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.
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.
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.
 Here, the technician has a...  Here, the technician has a connector removed from the Ford PCM to create slack in the harness, which makes the installation of Autoloc connections easier. Once the Autoloc wires are attached, the plug will be reinserted into the PCM. |  Potentiometers within the...  Potentiometers within the Autoloc box are used to adjust both the transmission-line pressure and the torque-converter lockup release rpm. |  The transmission line pressure...  The transmission line pressure is related to throttle position, so the BD Autoloc module increases line pressure when the throttle is released to apply greater pressure on the transmission's clutch packs. |
 Part of the BD brake's success...  Part of the BD brake's success story on the 6.0L is the control of the factory VGT's vanes. Intercepting the signal the factory computer sees keeps the vanes closed during exhaust braking and ensures good acceleration response when the throttle is reapplied. |  The backpressure sensor provides...  The backpressure sensor provides the signal to the PCM that controls the turbo vanes. The BD brake module intercepts the signal when the brake is activated to eliminate boost lag when the throttle is reapplied. |  The BD brake controller prevents...  The BD brake controller prevents the EGR valve from opening during engine braking. Otherwise, the valve would release the necessary exhaust backpressure. |