When used in Objects.con, this is the amount of acceleration per second an object can gain or lose when rotating in a plane of rotation. See SetRotation for an explanation of the planes of rotation. A lower value means it is slower to get the object moving in response to the mouse. For example, the defgun has 0/75/0 and tank turrets are 0/1000/0, meaning that you can whip the tank turrets around but it's slow going to get the defgun to start turning and to reverse its direction. SetMaxSpeed sets the maximum speed an object can turn in response to a mouse move.
When used in Physics.con in an Engine object, acceleration sets how quickly the engine accelerates when changing between speed settings, e.g., low acceleration will make turning sluggish but will increase in response the longer you hold the throttle key. Having it set very high should make the car start to alter speed settings more rapidly (which results in faster response). This controls how quickly the SetMaxSpeed part can get up to its set speeds, which in turn sets how quickly the engine changes between the individual speed segments. See SetMinRotation for more information.
All these settings (SetMinRotation, SetMaxRotation, SetMaxSpeed, and SetAcceleration) control how an engine responds to player input, they do not otherwise change the speed chracteristics of a vehicle in any other way.
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