This method of slowing down relies on the friction and manifold vacuum created inside an internal combustion engine when the throttle closes. By shifting into a lower gear and staying off the accelerator, the vehicle's momentum is forced to turn the engine against this resistance. It is used to maintain a safe speed on long downhill grades without overheating the friction brakes.
Drop it into third gear and use engine braking down this mountain pass so you don't boil your brake fluid.
While gasoline engines rely on throttle vacuum for braking force, diesel engines lack a throttle body and require exhaust restrictors or compression release brakes to achieve strong engine braking.
Related terms
Brake fade
The loss of stopping power caused by excessive heat buildup in the braking system.
Downshift
The act of selecting a lower gear in a transmission to increase engine speed and torque.
Compression
The phase in an internal combustion engine where the piston squeezes the air-fuel mixture.
Manual transmission
A gearbox requiring the driver to manually select gear ratios using a shift lever and a clutch.