This Is How Rev Limiters Work, And Why You Need One

Explaining a rev limiter isn’t rocket science. Sorry for stating the bleeding obvious, but they limit the maximum speed an engine can achieve. Equally plain is that they’re there to prevent an engine damaging itself by extending beyond its own limits.

Any engine is only designed to be able to function up to a certain speed, which we measure in revolutions of the crankshaft per minute. Out of the factory the rev limiter is set at the point the engine’s maker is happy for it to rev to on a regular basis. It’s not to be confused with the redline, although it can sometimes be set at the same point in the rev range.

While the redline is the start of the engine speed zone in which it’s best not to spend too much time, the rev limiter is there to stop the engine speed extending beyond what other parts in the engine are physically capable of withstanding.

One example is the valvetrain. Most engines use metal springs to maintain the valves’ intended function and return them to their proper positions even under hard use, but if an engine overspeeds, the valve springs can go beyond their limits and fail to bounce the valves back quickly enough. They effectively get held open.

That’s called valve float and it’s not good. It can cause a loss of compression, misfires, or it could even cause the valves to spring so far back that one hits a piston. Expect to be ordering fresh engine parts very shortly afterwards. Another possibility is that a connecting rod (commonly called a con-rod), the arm that joins a crankshaft to a piston, will snap or shatter. If that happens, you’re looking at a very big bill and potentially even a new engine.

You’ve probably heard of ‘hard’ and ‘soft’ limiters. Hard limiters are the ones that you slam into with an unpleasant jerk as the engine’s power is suddenly cut. Soft limiters slow the revs more gradually, like the engine is pushing against a coil spring.

It can be argued that a soft limiter is better for road cars because it places less stress on various drivetrain components and, if forced, will hold engine revs at a set maximum rather than delivering that ‘bounce’ you get if you keep the throttle pinned on a hard limiter. On the other hand, a hard limiter allows you to get to 1rpm from the figurative wall before it will cut in, which is better for flat-out racing. Soft limiters begin to rein things in sooner.

A hard limiter cuts either the spark or more commonly the fuel, halting combustion and robbing the engine of all its power in an instant. That’s what creates such a fierce effect when you hit one. Power is restored as the engine speed drops by a small amount, then gets cut again as it reaches the limiter. Soft limiters cut the fuel, but do it more gradually, decreasing the engine’s juice supply so it runs increasingly lean, slowing and ultimately halting the engine’s acceleration.

It’s worth noting that if you’re approaching your rev limiter in, say, second, then try to snatch third but get first instead, not even the limiter will be able to prevent sudden doom smashing the engine in the danglies. The forced overspeed means severe damage is almost inevitable unless you’re onto the clutch like lightning. You might also lock the driving wheels and fly off the road upside down/backwards/on fire/all of the above.

Variable rev limiters aren’t uncommon these days. These normally set the maximum revs lower while the car is stationary, for example, or while the engine is still in a warm-up phase. With modifications to your motor you can safely raise the limiter; a process that can even be done with aftermarket tuning software. Rev limiters are their to protect the heart of your pride and joy, so it’s best to respect them.