Redline

It has been suggested that this article or section be merged with rev limiter. (Discuss)

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2008)

This article discusses the term "redline" as it applies to internal combustion engine technology. For other uses of the term, see Redline (disambiguation).

Tachometer showing red lines above 14,000 rpm.

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.^[1] The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.

Variation of redline

Engines with short strokes can handle higher rpm because there is less force in reciprocating motion. Lighter components can increase the redline as well, since they have less inertia and decrease forces present in the engine.

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 Honda F20C engine used in the S2000 together with Ferrari 458 Italia has the highest redline of a production piston-engine road car rated at 9000 rpm. The Renesis rotary engine in the current Mazda RX-8 has a redline of 9000 rpm.

On the other hand, 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 60's all the way to the early 90's. Some other factors are that OHV engines tend to have heavy bottom ends, making the redline much lower, as the engine will become unbalanced and destroy itself, as with any engine taken over its redline. Usage of overhead valves also limits maximum engine speed, as the pushrods required to move the valves can warp or break at high speeds. 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 2600-3600rpms, yet the engine is a strong performer from take-off, all the way through to the redline.

Motorcycle engines can have even higher redlines because of their comparatively lower reciprocating mass. For example, the Yamaha YZF-R6 has a redline of about 16,200 rpm. (Yamaha heavily advertised the engine as having a 17,500 rpm redline, though for reasons of marketing and/or tachometer error, the engine was not actually capable of that speed.) Higher yet is the redline of a modern Formula One car. Regulations limit the maximum engine rotation to 19000 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 (an automatic transmission sport car's ECU 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 transmission), the engine will be forced to rapidly rev-up to match the speed of the drivetrain. If this happens while the engine was 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. When the Porsche 959 supercar was being evaluated by journalists, a mistake during a downshift caused the engine to be damaged after shifting from 5th gear to 2nd while at high vehicle speed^{[citation needed]}.

Examples of performance automobile piston engines

• Audi R8 V8 8000 rpm (rev limiter 8250 rpm)
• BMW E36 M3 I6 6500 rpm
• BMW E46 M3 I6 8000 rpm
• BMW E90 M3 V8 8400 rpm
• BMW E39 M5 V8 7000 rpm
• BMW E60 M5 V10 8250 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 F355 V8 8500 rpm
• Ferrari F50 V12 8500 rpm
• Ferrari 360 V8 8500 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
• Honda NSX V6 8000 rpm
• Honda Integra Type R I4 8400 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)
• Lamborghini Gallardo V10 8100 rpm
• Lamborghini Gallardo V10 LP560-4 8500 rpm
• Lexus LFA V10 9000 rpm (rev limter 9500 rpm)
• McLaren F1 V12 7500 rpm
• Mercedes-Benz SLR McLaren supercharged V8 7000 rpm
• Mitsubishi Lancer Evolution X turbocharged L4 7000 rpm
• Nissan GTR turbocharged V6 7000 rpm
• Nissan 370Z V6 7500 rpm
• Porsche Carrera GT V10 8400 rpm
• Porsche 997 GT2 turbocharged flat-6 6750 rpm

Examples of production rotary engines

• Mazda RX-8 1.3L 9000 rpm

References

1. ^ Car and Driver Magazine: Glossary of Terms - Redline