Does hitting the redlining of your car extract the last drop of power, or does it do more harm than good? We explain why it’s bad to redline your engine.
Updated 30 August 2022 by Practical Motoring
Redline. We know it as the red-shaded section of the tachometer and it refers to the maximum engine speed that an internal combustion engine can operate at before literally tearing itself apart.
So, just what is the redline?
Well, the movies would have us believe that riding the redline in a vehicle will squeeze out every drop of oomph but by revving your car to redline or, worse, holding it on the redline, all you really do is cause the engine to tear itself apart. See, car makers determine the redline of an engine based on several things like peak power and torque delivery but also on the structural strength of the componentry, including the piston, bearings, valves and so on. Thus, if you don’t exceed the engine’s maximum capability, it’ll happily rev without causing structural failure.
But, cars have rev limiters, right?
Yep. And these things tend to prevent you from either revving to redline or holding the car on the redline – but rev limiters are a reasonably modern thing. What the rev limiter does to prevent over-revving is to cut fuel flow to the engine or it might also fiddle with the ignition system but, in both instances the system aims to reduce revs to a safer level.
The danger of constantly revving to redline isn’t something owners of vehicles with an automatic transmission need to worry about, and that’s because the gadgetry controlling the transmission will make sure it upshifts before hitting the redline. In a sportier vehicle, the transmission will be allowed to rev closer to redline or even hit it briefly (usually only in Sports mode). And, in vehicles with a manual override function, like the Kia Stinger, the transmission will be allowed to hit the redline before upshifting…but they will upshift. I’ve read reports where journalists have claimed they’ve been able to ride the redline in the Stinger but this is nonsense.
What’s the benefit of revving to redline?
There isn’t one. See, while some people will tell you that the engine is producing power all the way to redline that’s not entirely true. Sure, there’s power and torque being generated but peak power and torque is really what you’re looking for. So, take the Mercedes-AMG 35, for instance, it’s redline runs from 6500-8000 but it’s making peak power (225kW) at 5800rpm and peak torque (400Nm) between 3000-4000rpm. Beyond those peak rpms, power and torque starts to drop away, so, revving beyond the 5800rpm peak power output is theoretically pointless.
However, say you’re in a drag race with the Mercedes-AMG 35, you might push towards redline and shift into the next gear at 6200-6400rpm to account for a drop in revs. This rev drop as you shift into the higher gear will then ensure the vehicle is revving at that peak power sweet spot but you’ve still shifted before redline.
So, then it sounds like I can’t rev to redline, so…
That’s almost true. It’s still possible to unintentionally rev to redline. Say, for instance, you’re in third gear racing towards redline and you want to shift into fourth but accidentally shift into second. The transmission will be going much faster than the engine and so the engine, when the gear is engaged, will rev and exceed the redline. This will cause the vehicle to feel like it’s hit a brick wall with engine braking and is the main way revving to redline can cause engine damage. This sudden and dramatic exceeding of the manufacturer tolerances is often referred to as the ‘money shift’ because of its potential to cause expensive damage to your vehicle.
What about the Italian Tune-Up?
Ever heard of this? Where you turn on the car and rev the bejezus out of it…the idea is that it’ll impress your mates and that it’ll burn of carbon deposits and make the engine run better. But it doesn’t. Not in normal driving anyway. See, while it’s theoretically possible for an engine to be revved hard enough and become hot enough to burn off carbon deposits the thing would probably have torn itself long before the deposits were burned off.
You know enough to sound like you know what you’re talking about, but not enough to give good advice. While it’s true that the rev limiter is there to protect your car, brushing up against the rev limiter in a type 1 over-rev isn’t really a big deal. When you bounce off the rev limiter during acceleration, the load on the motor in that case is within the stress limits of the motor and reasonably proportional to the rpm speed. It only just looks and sounds bad, and costs a fraction of a second at the finish line. Redlines are set with a buffer so that in any normal range of operation, the motor is within its stress limits. It’s only when you vastly exceed the limiter like when you downshift to the wrong gear in a type 2 over-rev that you’re looking at mechanical failures.
Italian tune-ups also work! The situation doesn’t always call for it and it’s not just “get the revs high once and you’re good.” But if you have a car that’s been driven too gently for a long time, it will have those carbon deposits because it never gets hot enough to burn them out. The goal is to get the motor close to max operating temperature and keep it there for a little bit. I do this by hitting the freeway in second gear for a couple exits. As long as your cooling system isn’t also busted, you’re gonna be fine.
Oh forgot to make this point too: yes in most cars you usually don’t make peak torque and horsepower at max RPM, but you still go all the way to redline sometimes because you make more torque and horsepower toward the top of the gear than you would after you shift into the next gear, so you linger in the lower gear until your RPMs will be at a more efficient value after you shift. It depends on the car, track, and current velocity, but it’s a commonly used driving technique in the professional world.