Over-Revving an Engine

It can be tempting to drive your Mustang into the red, but over-revving can cause serious harm to the components of your engine and even total your vehicle.

Max RPM

Every engine has a maximum RPM. Though max RPM can be increased by tuning and otherwise modifying the engine, there will always be a point where the engine is unable to turn faster without damaging itself.

There are engines that can reach 18,000 RPM, but they’re usually found on a Formula One track. Even for Mustangs equipped with the third generation 5.0L Coyote engine, 7,500 RPM is really pushing it unless you’ve tuned your engine.

Even then, no matter how much you tune your engine, there will always be a max RPM.

Ultimately, there are a few factors that limit your engine’s max RPM. These include air flow, engine displacement, and the bore and stroke of the pistons.

Air Flow

Engines rely on a series of minor explosions to provide the propulsion that pushes them forward. Explosions require ignition, fuel, and air, like any fire. Many people add a cold air intake or functional hood scoop in order to increase the air flow or decrease the temperature of the air that reaches the engine so that they can get more propulsion per each explosion, but ultimately air flow can only be increased so much, and the air available will create a limitation on air flow.

Engine Displacement

When we refer to engines in “liters” we’re referring to the total volume of the cylinders of the engine. Ultimately, the engine’s displacement becomes a limiting factor in max RPM. More displacement creates more torque and power.

Piston Stroke

The engine's pistons have four stages they need to hit – induction, compression, combustion, and exhaust, and that requires time. Admittedly the time it requires is minimal, but it does become a limiting factor.

Increasing Max RPM

It’s possible to increase your max RPM, but it’s challenging. Formula One racers are able to because their cars have such a low mass and their engines have a smaller displacement and shorter stroke. Even in Formula One, cars cannot surpass their max RPM without risking damage to their vehicles.

Rev Limiters

Modern engines are outfitted with rev limiters in order to make it more difficult to over-rev your vehicle and damage the engine. You really only need to worry about over-revving if you’re into classic cars or like to tune your car to drive it to next level performance (like most of the people at CJ’s). You can install rev limiters in older cars or buy ones that can be customized for your new tuned max RPM as well.

Most rev limiters are “fuel control” limiters. Essentially, they take the fuel away from your car, forcing it to slow down. Spark control limiters are the other type of rev limiter. They function by taking away the other thing you need for combustion: The spark. The problem with spark control limiters is that fuel is still injected into your engine’s cylinder; the fuel is just not getting burned. That fuel is still going to end up somewhere, and where it ends up is kinda a whole separate fun issue you’ll have to sort out after your engine decides to slow itself down.

Unfortunately, If you have a manual transmission vehicle and shift poorly, it doesn’t matter if you have a rev limiter installed. If you’re downshifting early, even cutting power won’t necessarily prevent your engine from rotating much faster than its acceptable range. Most often, over-revving occurs not because of a deliberate attempt to push a vehicle, but rather as a result of missing a gear while shifting.

Mechanical Over-Rev

When a manual car accidentally downshifts, a rev limiter will not be able to react quickly enough to prevent the engine from over-revving. This is what’s referred to as a “mechanical over-rev,” and even though it’s only a momentary mistake, it can have long-term devastating consequences for the engine.

The first thing most people notice is the sound. The car will immediately make a horrifying sound as it is thrown into an uncomfortably high RPM. This sound is a warning alarm letting you know that something has gone seriously wrong.

The best way to avoid over-revving if you are driving a manual transmission car and realize that you have under-shifted is to jam the clutch pedal in. When your clutch is depressed in a manual car it prevents the wheels from being connected to the engine.

Results of Over-Revving

Over-revving can cause serious harm to your engine, even in the few seconds it takes to realize that you meant to shift to fourth gear but wound up in second instead. The good and bad news is that if you seriously damage something you’ll know it almost instantly.

Valve Train Damage

Over-revving can cause damage to your valve train by causing a valve to stay open for too long. This leads to valve float. Valve float occurs when a valve is stuck in between open and closed. This will cause an immediate loss of power. Valve float is the only one of these problems that can be "quiet" and cause problems later, but you’ll be able to tell by either the engine misfiring or losing power at higher speeds. To fix valve float you just need to replace the springs, which is a great thing to do as part of routine maintenance anyway.

Throwing a Connecting Rod

Throwing a connecting rod is the number one cause of catastrophic engine failure. The connecting rod is a metal piece that looks almost like a wrench and connects your piston to your crankshaft. It’s crucial to the operation of the vehicle, but that’s not why it causes catastrophic engine failure. Connecting rods are located in the center of the magic that is an internal combustion engine.

The connecting rod is a solid piece of metal, with the pistons and cylinders above, and the crankshaft below. No matter which way you toss your connecting rod it’s going to take a lot of important stuff out with it, and it’s going to throw your engine out of sync. The only reason the thousands and thousands of mini-explosions that occur in your engine don’t destroy your car is a perfectly timed series of actions that contain and utilize every bit of power from those explosions.

If one thing falls out of choreography though, it doesn’t take more than a second for the entire routine to be ruined.

Flywheel/Clutch Damage

Out of all the damage that over-revving can cause, this maybe makes the most sense. Your clutch literally connects the wheels of your car to your engine. It’s the part of the car that’s going to take the most direct damage when the RPM is significantly greater than what the safe max output is rated as.

The clutch only works because of friction, but too much friction creates a lot of heat and tension in between the clutch plate and flywheel.

No Damage

Typically with over-revving, no damage showing really does mean no damage. If you had an accidental overrev and check engine lights didn’t suddenly come on and your engine didn’t grenade or suddenly fall into limp mode, you got lucky.

There’s no reason or logic to who gets lucky in these scenarios, but sometimes people do mechanically overrev and magically their engine escapes with only minor damage or even no damage at all.

If it seems that you have escaped over-revving with no lasting damage to your vehicle, you’ll still want to check the valves.

How to Avoid Over-Revving

Your best solution when it comes to avoiding the damage of over-revving is simply to avoid over-revving. There are a few ways to do that.

Push the Clutch

As noted, most people who cause mechanical over-rev do so when they are shifting from third, try to go to fourth, and somehow end up in second gear instead.

When that happens, you can push the clutch in on your car and that will disconnect the power of the engine from the wheels of the car. This will prevent the wheels from unleashing the full force of their weight on your engine, and if done quickly enough can prevent your engine from being severely damaged.

Upgrade Your Shift Knob

Keeping your shift knob in good working order and replacing it when it gets too worn can also help with accidentally shifting into a lower gear. Being able to grip your gear shifter helps you avoid mistakes. You can even try a short throw shifter to see if that helps you make tidier shifts.

New Rev Limiter

In an older car, you can install a rev limiter. This will allow you to set your max RPM, and while it won’t be able to get you out of a jam if you accidentally shift poorly, it will help you most of the time.

If you notice a loss of power following an accidental mechanical over-rev incident, getting your car to a garage or checking it out yourself can help to keep issues like valve float from becoming a blown cylinder.

Over-revving can be catastrophic, but by understanding it before it happens you hopefully will be able to keep your presence of mind and reduce the damage as much as possible. Though car engines are incredibly powerful and strong, they still rely on a complex system of parts that can become out of sync and cause extreme damage in mere seconds.