Why do cars have a redline and what is it for?

You might not notice it on your daily drive, but your engine has so much more power to give if you simply step on it, though there is a limit as illustrated by your tachometer. The term “redline” isn’t something that is used in movies only. In everyday speak for cars, it’s the top speed of your engine as stated by the manufacturer. 

Most engines in consumer-grade vehicles are built to a standard of reliability, and manufacturers know their motors inside and out enough to say “yep, this is the limit, no more.” There is a fine line between an engine being worked efficiently and properly, and a pandora’s box of what-ifs and potential problems. 

If your vehicle comes with a tachometer, you will typically see numbers between 0 to 5 printed or displayed in white with indices that have no filler in between. The redline is usually located towards the top of the end of the tachometer, and its denoted by red or bold fonts with the space of its indices filled out with a certain color (likely and appropriately in red). 

To keep your engine from over-revving, manufacturers install a redline, which is a point where the engine speed will refuse to climb. Rev limiters are often programmed into the car’s ECU and cut the fuel from the engine or alter the input to the throttle. In whatever case, the purpose of these systems is to make sure that the engine’s components don’t come into contact with each other and it helps ensure and tell the driver that the motor’s nearing its limit in terms of power production. 

TIn a traditional motor, you have multiple parts that are working in tandem to create kinetic energy. Chief of these two parts that get awfully close together during a typical combustion cycle is the valves and the pistons. At high engine speeds, the valves and pistons are moving at an extremely fast rate to produce power as demanded by the driver. Most cars come with a specific tune that allows their valves and pistons to work seamlessly together and not come into contact. Given how most modern engines are getting higher and higher in compression ratios, which is a fancy way of saying that the piston squeezes the air-fuel mixture closer to the valves, the risk of a valve coming into contact with a piston grows as the engine speeds up. 

That isn’t to say that running your engine at high speeds will break it. The redline is simply the “safe zone” for your engine to operate in. Most passenger cars like the Toyota Vios, Geely Coolray, and even diesel examples like the Kia Sorento come with redlines regardless of their engine type, be it naturally-aspirated gas, turbocharged gas, or diesel. The safeguards are set in place to keep the engine within its specifications and heat efficiencies, which ensures better longevity just in case a driver decides to wring it out all the way. 

On top of that, high RPMs without good air intake and fueling will cause your engine to either richen too much or lean out. Remember, you need a lot of fuel to make a lot of power and your fuel pump needs to keep up, your air intake must have good flow, and your injectors must supply the right amount of fuel to create power. If you go beyond the limits of your components and lean your engine out, there is a chance that you may cause a malfunction due to an improper and dangerously lean or rich air-fuel mixture. There’s a balance that must be met and upsetting that balance may cause your engine to skip a beat and grenade itself in the process. 

In truth, most car engines can rev out higher, it’s just that the manufacturer uses the red line to not only protect the engine from high-speed damage but also to maintain a level of heat that can still be handled by the vehicle’s cooling system and all of its fluids. 

More engine speed means more heat, and more heat tends to degrade oil, overwork the cooling system, and create an environment that makes the engine’s components prone to warping, thus causing a malfunction. 

Just because a manufacturer says that your engine has a redline, doesn’t mean that you can keep it there for long periods of time. Keeping your engine at redline will stress it out more than it should, and it will introduce a lot of heat to it if the cooling system can’t cope and isn’t made for long and sustained redline stints. Remember, however, you don’t have to bang on the limiter for your engine’s fluids to degrade, sustained load at high RPMs produces high heat, and not enough airflow means that the cooling system won’t be able to funnel enough cold air in and extract the heat out. Even at low engine speeds, traffic can cause your car to overheat given a faulty cooling system. 

You can hit redline once and get away with it, but keep it there for more than a few seconds at a time and you might need to worry. If you’re one of those guys that love revving and banging on the limiter between stoplights while refusing to shift up a gear, then it’s likely that your motor is a little more worn and your fluids are possibly heat-cycled, given enough trips to the redline. High heat without the proper airflow can be damaging to your engine and its fluids if it's neglected. If your motor has experienced a ton of heat for sustained periods of time, your oil life is likely to be reduced and you may need an oil change at an earlier interval to keep the fluids fresh and your engine properly lubricated. 

So should you worry about hitting the redline? Not all the time. Done a few times for a few moments, your engine won’t magically break on you, it just means that you hit the limiter and the ECU dropped you back into the safe zone. It’s when you keep your motor running at extremely high engine speeds that can cause damage and decreased longevity. Manufacturers know what their motors can take if well-maintained, and the redline is there to keep you driving and not leave you on the side of the road. If a motor didn’t come with a redline, it’ll likely keep revving out until the air going into it was not sufficient to spin it up faster and faster. With no limiter, you run the risk of over-revving your engine to the point of failure, which is beyond the manufacturer’s recommendation. 

That’s the thing, however, your manufacturer recommends that you can run up to that engine speed. There are times when it’s not good to, but given good driving conditions, your engine will be fine especially on more spirited drives. Just watch out for your oil health and your coolant levels since they’re both equally important for you to keep your engine running and running well.