Understanding Redlining

You probably know that “Redlining” an engine isn’t a good thing. It usually means you have jammed your foot on the gas pedal far enough that the engine is spinning into the “red zone” on the tach. When something is operating in any red zone, it’s usually a bad thing. But what happens when an automotive engine redlines, and can you really damage your engine if it occurs?

Engineers define “the redline” as the maximum speed that an internal combustion engine can operate at without causing damage to itself. The redline of an engine depends on factors such as the mass of the engine parts, composition of the parts (i.e. what type of metal) and the interrelated balance of all components.

So what happens when an engine goes into the red zone? The rate of change in piston velocity is the critical issue. This is directly proportional to the G-forces experienced by the piston-connecting rod assembly as it slams up and down. As long as the G-forces acting on the piston-connecting rod assembly is less than the strength of the materials they are made of, the engine can safely rev without breaking anything.

Engine redlines can vary a great deal. Big engines, such as in trains and ships can redline at just a few hundred RPM. Smaller, usually high-performance, engines such as motorcycles and sports cars normally have much higher redlines. The Honda CBR250 has a redline of about 19,000 rpm. Gasoline automobile engines typically will redline at around 5500 to 7500 rpm. By contrast, some older overhead valve engines had redlines as low as 4800 rpm. By the way, the 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. This is why overhead cam engines are popular today –no valve float.

The computer systems (CPUs) that control most modern cars prevent engines from straying too far into the redline by cutting fuel flow to the engine or by disabling the ignition system. This circuitry is known as a “rev limiting” and is usually set to an RPM value at the determined redline or a several hundred RPM above.

However, even with an electronic protection system, a car is not prevented from redlining when a driver “misses a shift”. 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. This can easily redline an engine. Fortunately, this is uncommon in normal driving but is a constant issue for race car drivers to be aware of.

Source: McLoughlin Chrysler Jeep in Portland


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