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Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine. The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.
The word is also used as a verb, meaning to ride or drive an automotive vehicle at its maximum engine speed.
Variation of redline
The acceleration, or rate of change in piston velocity, is the limiting factor. The piston acceleration is directly proportional to the magnitude of the G-forces experienced by the piston-connecting rod assembly. As long as the G-forces acting on the piston-connecting rod assembly multiplied by their own mass is less than the compressive and tensile strengths of the materials they are constructed from and as long as it does not exceed the bearing load limits, the engine can safely rev without succumbing to physical or structural failure.
Redlines vary anywhere from a few hundred revolutions per minute (rpm) (in very large engines such as those in trains and generators) to more than ten thousand rpm (in smaller, usually high-performance engines such as motorcycles and sports cars with pistonless rotary engines). Diesel engines normally have lower redlines than comparatively-sized gasoline engines, largely because of fuel-atomization limitations. Gasoline automobile engines typically will have a redline at around 5500 to 7000 rpm. The Ariel Atom 500 has the highest redline of a piston-engine road car rated at 10,600. The Renesis in the Mazda RX-8 has the highest redline of a production rotary-engine road car rated at 9000 rpm.
In contrast, some older OHV engines had redlines as low as 4800 rpm, mostly due to the engines being designed and built for low-end power and economy during the late 1960s all the way to the early 1990s. One main reason OHV engines have lower redlines is valve float. At high speeds, the valve spring simply cannot keep the tappet or roller on the camshaft. After the valve opens, the valve spring does not have enough force to push the mass of the rocker arm, push rod, and lifter down on the cam before the next combustion cycle. Overhead cam engines eliminate many of the components, and moving mass, used on OHV engines. Lower redlines, however, do not necessarily mean low performance, as some skeptics sometimes assume. For example, a Supercharged Buick 3800 V6 with a redline anywhere from 5500-6000 has a torque curve that peaks at 2600-3600rpm, yet the engine is a strong performer from takeoff all the way through to the redline.
Motorcycle engines can have even higher redlines because of their comparatively lower reciprocating mass. For example, the Honda CBR250 has a redline of about 19,000 rpm. (Though due to regulations in Japanese motorcycle manufacturing this was later lowered to 18,000) Higher yet is the redline of a modern Formula One car. Regulations in 2010 limit the maximum engine rotation to 18000 rpm , but during the 2006 season, engine speeds reached over 20000 rpm on the Cosworth engine.
Rev limiter and implementation
The actual term redline comes from the red bars that are displayed on tachometers in cars starting at the rpm that denotes the redline for the specific engine. Operating an engine in this area is known as redlining. Straying into this area usually does not mean instant engine failure, but may increase the chances of damaging the engine.
Most modern cars have computer systems that prevent the engine from straying too far into the redline by cutting fuel flow to the fuel injectors/carburetor or by disabling the ignition system until the engine drops to a safer operating speed. This device is known as a rev limiter and is usually set to an RPM value at redline or a few hundred RPM above. Most Electronic Control Units (ECUs) of automatic transmission cars will upshift before the engine hits the redline even with maximum acceleration (The ECU in a sports car's automatic transmission will allow the engine to go nearer the redline or hit the redline before upshifting). If manual override is used, the engine may go past redline for a brief amount of time before the ECU will auto-upshift. When the car is in top gear and the engine is in redline (due to high speed), the ECU will cut fuel to the engine, forcing it to decelerate until the engine begins operating below the redline at which point it will release fuel back to the engine, allowing it to speed operate once again.
However, even with these electronic protection systems, a car is not prevented from redlining through inadvertent gear engagement. If a driver accidentally selects a lower gear when trying to shift up or selects a lower gear than intended while shifting down (as in a motorbike sequential manual transmission), the engine will be forced to rapidly rev-up to match the speed of the drivetrain. If this happens while the engine is at high rpms, it may dramatically exceed the redline. For example, if the operator is driving close to redline in 3rd gear and attempts to shift to 4th gear but unintentionally puts the car in 2nd by mistake, the transmission will be spinning much faster than the engine, and when the clutch is released the engine’s rpm will increase rapidly. It will lead to a rough and very noticeable engine braking, and likely engine damage. This is often known as a 'money shift' because of the likelihood of engine damage and the expense of fixing the engine.
