|This article does not cite any references or sources. (December 2009)|
A rev limiter is a device fitted to an internal combustion engine to restrict its maximum rotational speed. This is usually carried out to prevent damage to the engine, however sometimes these devices are fitted to prevent an engine reaching the point at which it develops maximum power. This may be useful in a driving school car or similar, or utilised in systems preventing Valet parking operatives from joy-riding in high-performance vehicles placed in their charge. When fitted to racing or modified street cars these adjustable devices provide more precise control than the original components built in by the manufacturer. Limiters can be used in conjunction with a shift light, which illuminates when the desired RPM is reached, indicating to the driver that a gear change is required. This allows the driver to keep his eye on the road and not have to glance repeatedly at the tachometer to monitor the engine speed.
Limiters prevent damage to an engine by interrupting the power that is distributed to the spark plugs; this prevents the engine from operating above a pre-determined RPM level known as the redline. The RPM level that results with the spark being arrested can be a constant level, or, with the proper ignition control modules, variable. Variable rate ignition modules can be adjusted quickly and easily to achieve the appropriate RPM limit for different situations, such as street racing, drag racing, road course racing, highway driving, and lawn turfing.
Multiple stage ignition modules offer greater RPM limit control. The first stage can be used to limit RPM levels when launching a vehicle from a stationary position, providing maximum power and traction. The second stage is activated after launch to set a higher RPM limit for wide-open-throttle acceleration.
Engines with hydraulic tappets (such as the Buick/Rover V8) often have what is in effect a rev limiter by virtue of their design. The tappet clearances are maintained by the flow of the engine's lubricating oil. At high engine speeds, the oil pressure rises to such an extent that the tappets 'pump up', closing the valve clearance and preventing the valves fully closing. This sharply reduces engine power, causing speed to drop.
Fuel efficiency 
Limiters are used as a means of increasing the fuel efficiency of commercial motor vehicles. Limiters can be individually adjusted by fleet owners to meet the performance requirements of many different driving conditions. A limiter used for this purpose, however, especially if it is mechanical in nature, is more commonly categorized as a governor. Electronic adjustments require special diagnostic equipment to access control settings, and are limited to authorized personnel, such as a fleet manager or mechanic.
Due to their electrical nature, however, it is still possible to drive an engine beyond the maximum RPM level set by the limiter. This most commonly occurs in the case of selecting the incorrect gear with a manual transmission - for example, selecting second gear instead of fourth after shifting out of third. The speed of the vehicle and the drive wheels may drive the engine far beyond its intended range in this situation, likely resulting in rapid engine wear, and in extreme cases possible outright engine failure.
Limiters are manufactured by numerous companies and are available for most vehicle and engine combinations.