Throttle response or vehicle responsiveness is a measure of how quickly a vehicle's prime mover, such as an internal combustion engine, can increase its power output in response to a driver's request for acceleration. Throttles are not used in diesel engines, but the term throttle can be used to refer to any input that modulates the power output of a vehicle's prime mover. Throttle response is often confused with increased power but is more accurately described as time rate of change of power levels.
Formerly, gasoline/petrol engines exhibited better throttle response than diesel engines. This results from higher specific power output and higher maximum engine power, as well as the fact that lower-powered diesel engines were disproportionately heavier. Recently diesel engines became able to outperform similar-sized petrol engines. Most naturally aspirated gasoline engines have better responsiveness than supercharged or turbocharged engines for engines with similar peak power outputs. However, factors such as improper maintenance, fouled spark plugs or bad injectors can reduce throttle response. Diesel engines are less likely to lose throttle response, since their power comes from self-igniting fuel. Older diesel engines directly connect the accelerator pedal to the injection pump resulting in instant response.
Several tuning factors can affect engine responsiveness. Throttle response in manual cars can be enhanced by dropping to a lower gear before accelerating. This action is often used in smaller cars to aid in overtaking.
The advent of concern about fuel economy and emissions had major impacts on engine design. Some of the trade-offs reduced throttle response. Most new cars employ a drive-by-wire system, which can itself reduce throttle response. Earlier designs featured carburetors with an "accelerating pump" that made the fuel mix richer when the accelerator pedal was depressed suddenly, which increased hydrocarbon emissions. Later designs removed this feature.