Tech Article #1: Supercharger Vs. Turbocharger

For as long as people have been debating the domestic versus import topic, they have been debating the best form of forced induction; superchargers versus turbochargers. On both sides of the debate stand strong opinions and in some cases, serious hatred, but for both sides the quest remains the same, to increase power out-put and efficiency. This article will serve not to chose a side, but as an informational to demonstrate the pros and cons of both sides, allowing the reader to decide which they prefer.

Superchargers and Turbos are both designed to force air into the combustion chambers; compressing the air first rather than leaving it at normal atmospheric pressure. This enables the engine to burn more fuel per power stroke, converting more fuel into power and energy producing 40-100% more power than N/A (normally aspirated) engines. In order to understand the pros and cons, one must first understand the basic mechanics of both forced induction methods. [See Fig. 8].

In 1908 a man by the name of Lee Chadwick (USA) bolted on a 3-Stage centrifugal compressor onto the side of his race car and won the “Wilkes-Barr Hillclimb” and won. This device featured 12, twelve blade impellers  that were driven by a leather belt attached to the engine crankshaft. This was the birth of the automotive supercharger.

The supercharger (referred to from here on as the ‘s/c’), is mounted to the engine and is driven by a pulley that lines up with the crank/accessory belt [see Fig 1].  To operate, air in drawn into the s/c and is compressed either by  twin rotating screws [screw-type], counter rotating rotors [roots-type] or by an impeller [centrifugal type] [see Fig 7]. This compressed air is then forced into the engines intake. The faster the crank speed, or RPM, the faster the s/c spins. This allows the s/c to create more boost, the average ‘street vehicle’ s/c creates 6-9 psi. The peak operating speeds are between 15,000 – 40,000 RPM.

Built in 1909 by a Swiss man by the name of Alfred Buchi, the turbocharger took the basic idea of the s/c and harnessed the stream of exhaust from a diesel engine, that normally would have been wasted, to drive a turbine/compressor. He wanted a machine that would have the benefits of the s/c without the drain of power on the crankshaft. Thus the turbocharger was born. [See Fig. 5].

The turbocharger (turbo) works in a similar fashion to the centrifugal (internal impeller) s/c. [See Fig. 3] However the turbo is not attached to the engine, nor driven by the engine’s crank pulley system. A turbo is mounted to the exhaust manifold, and is driven by the exhaust gasses that would have otherwise been expelled by the engine. Exhaust flows through the turbine, driving the impeller and compressing the air and forcing it into the combustion chambers. Typical operating speeds for a turbo are in the neighborhood of 75-150,000 RPM.

Everyone who’s ever been to a muscle car show knows the distinct sound of a s/c. A high pitched “whine”, reminiscent of a hundred hamsters crying out at once. Screaming for the gates to open so that they can run free, the sound alerts drivers and onlookers a like that a supercharged vehicle is headed their way. Surprisingly easy to install, a s/c is comprised of one major component, the compressor [see Fig. 2] The compressor portion of the s/c is nearly identical to that of a turbo, and when paired with the proper pulleys and belts the system works in partnership with the engine. The only way to create boost from a s/c is to rev the engine. The benefit of this is that since the s/c is driven by the crankshaft, there is virtually no lag while you wait for boost to occur. With a s/c boost can even occur at very low RPM. As long as the s/c is working, air is constantly moving into the induction side of the engine, giving the driver a broader power-band while generating massive amounts of torque. A basic s/c kit can cause a gain of up to 40% in horsepower and 50% in torque.

However the price of having no lag is what some at what some would call a high cost. On the flip side of being driven by the crankshaft, the s/c takes away anywhere from 15-50% of the generated horsepower just to power the s/c itself. Due to this setback, a lot of drivers find the s/c is an inefficient means of forced induction.

While the s/c is the sound or warning for approaching american muscle, the gentle whirring of a turbo charger combined with that of a blow-off valve has become the signature sound of the ‘import’. Turbos are generally more complex than a s/c, and require manifold and exhaust modifications, intercoolers and extra oil lines upon install to operate correctly. [See Fig. 4]. A turbo consists of a compressor and a turbine working together to create power. [See Fig. 8] The compressor is attached to the air intake manifold, and is driven by the turbine, which is attached to the exhaust manifold, via an output shaft. As air passes through the turbine from the exhaust manifold the turbine spins or “spools” and uses the output shaft to turn the blades inside the compressor, therefore forcing more air through the intake. The pressure of this air is called “boost”. The larger the turbo the longer it takes to build boost, however it will produce more boost than a smaller turbo that may boost faster. In contrast to the s/c, a turbo is highly efficient, stealing almost no power from the engine to create boost; capturing “free” energy  to create more power.

The common price of a turbo, is the addition of higher temperatures in the engine bay. Turbo chargers are much more difficult to install and require supporting modifications as well. As boost increases, so does heat, so installation of an intercooler is recommended. An intercooler will work to prevent engine detonation and fire. While a turbocharger works to increase the amount of air going into an engine, it can also negatively effect the way air flows back out. This negatively effects the positive pressure difference between intake and exhaust making cam timing and exhaust system design incredibly important. While installation of a s/c can be done by a novice, installation of a turbo is usually left to the professionals who can tune the engine to peak boost numbers.

As you can see both the s/c and the turbo have benefits and draw backs, but each has their own dedicated fan base amongst the petrol-heads. A poll given here on Gear Head Haven shows that 10.3% of members polled chose a supercharger as their preferred method of performance; while 38.5% percent chose a turbo charged set up. [http://gearheadhaven.com/forum/index.php/topic,365.0.html].
Both methods also have specific racing types that subscribe to their benefits. The s/c is most commonly used in drag racing, top fuel racing [NHRA] and American muscle cars, ie. Chevrolet, Pontiac, Mopar, and Ford etc. These powerful engines benefit from constant boost and fight the power drain with the addition of raw horsepower. Turbochargers are seen more often in import or European vehicles, ie. Subaru, Mitsubishi, Volkwagen and Porsche, these vehicles are more commonly prone to auto-x or gymkhana events.

The debate for which is the best will continue to rage for as long as there are gear-heads, and the winner will still always be in the eye of the driver. I hope that this article has shed some light onto the subject for you, now all you have to do is chose which method you prefer!
[please forgive the quality of some of the images. Some were scanned from Street Turbo Charging.]

If you’d like to suggest a top for “Electra’s Editorials” feel free to post up in the forums [http://gearheadhaven.com/forum/index.php/topic,834.0.html], or via myspace.com/wankeldriv3r or facebook.com/wankeldriv3r.

Thanks for reading! XO Electra

Sources:

Boodasingh, Duane. “Turbochargers vs. Superchargers” 27 January 2005.
Ferrari, Brian. “Overview: Supercharger vs. Turbocharger.” 19 July 2004. www.automotiveartciles.com
Hotrodders.com “Turbo vs. Supercharger-Write up!” 7 January 2010
Superchargersonine.com “Supercharger Tech” 3 April 2002
Warner, Mark P.E. Street Turbo Charging. HP Books, New York, NY. 2006