By G.R. Whale, Courtesy Of Pacbrake
photographer: G.R. Whale, Courtesy Of Pacbrake
In his motor racing spoof, The Grand Prix of Gibraltar, Peter Ustinov noted in reference to less-than-impressive brakes that anybody can make a car go slowly. But making a truck go slowly, especially if it's at or near maximum capacity, is another story, and that's the reason exhaust brakes were invented.
The Pacbrake PRXB is an air-powered exhaust brake that uses air pressure to close a butterfly valve against the exhaust flow to create backpressure and slow the engine speed (and your truck).
How It WorksOn Duramax pickup applications, the brake is mounted inline in the exhaust pipe, inside the framerail, and under the passenger-side seat. What separates the PRXB from other designs is the pressure-regulator valve (or variable orifice), in the throttle butterfly. Closing the butterfly completely would-once the exhaust gas recirculation (EGR) system is overwhelmed-blow gaskets, bend clamps, or anything else to bleed the excess pressure. On the PRXB, the butterfly has a hole in it with a flat bypass plate in back that's held closed by a pivot arm connected to a spring outside the housing. That allows excess pressure to be blown off at high rpm without losing all backpressure at low rpm, and it's the primary reason the PRXB supplies good braking at low engine speeds.
Do It YourselfThe Duramax kit includes the assembled brake unit, high- and low-pressure air lines, a Viair compressor, a filter with replacement elements, an air tank, a switch, a controller, a wiring harness, and all the fittings. It also includes a bag with a 20-foot air hose, a blowgun, a tire chuck, and assorted air-powered accessories that hook up to the compressed quick-connect fitting.
The Duramax kit uses a 4-inch housing with collars for the stock 3.5-inch pipe. If you've upgraded to a 4-inch pipe, Pacbrake has the necessary adapters to fit that won't restrict your exhaust flow. The system will also work with aftermarket electronic engine upgrades, but check with Pacbrake first for recommendations on how to wire your truck.
The kit is suitable for do-it-yourself installation, and an extra set of hands is useful (but not necessary). There are no special tools required-just some hard-to-reach bolts. All the wiring is plug-and-play-except for simple 12-volt hot and ground leads-and the installation can easily be done in one day. Although one exhaust adapter must be welded for a good seal, it's just behind the 4-bolt exhaust flange and can be clocked anywhere on the pipe, so you can pull the pipe at the flange and visit your local welder, then reinstall with the new flange gasket provided in the kit. No adjustments or tuning are required.
With hundreds of installs behind them, the crew at Chino Valley Muffler needed only a few hours, including cool-down time.
We marked and cut a section of exhaust pipe just behind the 4-bolt downpipe flange.
The ResultsTo have an idea how much braking power the PRXB delivers, we hooked our Duramax truck to a 10,200-pound, low-profile box trailer. Testing was done at altitudes of less than 1,500 feet and ambient temperatures were in the low 80s.
At startup, the Pacbrake cycles a couple of times to ensure the butterfly isn't frozen or carboned up. It makes a swishing sound like an air-powered grease gun. When it's working, you will hear the air, but it's quieter than a screaming turbo, and that's a lot better than the smell of smoking brake pads.
You can use the exhaust brake anytime, although it will only drag you down to the low 40-mph range when the converter unlocks if the Allison is not in Tow/Haul mode. Still, this is useful for slowing for exits or long grades in a lightly loaded truck. In Tow/Haul mode, the converter will lock down through Second gear, and if you tap the brake pedal, it does the downshifts automatically.