Under the Hood
Old School Power Meets New School Efficiency
By: Joel Patel \ Associate Editor \ July, 25 2014
Horsepower is the quintessential reasoning behind automotive enthusiasts purchasing a car. Yes, horsepower does mean the ability to do work. In fact, one horsepower has the ability to lift 33,000lb-ft per minute. But it’s also much more than that. Horsepower is a bragging right.
For the majority of car owners out there, the amount of horsepower your car comes with from the factory is more than enough. But for enthusiasts, their car can never have enough. But how would one go about adding more power to their vehicle? Well, one radical way to do this is forced induction.
Forced induction systems, such as superchargers and turbochargers, compress the air coming to the engine. By compressing the air, forced induction systems allow the engine to cram more air into a cylinder. More air equates to more fuel, which allows for a more powerful explosion in each cylinder. Both types of forced induction systems have unique characteristics, as well as similarities. They both have the ability to completely alter a vehicle’s personality. Which one fits best is a tough decision, but one that is ultimately yours.
Superchargers are predominantly associated with muscle cars and have a rich history within the automotive market. This type of forced induction is belt or chain driven and is installed on top of the engine. The belt, or chain, wraps around a pulley connected to a drive gear. The drive gear then rotates the compressor gear, which forces air into a smaller space and discharges it into the intake manifold.
Superchargers, or blowers, can spin 50,000 to 65,000 rotations per minute (RPM). Spinning at 50,000 RPM, a compressor creates a boost of approximately six to nine pounds per square inch (psi). Since this boost takes place in addition to the atmospheric pressure (14.7 psi), a typical boost from a supercharger places roughly 50 percent more air into an engine. Overall, a supercharger is capable of adding 46 percent more horsepower and 31 percent more torque. These are general characteristics of superchargers and differ depending on the type of vehicles and supercharger.
There are three types of superchargers: Roots, twin-screw and centrifugal. The differences between the three are the way they move air to the intake manifold of the engine.
The Roots supercharger, named after the American inventors and brothers Philander and Francis Marion Roots, is the oldest of the three and can be traced back to the 1860’s. These types of superchargers act more like air pumps opposed to a compressor. The majority of roots blowers utilize a two or three lobe rotor design, which helps the supercharger create massive amounts of horsepower at lower RPMs all the way through the rev range.
While it may be an old design, roots blowers offer excellent reliability, high horsepower gain potential, consistent boost through the entire RPM range and can give the vehicle a unique look since the supercharger can literally stick out of the hood. However, this type of supercharger also has some associated negative aspects. Throttle response is known to be somewhat violent, the boost rating diminishes at higher RPMs, and engine temperatures run higher on this type of set up than the other types of superchargers.
Roots type superchargers can be found on a majority of drag cars and show vehicles. The 2014 Shelby GT500 utilizes an Eaton TVS 2300 (2.3-liter) Roots style supercharger to help produce 662hp from its 5.8-liter V8. The original Roots style blower may not appear on a lot of vehicles anymore, but its design has evolved and is now being used on a lot of new age muscle cars.
The second type of supercharger, the twin-screw, is a close derivative of the Roots blower. The twin-screw supercharger looks similar in its design to the Roots blower, but utilizes an axial-flow design on the inside. While the Roots supercharger pumps air into the three lobe rotors, the twin-screw supercharger compresses the air between the male and female rotor. These rotors have a conical taper, which means that the air pockets decrease in size as air moves from the fill side to the discharge side. As air enters the supercharger, the male rotor rotates clockwise, while the female rotor rotates counter clockwise. This process traps and compresses the air between them and channels (screws) the air to be discharged.
The twin-screw design has expanded from its predecessor – the Roots supercharger – and has a host of advantages above the Roots blower. One major advantage of a twin-screw blower is the amount of horsepower to run it. The twin-screw blower requires 10-16hp less to run than the Roots blower leaving more horsepower for the engine to power the wheels. The twin-screw blower also injects colder air into the engine, which allows the engine to achieve more horsepower on a more reliable basis.
