air into the cylinders at the full ambient pressure. More vacuum is created in some situations. On deceleration or when descending a hill, the throttle will be closed and a low gear selected to control speed. The engine will be rotating fast because the road wheels and transmission are moving quickly, but the butterfly valve will be fully closed. The flow of air through the engine is strongly restricted by the throttle, producing a strong vacuum on the engine side of the butterfly valve which will tend to limit the speed of the engine. This phenomenon, known as engine braking, is used to prevent acceleration or even to slow down with minimal or no brake usage (as when descending a long or steep hill). This vacuum braking should not be confused with compression braking (aka a "Jake brake"), or with exhaust braking, which are often used on large diesel trucks. Such devices are necessary for engine braking with a diesel as they lack a throttle to restrict the air flow enough to create sufficient vacuum to brake a vehicle. Uses of manifold vacuum This low (or negative) pressure can be put to use. A pressure gauge measuring the manifold pressure can be fitted to give the driver an indication of how hard the engine is working and it can be used to achieve maximum momentary fuel efficiency by adjusting driving habits: minimizing manifold vacuum increases momentary efficiency. A weak manifold vacuum under closed-throttle conditions shows that the butterfly valve or internal components of the engine (valves or piston rings) are worn, preventing good pumping action by the engine and reducing overall efficiency. Vacuum used to be a common way to drive auxiliary systems on the vehicle. Vacuum systems tend to be unreliable with age as the vacuum tubing becomes brittle and susceptible to leaks. Before 1960 Windshield wiper motors - Prior to the introduction of Federal Motor Vehicle Safety Standards in the USA by the National Traffic and Motor Vehicle Safety Act of 1966, it was common to use manifold vacuum to drive windscreen wipers with a pneumatic motor. This system was cheap and simple but resulted in the comical yet unsafe effect of wipers which operate at full speed while the engine idles, operate around half speed while cruising, and stop altogether when the driver depresses the pedal fully. Power lock motors "Autovac" fuel lifter, which uses vacuum to