Sexy-sounding equipment is really all about efficiency, cleaner emissions
A popular option as either factory equipment or an aftermarket add-on, a forced-air induction system is a relatively inexpensive and uncomplicated way to extract more power from an engine, particularly in the smaller, emissions-managed engines popular in modern vehicles. But to the average driver, the function and even the terminology of the system can be more than a little confusing.
The difference between a turbocharger, which is the more common system found on current or recent car models, and a supercharger is in how the unit itself is powered. Both types compress air with a small turbine; in a turbocharger, the turbine is driven by gases diverted from the engine’s exhaust, while in a supercharger, the turbine is driven by the engine itself through a belt-and-pulley arrangement.
Both function in the same basic way, by compressing intake air above atmospheric pressure; typical systems usually add from six to eight pounds per square inch (PSI). Stuffing more air into the engine boosts power without compromising fuel efficiency, which is why equipping models with a turbocharger has become a popular option for manufacturers.
Wave of the future
For many manufacturers, in fact, a turbo system is no longer an option but a necessity. According to a report in Auto Express last year, BMW will no longer offer a naturally-aspirated engine, and competitor Audi is also phasing them out; the only non-turbo engines available in Audi’s lineup for the current model year are in the RS5 and R8 models from the Quattro GmbH performance division.
A little closer to home, Toyota, which has long experience in turbodiesel engines, reportedly will add a turbo-equipped four cylinder gas engine to its lineup in the near future, as will Honda. Ford’s new EcoBoost engine, which equips the Fiesta, Focus, Mustang, Escape, Explorer, and Expedition, is essentially designed around a turbocharger, which allows credible performance from engines with a displacement as small as one liter.
Superchargers, while not nearly as common as turbochargers, are also becoming more popular as stock equipment, particularly among models positioned as performance cars. Jaguar offers a supercharger option on all its models, according to car buyers’ guide Autobytel, and the set-up can also be found on Audi’s S4, A4, and A6 models, the Chevrolet Camaro and Corvette, the Dodge Charger and Challenger, and many Volvo models, including the S60, which is unique in being equipped, in one version, with both a turbocharger and a supercharger.
Car and Driver magazine, in an article last year, identified the culprit behind the apparent rapid demise of naturally-aspirated engines as European Union Regulation No. 443/2009, which mandated strict emissions reductions in two stages: first, between 2012 and 2015, automakers were to reduce their CO2 average (an average of all models built by a particular manufacturer) to 130 grams per kilometer; and in the second phase, emissions are to be cut further to just 95 g/km by 2020. Some countries have even gone beyond that, like the Netherlands, Car and Driver pointed out, passed a regulation mandating a limit of 80 g/km by 2020.
But as Uwe Wolf, race engineer for Rahal Letterman Lanigan’s USCC team, pointed out, “Turbos really don’t do anything for emissions. What they do is make up for less power from the way these engines have to be designed to produce lower emissions.”
Wolf went on to explain that in addition to “downsizing” engines, manufacturers have also “down-tuned” them, lowering rev ranges by lengthening the piston stroke (which results in more efficient fuel burning). “Take the [BMW] M4, for example,” he said. “The stock 3.0-liter delivers its peak torque at around 1,800 rpm [It’s actually 406 Newton-meters at 1,850 revolutions per minute, according to BMW’s published specs]. Without a turbo, that just wouldn’t be possible.”
Which is better?
Preferences for turbochargers or superchargers seem to be largely matters of taste, as far as most mechanics and tuners are concerned. For the owner considering adding one to his or her own car – which is not a particularly difficult modification in either case – there are a couple characteristics of both that one needs to be aware of:
1. Turbos tend to be a little more emissions-friendly than superchargers: Because a turbo is powered by exhaust gases, it is equipped with a simple pressure relief mechanism – called a wastegate – that allows exhaust gas to bypass the turbine once a certain preset pressure is reached. This prevents the turbo from “over-boosting,” which would result in less efficient fuel burn, less power and dirtier emissions. The wastegate also allows the turbo to generate its peak boost at lower revs, whereas a supercharger, because it is driven by the crankshaft, only produces its peak boost at the redline.
2. Turbos are more efficient than superchargers: This is just basic logic; because the turbocharger is running off exhaust gases, it is not scavenging power directly from the engine as a supercharger does, and therefore produces more power from the same size package. But…
3. Superchargers are less complex, easier to install and maintain: For one thing, a supercharger has half the air plumbing of a turbocharger, because it isn’t connected to the exhaust side of the engine. Superchargers also typically do not run as hot as turbochargers – again, because of the lack of exhaust gases flowing through it – and so do not usually require an oil supply, which most turbos, especially larger ones, do need, along with the associated plumbing to the engine’s oil system.
4. Superchargers are not prone to “surge”: Because it’s driven by the crankshaft, a supercharger is essentially providing boost (although less than peak) all the time, meaning the transition to peak power is much smoother. In an exhaust-driven turbocharger, the turbine typically takes some time to “spool up,” providing a surge of power when it hits its rotational speed that actually compresses air. In certain situations, the extra jolt of power might actually make the car unstable or catch the driver by surprise; with most supercharged cars, the surge, if any, usually only occurs at the top end, where its effect is much less noticeable and unlikely to unsettle the car.