In last month's installment of our Building the Beast LS3 engine build series, we covered the assembly of the top end of our 416ci LS3, a process that involved bolting the RHS heads and FAST intake onto the previously assembled short-block. After a few days spent at Grimes Automotive Machine, watching Merlin James and Garry Scott Grimes machine and assemble the engine (and turning the odd wrench myself), the collection of high-end parts had finally become a single, tough-looking unit. The aluminum engine—crowned with the matte-gray intake, gleaming red fuel rails, and high-rise valve covers in crinkle black—looked nothing but purposeful. And since its purpose is to go fast, it was time to move it off the engine stand and onto the dyno, to see exactly how ill- tempered this beast could be.

By the time I got there, the motor was in the dyno cell, surrounded by the various black umbilici connecting it to the many inputs and outputs required to make it run: fuel lines, sensors for the dyno gauges, and wiring for the MSD ignition box and FAST XFI computer, along with a set of well-heat-cycled headers routed to the outside of the building. Garry Grimes sat at the controls, his left hand resting on the lever that controlled the throttle, the computer monitor above him filled with the brightly colored lines of a bar graph. John Lee, who was doing the actual tuning, sat on his right, leaning over his laptop and alternately scrolling through data-logging results and air/fuel tables.

The drill is simple: With the engine having previously been warmed to operating temperature, the Start switch is pressed until it comes to life and drops into its restless idle, provided courtesy of the 0.605/0.615 lift cam from Comp. Check the needles on all the gauges to be sure they're on the safe side of the middle; make sure all the inputs look right on the computer. Then, gently at first, Garry's hand begins smoothly pushing the lever forward, and that saucer-sized throttle body turns from a gold-colored disk to a flat line, and just for a moment as it hits wide-open throttle there's an expectant pause. Secretariat inhales. Then the engine's steady hammering changes tone as it sucks in air and goes from a snarl to a roar, the needle on the big, white tach hits the 6,800 rpm redline, and it's time to throttle it down and take a look at the charts.

Garry hands me a dyno sheet, and I can't help but scan the fine, dot-matrix type for the bottom number on the page, the bragging one. 595. As we continue to run the motor and Lee makes fine adjustments via his laptop, the peak horsepower continues to climb. It goes over six quickly. 625. Still climbing, and always hitting the peak around 6,700 rpm.

The volumetric efficiency (or VE) of a motor is basically a mechanical function determined by such things as cam, compression ratio, and other factors, and it can't really be changed by tuning. The VE table used in tuning is generally used as a fuel table: When you change its settings, you're effectively telling the engine computer that the engine is using more or less air, and the computer adjusts the fuel accordingly. This adjustment of air/fuel ratio is key in getting the engine to produce peak power while remaining in a safe range, so it doesn't run too rich (too much fuel) or too lean (too little fuel), either of which could cause damage.

To get there, the FAST XFI lets you read manifold pressure, spark timing, injector pulse, and other factors, as well as controlling such esoteric variables as injector phasing (similar to timing advance, only with the fuel injectors rather than the spark plugs). After the data is reviewed, changes are made to the calibration to adjust timing and fuel flow as required to optimize power.

Although we had originally planned to rely on the LS3's stock E38 computer, we eventually opted for the FAST XFI, in part because our stroker uses a cable-mounted throttle body, which is incompatible with the E38. The XFI's other advantages, however, are significant, making it a well-justified upgrade even without that consideration factored in.

Specifically, it gives the user an amazing amount of control over the engine, including everything from basic functions to ancillary items such as fan temperature and fuel-pump operation, torque-converter lockup, and controls for forced induction and nitrous oxide—with a provision for up to four stages of the latter.

Another unique feature that makes the XFI very appealing is its ease of tuning. In the event you need tuning help and no one suitable is available locally, you can connect your laptop to the XFI, establish an Internet connection, and allow another user to connect to your ECU, see what's going on, and change parameters as necessary.

As a matter of basic setup, the XFI can run the motor in several different modes, including speed density and a “load indexed” speed density that compensates for variations in barometric pressure. Ever driven in the mountains with a car tuned for the coast? That's the sort of problem load-indexed speed density is designed to avoid.