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What is a supercharger?

Superchargers are an increasingly rare breed, but what are they exactly and how do they work?

The word “super” is a real favourite of marketeers and writers of all stripes, because it makes just about anything sound more exciting. Super car, super hero, super food and even superannuation, they all just sound a bit more… awesome.

It stands to reason, at least in ad-speak, that superchargers must be just that little bit better than turbo ones, but is that the case?

As anyone who has ever overdone it on their New Year’s Resolution exercise will tell you, there comes a point where you just need more air than you’re getting.

Regular breathing is all well and good when the only weight you’re lifting is a Big Mac but, at full exertion, you need more air than you can suck in.

So it goes with engines. It’s all well and good to put petrol in the tank, but for every litre of fuel your engine needs 14.7 litres of air to work properly. So if you’re asking your car to get up and run, it makes sense to give it as much air as possible.

Enter the supercharger, essentially a whacking great air pump that shoves air into the engine, allowing it to burn more petrol, extract a bigger bang and deliver more power.

Superchargers operate in a similar way to turbochargers, but are, in fact, less efficient. Where a turbocharger uses otherwise useless exhaust gases to power its air compressor, a supercharger runs off the power of the engine.

In severe cases, the supercharger can use up to 30 per cent of the engine’s power to run, diminishing the gains made by forced induction.

If turbos are better, why use superchargers at all?

They may not be as efficient or deliver the peak power of their fume-powered brethren, but they offer something unique

If numbers told the whole story, superchargers would be on the mechanical scrapheap, next to magnetos, distributors and rotary-dial phones.

A good way to think about numbers versus reality is to look at Formula 1, where numbers, including those with dollar signs, make little sense.

While Sebastian Vettel and Lewis Hamilton have more race wins, F1 fans would still argue that Ayrton Senna was a better driver.

So it goes with superchargers – on paper, they may not be as efficient or deliver the peak power of their fume-powered brethren, but in the real world, they offer something unique.

Because superchargers run directly on engine power, they don’t experience lag. There’s no sense of waiting for the engine to gird its loins and fire you off into the middle distance; instead, there’s an instant surge of torque that’s always there, ready to propel you forward like a scrum-full of Springboks.

It’s a pretty intoxicating feeling, and is accompanied by a particularly sexy rising whine of a noise, which is far more attractive, or super, than a turbo whistle.

It’s because of this on-demand, low-down shove that some engines, such as Volkswagen’s 1.4-litre TSI, use a small Roots-type supercharger combined with a turbo, to improve low-down torque and eliminate lag. As the turbo gets up to speed, it can deliver top-end power; by this point, a clutch disengages the supercharger to free the engine from the parasitic loss of running the blower. This rarely used technology is known as ‘twin-charging’.

A Roots-type supercharger?

The Roots-type supercharger - named after its inventors, not the book about slavery - is the original huff-and-puffer. It’s also a curious case; where modern superchargers act as air compressors, the Roots-type operates as a set of mechanical bellows, accelerating air into the inlet manifold at a fantastic rate.

When the gale force of air backs up in the intake, it creates huge inlet pressures.

As befits something from the era of Irish potato famines and Jane Austen (first patented in 1860), the Roots supercharger is a fairly basic bit of kit, which makes it simple, cheap and mostly maintenance-free. It can create huge amounts of boost pressure from idle, which means it feels like the most muscular of all superchargers.

Inevitably, the old-school tech also comes with a series of drawbacks; Roots-type superchargers tend to run hotter and less efficient than other types, becoming progressively less efficient as the boost pressures increase. This means there’s a huge surge of bottom-end torque that tends to fall away as the revs rise.

Is there a more efficient way?

Centrifugal superchargers, on the other hand, more closely resemble a turbocharger and operate in a similar way.

The centrifugal supercharger draws its power from engine speed, but unlike Roots and screw-type blowers, the centrifugal option doesn’t offer a lot of boost pressure at low revs. Instead, because its air compressor is basically the same as a turbocharger’s, it needs to get up to speed before delivering boost.

Only a handful of cars offer the instant-access thump of supercharged induction

In practice, it doesn’t feel the same as a turbocharger, because there’s no lag between acceleration and boost. Instead, there’s a progressive increase in boost pressure across the rev range, making the engine behave in much the same way as a powerful, naturally aspirated engine. Like turbochargers, centrifugal superchargers can be used in tandem with an intercooler – basically an air-refrigeration unit – to cool down the air flowing into the cylinder. The hotter that air gets, the less a supercharger can compress it. So, by cooling the air, intercoolers can help centrifugal superchargers achieve efficiency unheard of by Roots-type puffers and on par with twin-screw blowers.

Twin-screw blowers? 

Similar in principle to the Roots-type blower, the twin-screw instead acts as a true compressor, offering good boost from idle that continues at a constant rate, all the way through the rev range.

Twin-screw superchargers don’t create anywhere near as much heat as a Roots-type blower so, while they can’t usually be used with intercoolers, they tend to require less cooling anyway.

To work as a compressor, twin-screw setups - popular on some American muscle cars - have to be engineered with surgical precision, which makes them a much more expensive proposition than centrifugal and Roots superchargers.

The last gasp

Superchargers are an increasingly rare breed. While you can buy anything from a Peugeot to a Porsche with a turbocharger, only a handful of cars offer the instant-access thump of supercharged induction.

While supercharging has been used on occasion for efficiency, its real talents lie in performance, which is why you’ll see them adorning such shrinking violets as the HSV GTS and Range Rover Autobiography.

For supplicants to speed, superchargers are better – because they’re worse. Superchargers are soulful because they’re imperfect, embodying a ‘because I want to’ spirit that flies in the face of the ‘because I have to’ efficiency drive of turbocharging.

So, before emissions numbers force manufacturers to ditch the supercharger forever, find a way to get in and indulge in their very fine whine.

Would you like to see more superchargers than turbos in hotted up cars? Tell us your thoughts in the comments below.