You probably know that "Redlining" an engine is a bad thing.  It means you have punched the throttle enough that the engine is spinning at a dangerously high speed.  Everyone knows that redlining is to be avoided at all costs. Or is it?  Exactly what is redlining, what happens when an engine redlines, and can you really damage your engine if it occurs? 

What does redlining the engine mean?

The actual term redline comes from the red bars that are displayed on tachometers in cars and trucks.  The redline refers to the maximum engine speed that an internal combustion engine is designed to operate without causing damage to any internal components. The redline of an engine depends on various factors such as the mass of the engine parts, composition of the parts and their interrelated balance.  Engineers design engines to operate within a range of speeds such normal operation rarely reaches redline levels. 

What is a car's redline limit?

Engine redlines can vary a great deal.  Very large engines, such as in trains and stationary generators can redline at just a few hundred RPM.  Smaller, usually high-performance, engines such as motorcycles and sports cars normally have higher redlines. Gasoline automobile engines typically will have a redline at around 5500 to 7000 rpm. 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. 

Can redlining be prevented?

The computer systems in most modern cars prevent the engine from straying too far into the redline by cutting fuel flow to the engine or by disabling the ignition system until the engine drops to a safer operating speed. This circuitry is known as a rev limiter and is usually set to an RPM value at redline or a few hundred RPM above.

However, even with an electronic protection system, a car is not prevented from redlining through accidental 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, the engine will be forced to rapidly rev-up to match the speed of the drivetrain. 


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