Take Charge - Engine Supercharging

Supercharging for the Masses

By Bob Mehlhoff

They look awesome, whine, and make huge power. Yes, we're talking about superchargers, and in the world of hot rodding they are the ultimate in cool. Unlike a normally aspirated engine that relies on both atmospheric pressure (14.7 psi at sea level) and vacuum to fill the cylinders, a supercharger--also known as a blower--feeds both air and the delivered fuel at high pressure into the cylinders. The result is a level of volumetric efficiency surpassing that of a normally aspirated engine to produce more power for a tire-frying experience. So actually, the topic of supercharging is really about increasing volumetric efficiency.

Like all other forms of engine and vehicle modification, a supercharger unit should be matched to the vehicle's performance level, type of driving, and budget for optimal results. This means improving the cooling system, fuel system, ignition, engine components, drivetrain, suspension, and other parts to support and enable the added power.

So here's your short course in macro superchargers: how they work, the different types, and the components you'll need to change on your vehicle. So hang on because the info will blow you away.

Blower Basics
Superchargers fall largely into three design categories: Roots, centrifugal, and screw, the most traditional being the Roots. As noted, the key to making good power in an internal combustion engine is to increase the amount of air and fuel stuffed into each cylinder before every power stroke. This improves the combustion that produces the heat that is transferred into energy, translating to greater force on the piston and the connecting rod so that crankshaft turns with added power. This is basically the same reason larger-displacement engines make more power and torque than smaller ones; there is a larger supply of air and fuel within the cylinders.

Roots
A Roots-style blower is probably the most commonly noticed supercharger at cruise-ins. On some applications these stick out of the hood with one or two carburetors attached. These units act as an air pump so that the compression of the inlet charge (boost) takes place inside of the manifold and cylinders, external from the blower. Because every full rotation of the Roots compressor element generates a specific amount of air pumped from the inlet side to the exhaust outlet (directly into the intake manifold), the Roots style is considered a positive-displacement blower.

Inside the case of a Roots blower are intermeshing, rotating rotors. A crankshaft-mounted pulley spins a drive pulley through a belt (typically cogged on larger blowers to eliminate slippage). The drive pulley, mounted on the front of the supercharger, is connected to internal gears that turn the rotors. The spinning of the rotors compresses air and fuel supplied from either the carburetor(s) or a fuel-injection system mounted above the blower case. This air/fuel mixture is pumped between the supercharger case and rotors.

Traditional Roots-style superchargers generally produce on-the-spot boost pressure down low in the rpm band and maintain it as engine speeds increase. However, efficiency generally tapers off at higher rpm due to heat buildup inside the case and leakage past the rotor seals. The added temperature may make the engine more prone to detonation. Two ways to counteract this problem are to run a slightly larger blower, which helps move more air, and underdriving the blower slightly to reduce boost.

Not all Roots-style superchargers operate with the same technology. Some are newer with hybrid designs, such as the Eaton or Magnuson models. These units use a high-helix set of involuted rotors that produce an axial airflow. In other words, they run the air movement from the back of the unit and discharge it at the front. This differs from the traditional Roots supercharger that pulls air in from the top and delivers it through the bottom. A Magnuson-style unit also incorporates a bypass or unloader valve that bleeds off air pressure at part-throttle operation, thus reducing mechanical losses.

Twin-Screw
The outward appearance of a twin-screw supercharger appears very similar to a traditional Roots-style unit, but there are a few distinct differences. A twin-screw's air compression takes place inside the supercharger, making it an internal-compression unit. Second, a screw-type blower uses rotors with tighter clearances that interleave to pull in and compress the air as it passes through. Third, the incoming air enters the twin-screw supercharger through the rear or top rear. The shorter airflow path of a twin-screw minimizes the high turbulence, friction, heat (reduced often by 50 percent), and pumping losses commonly found with the traditional Roots designs. Like all superchargers, the twin-screw is beltdriven. Because of the closer tolerance design, a twin-screw supercharger is often more expensive than a standard Roots type.

Centrifugal Supercharger
Centrifugal superchargers are installed along the front of the engine in line with other driven accessories (e.g., alternators and A/C compressors). A step-up drive inside the blower increases the speed of the internal impeller. Depending on the size and design, centrifugal blowers are capable of large power increases, while their compactness allows them to fit a variety of engines. Since a dedicated intake manifold isn't needed, they're easily adaptable to late-model, fuel-injected vehicles. Systems for late-model cars are available from ProCharger and Vortech. These packages typically come with the supercharger, ducting, oil lines, and brackets. Some race units enable big cubic-inch, high-boost applications to make over 1,300 hp.

While the noncentrifugal blower typically builds boost early and maintains it as rpm increases, a centrifugal blower typically builds boost exponentially. This means that as the blower's rotational rpm increases, the boost increases at a quicker rate. Key advantages of the boost increasing at higher engine speed are that there are fewer traction and detonation problems.

Selecting the right centrifugal supercharger requires a little research since matching the system correctly to the engine's performance level is key. Centrifugal superchargers develop added power from maximum shaft speed, the overall compressor housing size, and impeller trim configuration. This influences the amount and rpm level that maximum boost occurs at on various engines. Centrifugal supercharger manufacturers can provide excellent help in selecting the proper system for your vehicle's performance requirements.

Give it a Boost
A Supercharger's boost is defined as the amount of air pressure produced to fill the cylinders over and above the air pressure in a naturally aspirated engine. The amount of boost and the rpm at which boost begins is a function of the throttle, the blower size and drive ratio (overdriven or underdriven), engine displacement, camshaft grind, and exhaust system. Assuming an even speed ratio between the engine and the blower, a larger blower will produce a higher level of boost than a smaller one on an equal-displacement engine. By operating the blower faster in relation to the engine's speed (overdriving) boost can be increased. Conversely, running the blower slower will decrease boost (underdriving) and allow a level appropriate for engine compression.

Cam Accordingly
Just as you don't want a high static compression with a supercharger, you don't want a long-duration, big-overlap camshaft, either. Since the incoming air is under pressure, the intake valve opens as the mixture rushes into the cylinder. If the exhaust valve is open, a portion of the air/fuel mixture will be pumped out of the exhaust system by the supercharger. In general terms, a camshaft with less than 240 degrees duration (measured at 0.050 inch lift) will work well. Camshaft overlap should be minimized with lobe centers in the area of 112-115 degrees. Many street-blower cams are ground on a dual-pattern profile to allow the exhaust more duration than the intake (with the intake closed). This is because the blower pumps more air/ fuel mixture into the cylinder on the intake stroke, but the exhaust needs to exit on its own, so the exhaust duration is increased. Most camshaft manufacturers offer cams specifically designed for blower motors.