engines now have a separate vacuum pump ("exhauster") fitted to provide vacuum at all times, at all engine speeds. Many new BMW petrol engines do not use a throttle in normal running, but instead use "Valvetronic" variable-lift intake valves to control the amount of air entering the engine. Like a diesel engine, manifold vacuum is practically non-existent in these engines and a different source must be utilised to power the brake servo. References See also Vacuum delay valve Internal combustion engine Engine technology Vacuum