The BD Advantage
As engine rpm drops, so will backpressure. But here's where the BD brake has an advantage: the company's patented variable-orifice technology. The BD brake butterfly valve is solid and only opens enough to allow excessive backpressure to escape. BD has found that this method provides strong braking throughout the engine's operating range, not just at higher rpm.
Automatic-Transmission Issues
The final components of the BD brake system are its Autoloc and Pressureloc modules, which control torque-converter lockup and transmission-line pressure. A manual transmission always maintains a mechanical connection between the engine and transmission; with an automatic, this isn't the case. The transmission's torque converter has two modes: unlocked (or fluid coupling) and locked, where the clutches in the torque converter apply, creating a mechanical coupling. When the torque converter is in an unlocked mode, an exhaust brake can't function to its full potential because there is a certain degree of naturally occurring slippage in the fluid-coupling mode. To draw a comparison, imagine downshifting a manual transmission then letting the clutch out only halfway; there would be some slowing, but not as much as if you released the clutch pedal completely. The Autoloc maintains torque-converter lockup in all gears when the BD brake is applied, resulting in maximum retarding power.
Since much of the transmission's function is based on throttle position, transmission-line pressure drops when the throttle is released because the computer thinks the vehicle is coasting. To rectify this issue, the Pressureloc increases line pressure to 90 percent of maximum when the BD brake is activated, keeping the transmission clutches firmly applied to prevent slippage.
Road Testing
We recently visited BD to get an idea of what's involved with the installation and to take a testdrive in the company's Ford F-350. Without the BD brake activated, the truck slowed down almost as if it were shifted into Neutral, but with it activated, the difference was remarkable. If you can imagine downshifting a big-block gas engine into Low gear from 60 mph, you're starting to get the picture. But instead of a violent action, the truck felt as though it were being dragged to a stop by a powerful magnet-a progressive, controlled, yet very powerful slowing.