The influence of driving procedures and extraneous car noise on perception of engine knock has been studied with late model cars. Year to year changes in reports of knock by both customers and raters and changes in octane requirements were also reviewed.
Customers generally accelerated cars less vigorously and tended to perceive knock less often than raters. However, in cars that had maximum octane requirements at part-throttle, customers perceived knock about as frequently as raters.
Octane requirements, as determined by raters, were reduced by about 2 Research Octane numbers when the car radio was turned on and the windows closed.
Both customers and raters reported more knock with 1975-1977 model cars than with earlier models. Since 1975, octane requirements of certain car models tended to be adjusted to the octane quality of available gasoline.