When the mechanical diesel engine days came to an end in the mid '90s, it was the dawning of the electronically controlled diesel era, which has ballooned into the current plug-and-play culture we're now in the midst of. From the early days when six-position chips were used to hijack your engine's Powertrain Control Module (PCM) to today's endless custom tuning options for any make and model, the aftermarket has come a long way. Whether it's a programmer, tuner, pressure box, module, or chip, there is something out there for everyone, as electronic power adders are now just as common as the diesel engines they interface with.
Chips
In its most basic form, an aftermarket computer chip plugs into the factory computer, effectively overriding preset parameters. It allows the engine to make more power through increased fuel injection timing and injection pressures. In addition, the rev limit of the engine can be increased. The result is that you gain fuel efficiency, and more power is on tap. However, the result is two-fold: while better fuel economy can be achieved, driver input (via the right pedal) is the sole determining factor.
An early example would be the six-position chips for '94-'03 7.3L Power Strokes, which, due to OBD-II not being mandated until 1996, took advantage of the existing PCM location. Without downloading any information onto the computer, the chips simply piggyback to the back of the PCM's circuit board (J3 service port). Through the use of automotive tuning software, these hardwired chips can be re-programmed with different tunes by many different aftermarket companies.
Programmers
Changing the factory-programmed settings is possibly the easiest modification you can make. Also called downloaders, or tuners in some instances, a handheld programmer downloads custom tuning by connecting to the vehicle's Assembly Line Data Link (ALDL) or OBD-II port under the dash. Programmers download information directly onto the PCM, making power output the main concern, rather than efficiency, emissions, and drivability. Along with increasing injection timing, injection pressures, pulse width, boost, and the engine's peak operating range, some programmers can also raise the top speed limiter, recalibrate the speedometer to accommodate larger tires, firm up transmission shifts, and read diagnostic trouble codes (DTC).
Some programmers can be adjusted on-the-fly, while others can only operate on one setting at a time, requiring you to turn the truck off, or pull over in order to download a different program. Off-the-shelf programmers with pre-loaded tuning will offer considerably more power over stock for the entry-level enthusiast. On the other hand, programmers that can be uploaded with custom tunes have the potential to unlock serious amounts of power over stock. Best of all, many programmers currently on the market can be updated using install software, and the Internet. New and improved tunes can even be sent and received by email.