At what engine RPM are engine brake retarders most efficient?

Engine brake retarders, often referred to as Jake brakes, are most efficient at high RPMs. This is because the primary function of these devices is to utilize engine compression to slow the vehicle down, and this process is more effective when the engine is operating at higher revolutions. At high RPMs, there is greater airflow moving through the engine, which allows for a more effective transfer of energy from the engine to the braking process.

In contrast, at low RPMs, the airflow and compression levels are insufficient to create the same degree of retarding effect, making the system less effective. Similarly, while moderate RPMs can provide some retarding effect, they do not capitalize on the full potential of the engine brake retarder like high RPMs do. The efficiency of these systems is not fixed and varies depending on the engine speed, which emphasizes the importance of using them at optimal RPM levels for maximum effectiveness.