The time so taken to change the gear due to notchiness is called turbo lag. Turbo lag or Turbocharger lag is the time taken to change power output in response to a gear change in which a resistance is felt which slows the throttle response when accelerating as compared to a naturally aspirated engine. This is because of the time required by the exhaust system and turbocharger to produce the required boost. Compressor, Inertia and friction are the primary contributors to turbo lag. Superchargers do not have the tendency to suffer from this problem as the turbine is eliminated because of the compressor being directly powered by the engine.
Turbocharger applications can be categorized into those which require changes in output power (such as automotive) and those which do not (such as engine – generators, commercial automotive, marine, aircraft, industrial and locomotives). While important to varying degrees, turbo lag is most problematic in applications which need rapid changes in power output. Engine designs reduce notch (thereby, reducing lag) in many ways: firstly, it can be done by lowering the rotational inertia of the turbocharger by using lower radius parts and ceramic and other lighter materials, secondly, by changing the turbine's aspect ratio or by increasing the upper – deck air pressure (also called the compressor discharge) and improving waste gate response. It can also be done by reducing bearing frictional losses, like, by using a foil bearing rather than a conventional oil bearing or by using antilag system. Then we can also use variable – nozzle or twin – scroll turbochargers and even multiple turbochargers sequentially or in parallel. It can also be achieved by decreasing the volume of the upper – deck piping and lastly by using a turbocharger spool valve to increase exhaust gas flow speed to the (twin – scroll) turbine.