feedback fuel-air-ratio control: a feature of a computer-controlled fuel system. By using a sensor to measure the oxygen content of the engine's exhaust, the system keeps the fuel-air ratio very close to the proportion for chemically perfect combustion. Such tight control of the fuel-air ratio is mandatory for the proper operation of three-way catalysts.

fiberglass: a composite material that relies on small glass fibers for its strength.

final-drive ratio: the reduction ratio, found in the gearset of a drivetrain, that is furthest removed from the engine. Typically, the differential ratio.

floorpan: the largest and most important stamped metal part in a car's body. Usually assembled from several smaller stampings, the floorpan forms the floor and fixes the dimensions for most of the car's external and structural panels. It is also the foundation for many of the car's mechanical parts.

fluid coupling: any device that transfers power through a fluid between its inputs and outputs. A fluid coupling basically consists of two fans in a sealed, oil-filled housing. The input fan churns the oil, and the churning oil in turn twirls the output fan. Such a coupling allows some speed difference between its input and output shafts.

flywheel: a heavy disc attached to an engine's crankshaft to increase its rotary inertia, thereby smoothing its power flow.

four valves per cylinder: a valvetrain with a total of four valves in the combustion chamber, typically two intakes and two exhausts. Compared to the more common two-valve-per-cylinder designs, a four-valve layout offers improved breathing and allows the spark plug to be located closer to center of the combustion chamber.

four-wheel drift: a somewhat imprecise term that describes a cornering situation in which all four tires are operating at large slip angles.

four-wheel steering: a steering system that actively steers the rear wheels as well as the fronts to improve handling and maneuverability.

fuel injection: any system that meters fuel to an engine by measuring its needs and then regulating the fuel flow, by electronic or mechanical means, through a pump and injectors. Throttle-body injection locates the injector(s) centrally in the throttle-body housing, while port injection allocates at least one injector for each cylinder near its intake port.

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