Two-stage variable twin-turbos employ a small turbocharger at low speeds and a large one at higher speeds. They are connected in a series so that boost pressure from one turbocharger is multiplied by another, hence the name "2-stage." The distribution of exhaust gas is continuously variable, so the transition from using the small turbocharger to the large one can be done incrementally. Twin turbochargers are primarily used in Diesel engines. For example, in Opel bi-turbo Diesel, only the smaller turbocharger works at low speed, providing high torque at 1,500–1,700 rpm. Both turbochargers operate together in mid range, with the smaller one pre-compressing the air, which the larger one further compresses. A bypass valve regulates the exhaust flow to each turbocharger. At higher speed (2,500 to 3,000 RPM) only the larger turbocharger runs.