Written by Guest Blogger – Anthony Schwaller, www.streetrod101.com
Question: Some street rod owners have vehicles that are supercharged. What is the definition of supercharging and how is an engine supercharged?
Answer: Most street rod engines are called naturally aspirated. On a naturally aspirated engine, as the piston moves downward on the intake stroke, a vacuum is created inside the cylinder. This vacuum draws air and fuel into the engine. The amount of air and fuel entering the engine is based on the atmospheric pressure. In most cases the amount of horsepower and torque is sufficient for most driving conditions. However, in some cases, especially for high performance and racing conditions, engines are supercharged to get more power.
Supercharging a gasoline engine is accomplished by forcing a greater volume of air and fuel into the cylinders. This means that the engine is not naturally aspirated, it is supercharged. By supercharging an engine, horsepower and torque can be increased from 50%-100%, depending upon the exact design.
The most popular way to supercharge an engine, especially in the street rod industry, is by using a blower. A blower is an air pump placed on top of the intake manifold to increase the volume of air and fuel going into the engine. The most common type of blower is called the Roots-type blower, named after its inventors, Philander and Francis Roots. Roots-type blowers are used by General Motors on their 2 cycle diesel engine design. Often referred to as the 4-71, 6-71 or 8V-71 engine, each had a gear driven Roots-type blower to force increased amounts of air into the diesel engines. A Roots-type blower is considered a positive displacement pump. It pumps a specific volume of air each revolution of its rotors.
Referring to the illustration above, the Roots-type blower used on street rods is placed on top of the intake manifold. There are two rotors, each having three lobes made from aluminum alloys. The rotors also have a slight angle to them from the front to the back to increase volumetric efficiency. Note that some designs use a two lobed rotor. The two rotors are connected by gears (not shown) in front of the blower. On street rods, the gears are driven by a cog belt which runs off a crankshaft pulley as shown above. As the rotors turn, they do not touch each other. There are small clearances between each rotor that keep them from touching. The left rotor turns counter clockwise while the right rotor turns clockwise in the drawing. As the rotors turn, they draw in huge amounts of air and fuel, pressurize it, send it to the intake manifold, and finally to the valves.
One of the disadvantages of a Roots-type blower is that it consumes a large amount of frictional horsepower from the crankshaft. However, the total increase in horsepower and torque is still very high. Operating a supercharged engine produces greater power, but not necessarily better fuel economy.
Also, if a blower is going to be installed on a street rod, many other components also have to be changed. Since there is a large amount of power increase, stronger pistons, gaskets, cylinder heads, valves, piston rings, fuel pump, carburetors, camshaft, etc. must be changed to allow for the increase in air/fuel volume as well as combustion pressures. For example, many supercharged engines have three, four barrel carburetors to handle the increase fuel requirements. Also, the intake manifold needs to be changed so that the blower will fit on top, and also allow lubrication for the rotor bearings.
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