A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings.
The turbine's A/R (area/radius) ratio basically determines where the turbo starts to accelerate.
VFT (Variable Flow Turbocharger)
With conventional wastegated turbochargers or fixed geometry turbochargers (FGT), a turbine flow area cannot be adjusted during operation.
Knocking is abnormal combustion in which the air-fuel mixture ignites prematurely due to exposure to high temperature and pressure, creating an unwanted high-frequency noise. When the compression ratio is increased, the temperature at compression top dead center (TDC) also rises, increasing the probability of knocking.