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SCREW TYPE

Mechanical superchargers are either positive displacement or not. A screw compressor is the most efficient type of positive displacement type supercharger available. The screw is just as the name insinuates the rotors appear to screw into each other. A screw compressor has a male and female rotor where only one rotor displaces air. Because of this, the rpm at which these units run is greater than a roots design. The internal tolerances of the screw compressor are very tight, and the inter-twined rotors act as a seal against any leakage up through the rotors. The screw has an inlet cavity that decreases in size as the rotors rotate. This compresses the air charge internally, because of this it can be called an air compressor. The screw is known to be a very powerful supercharger design, while the rotors are large and can be heavy it is commonly over 200 horsepower over a roots supercharger. The reason for this power difference is the combination of lower discharge temperatures and lower parasitic drag because of no rubbing internal parts.

ROOTS SUPERCHARGER

The Roots supercharger is the old tried and true positive displacement design used since the GMC introduction on diesel trucks and even earlier in coal mines to move valuable air to the soon to be cancer victims at the bottom of the mine.

The Hi-Helix (left) and Standard Helix superchargers.

The Roots has two rotors with three lobes per rotor. The Roots supercharger has two rotor types, the Hi-Helix and the Standard Helix. The high helix rotors are called 120-degree rotors and the standard rotors are called 60 degree. This is the amount of rotor twist over the length. Both rotors in the Roots design displace air, giving the roots a large displacement per rotation.

The size of the supercharger is based from the rotor length. A 6-71 supercharger has a 15-inch long rotor, and an 8-71 has a 16-inch one. This keeps going until the 16-71, which has a 20-inch long rotor.

This is Kobelco's 14-71 supercharger.

Roots superchargers use Teflon sealing strips to help seal small clearances between the rotors and also to seal rotor to bore. These Teflon strips need replacing from time to time to keep the blower to its peak efficiency.

CENTRIFUGAL SUPERCHARGERS

Centrifugal superchargers are belt-driven turbine wheels that compress air centrifugally (outward from the center) with rpm. The impeller in the supercharger causes air to move from the center of the turbine and is accelerated outwards towards the turbine housing and is then directed to the outlet ducts. The rpm at which these superchargers operate is very high, upwards to 30,000 plus rpm. The centrifugal supercharger uses the same compressor theory as a turbocharger except that the means for driving the compressor is from the crankshaft via belt. The supercharger has a large gear ratio internally to bring the unit to its needed rpm.

This type of supercharger notoriously makes good boost pressures, but is much slower than the positive displacement type superchargers at building this valuable intake pressure.

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