Even though the twin-screw supercharger is an amazing design, it still has its obstacles. While the setup has tremendous power levels at low RPMs, the power curve remains relatively flat throughout the RPM range. It’s also a challenge for a twin-screw blower to achieve high levels of boost. However, largely, the twin-screw supercharger remains a tried and true, reliable option for racing and street use.
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The last type of supercharger is the centrifugal. Unlike the aforementioned blowers, centrifugal superchargers power an impeller – similar to a rotor – at extremely intense speeds to draw air into a compressor housing. As air is drawn at center of the impeller, centrifugal force pushes the air to radiate towards the discharge side. With impeller speeds reaching up to 60,000 RPM, the impeller forces air out at a high speed, but low pressure. The low-pressure, high-speed air is channeled through a diffuser. The diffuser’s job is to convert the airflow into a high-pressure and low-speed air. After this, the air is forced into the engine.
While a Roots style blower requires the most amount of horsepower to run, (referred to as parasitic loss) the centrifugal blower requires the least. Individuals and automakers alike perceive parasitic loss as insignificant, since they produce much more power than they take.
Centrifugal blowers have become arguably the best blower of choice for street and light-duty racing. Their flexibility for power adjustments, low discharge temperatures and exceptional reliability make these superchargers a popular choice. However, due to the belt system driving the impeller, there is not as much power at low RPMs unlike the aforementioned superchargers. The power comes on at higher RPMs further down the rev range.
Superchargers have been the go-to source of forced induction for some time. They’ve provided owners with copious amounts of horsepower, while creating an infamous whine. But as fuel economy and emissions become a larger part of the auto industry, car manufacturers may be forced to utilize a different type of forced induction – hence the turbocharger.
Just like superchargers, turbos compress the air flowing into the engine. Instead of drawing power from the crankshaft, like a supercharger; turbochargers utilize exhaust gases to power the compressor. The turbocharger employs the exhaust flow to spin a turbine, which in turn spins an air pump. The compressor then pressurizes the air before pushing it into the cylinders. The compressor works like a centrifugal pump, drawing air in at the center of its blades then hurls it outward as it spins. A turbine in the turbocharger can reach speeds of up to 150,000 RPMs, much higher than a supercharger. Turbochargers typically provide six to eight psi of boost, therefore providing approximately 30-40 percent in performance gains.
At speeds of up to 150,000 RPM, the turbine shaft needs to be supported carefully. To cope with these intense speeds, turbochargers utilize a fluid bearing, or high precision ball bearings. The fluid bearing supports the shaft with a thin layer of oil constantly pumped around it. The oil provides cooling for the shaft and other parts of the turbo, as well as reduces friction.
For individuals and automakers looking for even more power, a twin turbo setup has become very common. The two types of twin turbo setups include parallel and sequential turbos. A parallel twin turbo design involves both turbos sharing half of the engine’s exhaust gases. Parallel twin turbos are predominantly applied to V-shaped engines and are usually mounted with one turbo assigned to each cylinder bank.
The parallel twin turbo design has become popular with many muscle car enthusiasts and even aftermarket tuners. Hennessey Performance, a diehard muscle car tuning company based in Texas, utilizes twin ball bearing turbochargers in their 1,000hp monsters.
On the other hand, a sequential twin-turbo setup utilizes a small and large turbo. The smaller turbo operates at low speeds, while the larger turbo stays dormant until higher speeds. Due to the overly complicated setup, the sequential twin-turbos have become obsolete in gasoline-powered vehicles.
While turbochargers are known to operate more efficiently than superchargers, they are not faultless. One major downside with turbos is the concept of turbo lag. This especially occurs with larger turbos and refers to the time a turbo takes to ‘spool’ as it generates high levels of boost. Just like superchargers, turbocharged engines require copious amounts of cooling and reinforced engine components to deal with the increased levels of horsepower.
Factory equipped superchargers may be all the rage today but they have utilized by American automakers decades. The 1966 Ford Mustang Shelby GT350 boasted a Paxton supercharger and was rumored to increase horsepower from 271 to 395, still quite a lot by today’s standards. In recent time, we’ve seen forced induction introduced into the 2013 Chevrolet Corvette ZR1 and 2014 Camaro ZL1. The 2015