Overboost is becoming a common feature of turbocharged vehicles, providing turbocharged engines with an extra kick of power.

HOW MANY TIMES have you seen the word ‘overboost’ in a car review when talking about ‘extra’ power available from a turbocharged engine. For those without grease under their fingers, it could seem like under-bonnet black magic. So, what is overboost?

Before we answer that, we need to explain what a turbocharger is…

A turbocharger is designed to increase the amount of compressed air into an engine. A turbocharger is like a mouth blowing air onto a fire to get a flame going.

Typically, a turbocharger will pump around eight pounds per square inch (8psi) of compressed air into the engine. This is around twice normal atmospheric air pressure which is 14.7psi. This means, you can generally expect to increase performance of an engine, via turbocharging by around 40% or a little less (there are efficiency losses during the process).

A turbocharger uses exhaust gases to spin its turbine/fan which can spin at speeds of up to 200,000rpm – the more exhaust gases you pump into the turbine, the faster it will spin. The pressurised and compressed air produced is then forced into the cylinders which, along with more fuel being injected allows more power to be produced because of the bigger explosion.

What’s the advantage of a turbocharger?

There are widely considered to be a couple of key basic benefits to turbocharging an engine. And these are that a relatively small engine, and some makers are turbocharging engines as small as 1.0-litre with three-cylinders, can make much more power than without the turbocharger.

Also, using a turbocharger doesn’t add much to the weight of the engine and unless the performance is being used, a turbocharged engine isn’t that much thirstier than a naturally-aspirated (non-turbo) engine of equivalent size and output. And that’s because turbochargers rely on reusing exhaust gasses to spin the fan that aren’t being used in either naturally aspirated or supercharged engines.

What is overboost?

No doubt you’ll have read about this in a car review where it says the vehicle makes X amount of power but Y on overboost. The idea with overboost on a turbocharged engine is about getting the most amount of power out of the thing but while keeping the engine safe and the emissions down. Indeed, overboost is something we’ll see more and more of on turbocharged engines as car makers work towards meeting ever stricter emissions laws.

As the name suggests, overboost is when the turbo runs at a higher pressure (forcing more air into the engine) than would be safe to the engine on a constant basis. Overboost is usually limited to a set time of around 10-20 seconds with the hike in power around 5% or more.

Overboost isn’t something the driver can control and is dependent on set scenarios. For instance, it’s usually an automatic response when the vehicle is in a certain gear, at certain revs and the throttle is pinned. The idea is to provide a burst of extra oomph when the vehicle determines it’s needed, like when you’re overtaking. The new Hyundai i30 N features overboost and so does the Volkswagen Amarok V6.


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