Throttle Response

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, such as a pedal being pressed. Throttles are not used in diesel engines, but the term throttle is often used to refer to any input that modulates the power output of a vehicle's prime mover. Throttle response is often confused with better power, but throttle response is rather related to time taken for change in power level.

In general, gasoline/petrol engines used to exhibit better throttle response over diesel engines. This was given by their higher specific power output and higher maximum engine power, as well as that the lower-powered diesel engines carried disproportionally higher inertial mass (heavier crankshaft, steel reinforced pistons, as in Mercedes-Benz OM615. This changed recently with some modern diesel engines outperforming similar engine size petrol cars (V8 VW Audi, etc.) (Also the sound and 'feel' of such cars is much like that of petrol-engined cars.) Also, many naturally aspirated gasoline engines have better responsiveness than supercharged or turbocharged engines, for engines with similar peak power outputs. However, petrol engine is more likely to suffer from lackluster throttle response due to bad maintenance, fouled spark plugs, bad injectors, etc. Diesel engines are very unlikely to develop delays in throttle response, since their power comes from self-igniting fuel, plus older diesel engines have accelerator pedal directly connected to injection pump which results in instant response, which often makes driving of an older diesel engine car with manual transmission more enjoyable than similar performance petrol engine. Since second half of 1990s, much of the amount of injected fuel is controlled by the lambda probe (more properly named heated oxygen sensor, HO2S) which can delay the throttle response, as well as some other faulty sensors on an petrol engine.

Several tuning factors can have an effect on the responsiveness of an engine.

Throttle response in manual cars can be enhanced by dropping to a lower gear (e.g. 5th to 4th) before attempting to accelerate. This is often used in smaller cars to aid in overtaking.

Most new cars are equipped with a drive-by-wire system. Throttle response is often affected badly by this and drivers feel a noticeable delay when quickly changing the position of the gas pedal. This may be either caused by faulty components, which if a very frequent reason (sensors, ignition), it may be intentional by design (lazy engine developers, decreasing emissions by not allowing an old engine design to do anything harsh), or by emission controls engaging in the process. Before car emission became a general health hazard, carburettors used to have a "accelerating pump" in their designs, which made the fuel mix richer when the accelerator pedal was depressed suddenly. The resulting hydrocarbon emission from exhausts are one of the reasons of 'Los-Angeles type smog'. Due to the sheer number of cars being used, emission controls became mandantory and most petrol-engined cars operate on average in stoichiometric mix mode. Some engine manufacturers resigned to the fact and offer lackluster performance engines, which don't offer any excitement from driving. R&D of engines which conform to emission legislation, while still offering desirable throttle response is one of the many reasons why the number of engine manufacturers for personal cars drastically shrunk in the past 30 years.