Examples of performance automobile piston engines
- Ariel Atom 500 V8 - 10,600 rpm
- Audi R8 V10 - 8700 rpm
- Audi Q7 6.0 V12 Diesel - 4750 rpm
- Bentley Azure 'T' 6.75 V8 - 4500 rpm (is a petrol, but low redline)
- BMW E46 M3 I6 - 8000 rpm
- BMW E90 M3 V8 - 8400 rpm
- BMW E39 M5 V8 - 7000 rpm
- BMW E60 M5 V10 - 8250 rpm
- BMW F10 M5 V8 - 7200 rpm
- BMW M6 V10 - 8350 rpm (same engine as the M5)
- BMW E60 535d 3.0 CR Diesel I6 - 5000 rpm
- BMW F10 520d 2.0 CR Diesel I4 - 5500 rpm
- Bugatti Veyron turbocharged W16 - 6500 rpm
- Chevrolet Corvette C6 Z06 V8 - 7000 rpm 
- Chevrolet Corvette C6 ZR1 supercharged V8 - 6500 rpm
- Ferrari 348 V8 - 7500 rpm
- Ferrari F50 V12 - 8500 rpm
- Ferrari FXX V12 - 8700 rpm
- Ferrari Enzo V12 - 8000 rpm (rev limiter 8250 rpm)
- Ferrari F430 V8 - 8500 rpm
- Ferrari California V8 - 8000 rpm
- Ferrari 458 Italia V8 - 9000 rpm
- Ferrari LaFerrari V12 - 9250 rpm
- Ford Mondeo ST TDCi - 4500 rpm (rev limiter 4900 rpm)
- Ford Focus RS500 l5 Turbo - 8600 rpm (same engine as in the Volvo S60R but modified from the factory)
- Ford Mustang Shelby GT350 V8 - 8250 rpm 
- Honda NSX V6 - 8000 rpm
- Honda Integra/Accord Type R I4 - 8800 rpm
- Honda Civic Si I4 - 8000 rpm
- Honda S2000 I4 - 8800 rpm (2.0L) (rev limiter 9000 rpm)
- Honda S2000 I4 - 8000 rpm (2.2L) (rev limiter 8200 rpm)
- Koenigsegg One:1 - 7500 rpm (rev limiter 8250 rpm)
- Lamborghini Aventador V12 - 8170 rpm
- Lamborghini Gallardo V10 - 8100 rpm
- Lamborghini Gallardo V10 LP560-4 - 8500 rpm
- Lamborghini Murcielago V12 - 8000 rpm
- Lexus LFA V10 - 9000 rpm (rev limiter 9500 rpm)
- Lotus Elise I4 - 8000 rpm (rev limiter 8500 rpm)
- Mazda MX-5 Miata I4 - 7200 rpm (7500 rpm for some markets)
- McLaren F1 V12 - 7500 rpm
- Mercedes-Benz SLR McLaren supercharged V8 - 7000 rpm
- Mitsubishi Lancer Evolution X 2.0 turbocharged I4 - 7000 rpm (fuel cut off at 7600 rpm)
- Nissan GTR turbocharged V6 - 7000 rpm
- Nissan Skyline GT-R turbocharged I6 - 8000 rpm
- Nissan Micra K11 I4 - 7000 rpm (rev limiter 7300 rpm)
- Nissan VQ37VHR V6 - 7500 rpm
- Pagani Zonda F V12 AMG 7.3 48v - 7500 rpm
- Pagani Zonda R V12 AMG 6.0 48v - 8000 rpm
- Peugeot 309 GTI16 1.9 16v - 7300 rpm
- Porsche 911 Carrera (2012 991) B6 - 7800 rpm
- Porsche 911 GT3 4.0 (2012 997) B6 - 8500 rpm
- Porsche Carrera GT V10 - 8400 rpm
- Subaru Impreza WRX STI H4 2.0 - 8250 rpm
- Suzuki Swift GT-i 1300 16V - 7600 rpm
- Volkswagen Golf GTI 2.0TSI I4 - 6500 rpm
- Toyota RunX (RSi)/Toyota Matrix (XRS) 1.8 (2ZZ-GE) - 8200 rpm
- Toyota Corolla 1.6 (4A-GE) - 8200 rpm
- Toyota 86 H4 - 7400 rpm
- Wiesmann GT MF5 BMW M V10 - 8350 rpm
- 2006 Williams Cosworth CA2006 V8 F1 - 20000 rpm
- 2006 BMW Sauber P86 V8 F1 - 20000 rpm (rev limiter 21000 rpm)
Examples of production rotary engines
- Mazda RX-8 1.3L twin rotor 9000 rpm (rev limiter 9400 rpm) - although the engine has no reciprocating components and could turn much faster, the speed has to be limited to avoid the risk of the clutch breaking up under extreme centrifugal force, which would be highly dangerous as parts could penetrate the passenger cabin.
Examples of motorcycle engines
- Kawasaki ZXR 250: 19,000 rpm
- Honda CBR250RR: 19,000 rpm
- Yamaha FZR250: 18,500 rpm
- Yamaha YZF-R6: 16,200 rpm
- Honda RVF400: 14,500 rpm (With HRC ECU 15,500 rpm)