—not for overall speed, but for change in speed, since sudden wheelspin will cause a sharp increase in driveshaft speed. The degree of driveshaft acceleration at which tire slippage occurs is among the variables you can input into the computer based on your particular setup and needs. Once the motor goes in the car, we'll be focusing pretty heavily on getting this right, to make sure the car is as streetable as reasonably possible.
And exactly how much power are we expecting to tame with ITC? Our best numbers came in at 635 hp at 6,700, with a peak torque of 543 lb-ft at 4,800 rpm. On the final dyno sheet (which doesn't read below 3,178 rpm), the torque comes on at 476, never really dropping much below that, and stays above 500 from 4,400 to 6,600, making for a pretty stout midrange and top end.
Once testing was done, the engine was put to bed in a crate and couriered over to Tray Walden's Street Shop in Alabama to meet the six-speed trans, custom fuel tank, and all the other bits and pieces required to shoehorn it between the rails of our '72 coupe project car, aka Scarlett.