One of the worst kept secrets in the aftermarket performance products world is the fact that enormous power gains can be achieved in a diesel truck through the installation of digital tuners. With the alteration of air-fuel maps in the PCM, the engine will have improved throttle response, horsepower, and torque.

Diesel engines are capable of running on a wide range of air-fuel maps, and huge gains can be made. This is most noticeable in the low- to mid-RPM range, where it is not uncommon to have torque increases in excess of 100 lb-ft. This gives the truck better towing and acceleration characteristics. Above and beyond adding fuel and air, these tuners often also offer a truck load of other features.

One such tuner is the Bully Dog GT Platinum Diesel tuner, and this handy little device is the company’s best-selling product. We grabbed a GT Platinum Diesel Gauge/Tuner, and decided to check it out thoroughly by installing it on a 2005 Ram with the 5.9-liter Cummins. The Bully Dog unit for diesels combines four primary features into one controller: engine tuning, engine monitoring/safeguards, gauges/performance-capturing, and diagnostics. In addition, 49-state and 50-state versions of the GT Diesel are available.

Diesel engines in late-model pickups can gain a lot more from the Bully Dog tuner than power. Amy Kress of SCT/Bully Dog says, “We also deliver a digital monitor with gauges. When you modify a diesel it’s important to monitor key performance parameters. You want to make sure everything is running correctly and your truck is functioning properly.”

The Bully Dog GT Platinum Diesel tuner power tuning module features a choice of three power modes, each of which is easily selectable by the driver.

The first is mode Tow/Economy, the basic mode you would most likely run day-to-day on the street. It is claimed to add as much as 50 hp and 100 lb-ft of torque.

From there, you can progress up to the Performance mode, Bully Dog says this will deliver up to 75 hp and 125 lb-ft of torque, and Bully Dog advertises that the Extreme mode will net you an increase as high as 120 hp and 250 lb-ft  of torque. These numbers are for its 49-state model of the GT Diesel tuner.

Prep For Installation

Your first stop at installation of the Bully Dog GT Platinum Diesel Tuner will be at your computer to perform the recommended update to the tuner. You can find the update at www.BullyDog.com/updateagent.php. The Bully Dog website is optimized for newer versions of Windows, however, it will work for Macs as well.

Regardless of what type of computer used, you should perform software updates during normal Monday through Friday business hours, in case a phone call to Bully Dog’s tech support is required when uploading to the SD card. After the SD card is refreshed, you will need to transfer this information to the tuner itself. For these tuner updates, gather the GT Tuner, SD card, card reader, and the USB cable included in the kit.

Wiring The Cab

The GT Platinum Diesel can be physically installed by most hands-on enthusiasts in about 30 minutes. Once a key-on (ignition) circuit is located, the kit’s power cable is routed from that fuse. Next the adapter plug cable must be installed. The adapter plug cable is routed so that the power cable could be plugged into one end of the adapter cable.

The HDMI cable is then plugged into the appropriate end of the adapter cable, and then the HDMI cable is snaked up the inside of the driver’s A-pillar so it can be connected to the tuner. Once this had all been squared away, the switch on the adapter plug was moved into the BATT (battery) position. There is an OBD II plug on the adapter cable too, and this was plugged into the OBD II port located underneath the dash near the steering column.

A number of options are available for mounting the Bully Dog GT Platinum Diesel tuner. We found that the best solution for us was to use the Universal Windshield Mount, the flexible stalk mount that is included in the GT tuner kit. It attaches to the back of the Tuner through a T-shaped socket, then mounts to the windshield using a suction cup.

The Universal Mount and GT Diesel tuner were mounted in a location that placed the tuner’s screen and switches within inches of the steering wheel for easy manipulation. Once it was secured to the windshield, the power cable was hooked up to back of the tuner.

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A key-on hot fuse was located (top) for the power cable using a test light. Removing the dash side panel made it easier to route the HDMI cable (lower left) through the dash and up the A-pillar. The HDMI cable and the power cable were plugged into (lower right) the ODB II adapter cable.

Time To Program

The programming sequence in the Bully Dog GT Platinum Diesel tuner can stall if battery voltage drops too much while the computer is being re-flashed. If you are concerned about your battery, the hot tip is to attach a battery-charger with a 20-amp boost to the battery during programming. Also, Bully Dog recommends that any electrical accessories be turned off, they don’t have to be uninstalled, just not powered while you’re programming the tuner.

