What Is A Rev Limiter? - Roadrunner Converters

Engines need rev limiters to stop damage from over speeding, but what are the different ways in which they will work, and what could happen without one?

Explaining what a rev limiter is, isn’t rocket science. Obviously, they limit the maximum speed an engine can get to. Equally obvious is that they’re there to stop an engine from damaging itself by going past its own limitations.

Engines and Speed

Engines are only designed to be able to operate up to a specific speed, which is calculated in revolutions of the crankshaft each minute. From the assembly line the rev limiter is fixed to the point the engine’s manufacturer is content for it to rev to on a continual basis. It should not be confused with a redline, even though it can oftentimes be fixed at the same point in the rev extent.

Whereas the redline is the beginning of the engine speed area in which it’s a good idea not to spend a lot of time in, the rev limiter is there to stop the engine speed going further than what other parts in the engine are structurally capable of withstanding.

Rev Limiters and Your Valve Train

One instance is the valve train. A lot of engines use metal springs to sustain the valves’ intended function and return them to their correct positions even under strenuous use, but should an engine over speed, the valve springs can go further than their boundaries and can fail to rebound the valves back fast enough. They effectively get kept open.

That’s known as valve float and it’s bad for your engine. It could cause a loss of compression, mis-fires, or it could even make the valves spring so far back that one strikes a piston. Anticipate ordering new engine parts soon following that happening. One other thing is that a connecting rod (commonly referred to as a con-rod), the arm that joins a crankshaft to a piston, is going to shatter or snap. If that occurs, you’re looking at a very big price tag and possibly even a new engine.

Hard and Soft Rev Limiters

You’ve most likely heard of ‘hard’ and ‘soft’ rev limiters. Hard limiters are ones that you bang into with a nasty jerk as the engine’s power is suddenly stopped. Soft limiters slow the revs more progressively, like the engine is bumping against a coil spring.

Hard Limiters

A hard limiter severs either the spark or more usual the fuel, stopping combustion and stealing the engine of all its power instantaneously. That’s what produces such a violent effect when you hit one. Power is returned as the engine speed drops by a minute amount, then gets cut again as it hits the limiter. Soft limiters cut the fuel, but do it more steadily, decreasing the engine’s fuel supply so it runs progressively lean, slowing and ultimately stopping the engine’s acceleration.

Soft Limiters

It can be claimed that a soft limiter is better for road vehicles since it places less stress on different drivetrain components and, when forced, is going to hold engine revs at a fixed maximum instead of releasing that ‘bounce’ you get when you hold the throttle pinned on a hard limiter. Nevertheless, a hard limiter allows you to get to one rpm from the metaphorical wall before it is going to break free, which is ideal for full throttle racing. Soft limiters start to rein things in more quickly.

Rev Limiters and Manual Transmissions

It’s important to note that if you’re getting close to your rev limiter in second gear, then try to grab third but get first, the limiter is not even going to be able to stop sudden doom smashing the engine in its intricate parts. The forced overspeed means severe damage is almost certain unless you’re on the clutch in a flash.

Variable rev limiters are not uncommon currently. These typically set the maximum revs lower while the car is motionless, for instance, or while the engine is still in a warm-up mode. By modifying your engine, you can safely upend the limiter; a process that can even be done with non-OEM tuning software. Rev limiters are there to safeguard your vehicles engine.