Redline

Redline is the maximum engine speed at the which an internal combustion engine, traction motor or its components can operate without causing damage to other parts or to the components. Redline for an engine is dependent on many factors, including stroke, mass, displacement, composition, balance, and weight of components.

This word can also be used verbally to mean to drive or ride an automobile above the redline. Redline is actually derived from the red bars on tachometers of cars that indicate the engine's redline. Redlining is the process of operating an engine in this area. Although it is not common for an engine to fail in this area, it can increase the chance of damage.

Modern cars have computer systems that prevent an engine from exceeding the redline. This is done by cutting fuel flow through fuel injectors/fuelrail (in direct-injected engines)/carburetor, or by disabling ignition until the engine drops below a safe operating speed. This is called a rev limiter. It is usually set to a redline RPM or a few hundred more. The majority of automatic transmission cars' electronic control units (ECUs), will increase the speed before the engine hits redline. However, an ECU in a sports car will allow the engine go closer to the redline or hit redline before it upshifts. The engine may reach redline briefly if manual override is applied. If the ECU cuts power to pull it back, or auto-upshift, the ECU will stop the engine from reaching redline again. The ECU will cut fuel to your engine when you have the car in top gear. This is due to high speeds. Once the engine starts operating below the redline, it will decelerate until it stops running. At that point, it will release fuel to the engine and allow it to continue to operate.

Even with these electronic protection systems, redlining can still occur due to inadvertent gear engagement. The engine will rapidly rev up if the driver selects a lower or incorrectly selected a lower gear while trying to shift up. This can happen when the engine is revving at high RPMs. It may be dangerously over the redline. If the operator is driving in 3rd gear, close to the redline, and attempts to shift to 4th, but accidentally puts the car in 2nd, the transmission will spin much faster than the engine and the engine's rpm will rapidly increase when the clutch is released. This will cause engine damage and engine braking to become very noticeable. This is commonly known as a "moneyshift" due to the possibility of engine damage and the cost of fixing it.