Here again, just in case a phone call to Bully Dog’s tech line is needed, we recommend you perform the programming during normal business hours. If something goes haywire, do you really want to be stuck until Monday?

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A switch on the OBD II adapter (left) allows for battery power or ignition key power. During programming, the switch needs to be in the BATT position, afterwards switching it to KEY stops the tuner from draining the battery. The OBD II plug is inserted (right) into the OBD II diagnostic port.

Turning the ignition on, but not starting the engine, is the first step in the programming sequence. There are a number of times during programming when the tuner screen will ask that the ignition be turned off or on, so stick around and pay attention. The GT Platinum Diesel tuner turned on with the ignition, and then after a few moments of initialization, displayed the Main Menu.

From here on, it was a matter of selecting certain functions and parameters of the tuner’s capability to control or modify the truck’s engine capabilities as well as other functions to vehicle systems. The Bully Dog GT Platinum Diesel tuner can recalibrate the speed sensor for non-stock tire diameters and axle ratios, it offers a Driving Coach feature that makes suggestions for improving fuel economy, and a Mobile Desoot/DPF Burn-Off function (on some later-model applications) is also available.

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A diagnostic mode (left) in the Bully Dog GT Platinum Diesel tuner reads and clears Check Engine Diagnostic Trouble Codes (DTCs). The tuner can also record acceleration times (right) through two different modes.

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Recalibrating the speed signal for different tire sizes and new axle gear ratios (left) corrects the speedo and restores factory tranny shift points (unless otherwise adjusted in the tuner's options), ABS, and other powertrain functions. A Driving Coach feature (right) can help you maximize fuel economy for day-to-day driving or towing.

It also provides specifiable warnings and safeguards when certain parameters (such as exhaust and fluid temperatures) exceed user-set levels. The Bully Dog GT Platinum Diesel tuner can also be used to set other electronic functions, too, such as one- or two-click remote door unlocking, headlight off delay, accessory off delay, and automatic door locking.

The gauge pack that is an integral part of the Bully Dog GT Platinum Diesel Tuner that monitors EGT, boost, fluid temperatures (oil, trans, and coolant), fuel economy, and a host of other vitals. It also reads and clears Check Engine codes, and can be used as a performance tester through its ability to record ¼-mile and 0-60 mph times.

These performance tests can be done either as speed-sensitive (testing begins on movement) or from a standing start using the display’s drag-race Christmas Tree. The tuner can also be moved from truck to truck. “The GT Platinum Diesel Tuner has applications for more than 700 trucks. There’s no need to purchase a new tuner when you buy a new truck,” Kress said.

Wrapping It Up

As part of any of our performance product installations comes the most important aspect of the job, the testing procedures. After all, what good is to you if we just bolt something on to a diesel truck and leave it at that. You (and we) want to see the “proof in the pudding.” That is why this installation was performed with the 5.9L Cummins-powered 2500 securely strapped to our Dynojet dynamometer.

Our initial dyno runs prior to the installation of the Bully Dog GT Platinum Diesel tuner and the re-programming of the truck’s PCM shows very typical numbers for a 5.9L Cummins turbo diesel of its vintage. In stock from the engine made 290 peak hp at rpm and 516 lb-ft of peak torque at rpm. With the tuner’s programs flashed onto the brain controlling the engine, it was an entirely different story.

We could see the truck lurch on the dyno in a much more aggressive manner as we made our after-installation runs. After the stock baseline, we tested the Performance and Extreme power modes on the Bully Dog GT Platinum Diesel Tuner. The roar alone told the story, but the numbers (seen in the dyno chart at bottom) were dramatic. Once the Bully Dog tuner was all hooked up and running, we tested the truck with the tuner in Economy/Tow mode and peak power output spiked to 645 lb-ft of torque and 349 hp!

We also saw bountiful power increased in big steps through the next two power modes of the Bully Dog tuner. In Performance mode, we recorded power peaks of 691 lb-ft of torque and 363 hp. The whopper, though, was the Extreme mode, in which the truck was putting out 724 lb-ft of torque and 401 hp peak power to the rear wheels.

Admittedly, the cost of these electronic engine tuning wonders can be hard to swallow upon first consideration, but seriously, where are you going to get the same hp or torque gains per dollar? A set of headers and a performance exhaust system are not going to get you more than 100 hp and 200 lb-ft of torque. You would have to dig deep into the block to make that big a difference. Think about that the next time you eyeball a Bully Dog GT Platinum Diesel Tuner for your turbodiesel.