refers to delaywhen the engine rpm is within the turbocharger's operating rangethat occurs between pressing the throttle and the turbocharger spooling up to provide boost pressure. This delay is due to the increasing exhaust gas flow (after the throttle is suddenly opened) taking time to spin up the turbine to speeds where boost is produced. The effect of turbo lag is reduced throttle response, in the form of a delay in the power delivery. Superchargers do not suffer from turbo lag, because the compressor mechanism is driven directly by the engine. Methods to reduce turbo lag include: Lowering the rotational inertia of the turbocharger by using lower radius parts and ceramic and other lighter materials Changing the turbine's aspect ratio Increasing upper-deck air pressure (compressor discharge) and improving wastegate response Reducing bearing frictional losses, e.g., using a foil bearing rather than a conventional oil bearing Using variable-nozzle or twin-scroll turbochargers Decreasing the volume of the upper-deck piping Using multiple turbochargers sequentially or in parallel Using an antilag system Using a turbocharger spool valve to increase exhaust gas flow speed to the (twin-scroll) turbine Using a butterfly valve to force exhaust gas through a smaller passage in the turbo inlet Electric turbochargers and hybrid turbochargers. A similar phenomenon that is often mistaken for turbo lag is the boost threshold. This is where the engine speed (rpm) is currently below the operating range of the turbocharger system, therefore the engine is unable to produce significant boost. At low rpm, the exhaust gas flow rate is unable to spin the turbine sufficiently. The boost threshold causes delays in the power delivery at low rpm (since the unboosted engine must accelerate the vehicle to increase the rpm above the boost threshold), while turbo lag causes delay in the power delivery at higher PRM. Use of multiple turbochargers Some engines use multiple turbochargers, usually to reduce turbo lag, increase the range of rpm where boost is produced, or simplify the layout of the intake/exhaust system. The most common arrangement is twin turbochargers, however triple-turbo or quad-turbo arrangements have been occasionally used in production cars. Turbocharging versus supercharging The key difference between a turbocharger and a supercharger is that a supercharger is mechanically driven by the engine (often through a belt connected to the crankshaft) whereas a turbocharger is powered by the kinetic energy of the engine's exhaust gas. A turbocharger does not place a direct