in continuous use. As a result, compounded boost is always produced as the pressure ratios of the two compressors are multiplied, not added. In other words, if a turbocharger which produces on its own feeds into a supercharger which produces 10 psi on its own, the resultant manifold pressure would be rather than . This form of series twincharging allows for the production of boost pressures that would otherwise be inefficient or unachievable with other compressor arrangements. However, turbo and supercharger efficiencies do not multiply. For example, if a turbocharger with an efficiency of 70% feeds into a Roots supercharger with an efficiency of 60%, the total compression efficiency would be somewhere in between. To calculate this efficiency, it is necessary to calculate the efficiencies of the 2 stages, first calculating the conditions of pressure and temperature at the exit of the first stage and starting from these to calculate for the second stage. Following the previous example, for a first stage of the turbocharger with an efficiency of 70%, the temperature would reach after the first stage, to then enter the supercharger with an efficiency of 60% and leave at a temperature of , resulting in a total efficiency of 62%. A large turbocharger that produces by itself, with a thermal efficiency of around 70%, would produce air only in temperature. In addition, the cost of energy to compress air with a supercharger is higher than that of a turbocharger; if the supercharger is not compressing air, there remains only a small parasitic loss of rotating the working parts of the supercharger. This remaining loss can be eliminated by disconnecting the supercharger further using an electromagnetic clutch (such as those used in the VW 1.4TSI or Toyota 4A-GZE to bypass the supercharger in low-load conditions). With series twincharging, the turbocharger can be of a less expensive and more durable journal bearing variety, and the sacrifice in boost response is more than made up for by the instant-on nature of positive-displacement superchargers. While the weight and cost of the supercharger assembly are always a factor, the inefficiency of the supercharger is minimized once the turbocharger reaches operating speed and the supercharger is effectively disconnected by the bypass valve. Parallel Parallel arrangements typically require the use of a bypass or diverter valve to allow one or both compressors to feed the engine optimally. If no valve was used and both compressors