What is a soft limiter, and why do cars have them?

In some countries there is a legal lower limit of sound output from the exhaust. In some cases this is imposed as a general law, in others it’s a voluntary agreement with local authorities or car manufacturers. For this reason many cars have a soft limiter that will only allow a fraction of final torque to be transmitted.

The main reason is to protect the engine and drive line from overloading. If you were to provide full pedal pressure at all times, this would create very high loads on several components in your car. You’ll see this effect when someone jams their car into a lower gear too early or too aggressively, causing the engine to lug and drone until it can rev up enough to bring itself back into its comfort zone.

When full pedal pressure is applied, it places a very high load on the engine and drive train, particularly in lower gears. The classic example of lugging an engine because you’re in too low a gear occurs when a novice tries to power down a one-way street with a stop sign at the bottom, and jams his car into a too-low gear. When the light turns green, he gets nothing but high-pitched engine screaming as the car only moves slightly faster than if he was pushing it with his feet.

What can happen if there is a hard limiter?

A hard limiter imposes an absolute limit that is not adaptable to the speed of the car and as such it will lock up very quickly, giving just a brief moment where power can be felt. This has many disadvantages:

• A number of fast cars will adapt their gearbox to work with the engine to give the best performance over a range of speeds. This means that they will not be able to deliver full power at their optimum speed, which it turn will mean loss of traction and slower acceleration.
• A hard limiter can cause jerky gear changes as the car picks up speed, causing it to lose time both in terms of 0-60 times and in improving its position on the racetrack.
• The limiter effectively increases wear on the gearbox, causing more friction within it and wearing parts down more quickly.

What can happen if there is a soft limiter?

A soft limiter will allow for much smoother power delivery, meaning that the car will not jerk around as power is applied. In terms of accelerating very slowly this will give a slight disadvantage as the car only just reaches the limit before it starts to accelerate again.

In terms of acceleration from higher speeds there are three main benefits:

• The limiter allows for full power with smooth linear delivery, meaning that the car does not suffer any loss in performance.
• If the limiter is set too low then there will be a brief moment where the driver can feel a slight reduction in power before it starts to pick up again. This causes no real problems for accelerating or maintaining speed, but it may cause a loss of position at a critical point on a race track.
• The limiter does not cause extra wear or damage to the gears as power is quickly applied and therefore it will reduce general wear and tear on other parts, extending service intervals.

Conclusion

So a soft limiter gives full power for better acceleration but does not give a significant advantage in terms of top speed. A hard limiter means loss of performance at the optimum car speed and therefore you will lose out in terms of top speed and acceleration.

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If a soft limiter is used then the only real advantage is reduced wear and tear on other parts and reduced wear on clutch plates and gearbox components. This means that they are most likely to be found on cars with electronic gearboxes or those where there has been significant investment in creating smooth power delivery.

As ever, the choice of limiter will always be down to personal taste since some customers want maximum performance no matter what it takes, while others are more interested in the service interval or how their car feels to drive.

Some cars do not have soft limiters fitted. If you would like one then speak to your supplier about fitting one when you next have service intervals carried out.