Lots of misinformation in this thread, this will help!
Twin Screw vs. Roots SC arrangement
It's in PDF format so you need adobe reader but I'll post a few highlights here...
TWIN SCREW vs. ROOTS
Because of their ability to produce an abundance of boost (HP and torque) at virtually any engine rpm, the Twin Screw and Roots type are the two
most logical choices for supercharging. Both are essentially positive displacement SUPERCHARGERS (each revolution produces "X" cfm
regardless of engine rpm). Although their appearance is similar, one should not confuse the two. Internally they are as different as night and day.
LOWER POWER CONSUMPTION: The Twin Screw requires 10-16 less HP (depending on boost) to drive, leaving more horsepower for the
engine to transmit to the vehicle wheels for increased acceleration, passing, towing, hill climbing.
COOLER CHARGE TEMP: It also discharges air into the engine at a much lower temperature. The cooler denser air charge from the Twin Screw
equates to even more engine horsepower and torque potential with less thermal stresses on the engine.
INTERNAL COMPRESSION: The Twin Screw compresses the air between the rotors. This "internal compression" means less work to boost the
air pressure and quicker boost delivery to the engine. The compressed (boosted) air resides in the supercharger and makes it behave like an air
tank . . . squeeze the throttle and out comes the boosted air. The amount and boost level depends on throttle depression.
Eaton Corp. who is currently manufacturing the Roots type - and is also licensed to produce the Twin Screw - has this to say in their literature when
comparing them: "A Screw Compressor [Twin Screw] also provides more air for an engine's cylinders but it first compresses the air, thus
providing more boost than a Roots [Eaton] type supercharger. The additional boost allows engine manufacturers to extract more power
from an engine." ...Eaton Corporation
It has come to our attention that our potential customers, enthusiasts and the media often confuse the "Roots" style superchargers with the "Twin
Screw." They all labor under the false misconception that both of these positive displacement superchargers are identical in operating principle and
efficiency! Frequently the Twin Screw is referred to as a "Roots." It IS NOT a Roots. These products ARE NOT THE SAME! (see Twin Screw vs. Roots
It has been well documented in numerous tests by automotive engineers around the world that the Twin Screw concept is, unquestionably, more
efficient (cooler air charge temperature and lower parasitic loss). Ford, GM, Chrysler, Saab, Volvo etc. are among the many who have tested the Twin
Screw. The Mazda Millenia has used one for years and in 2001 Mercedes selected the Twin Screw for their latest high performance cars.
It is not our intent to knock the Roots type. Both are proven, reliable OEM quality superchargers. However, the Twin Screw is clearly more efficient.
Over 80 companies, including Eaton, have been licensed to produce the patented highly efficient Twin Screw rotors for compressors and
superchargers. Millions have been produced. The Twin Screw is a product who's time has come.
The superchargers tested were the Kenne Bell/Autorotor 3150 (Twin Screw) and a popular comparable Roots type. Both are rated at 1.5L and
recommended for applications up to 450HP. They were tested at 12000 rpm and 70° at sea level with the 3 boost levels indicated. Both are commonly
used superchargers. Two larger superchargers - a Roots type and a Kenne Bell/Autorotor 420 Twin Screw with ratings of 600HP - were also tested.
Again the test results confirmed the Twin Screw enjoyed up to 30% lower air charge temp and 30% less parasitic loss at all 3 boost levels.
Hope that helps, it is from Kenne-Bell (who sells twin-screw) but pretty accurate.
They do include the quote form Eaton, that produces the Roots units, but most
of the Supercoupe community recognizes the superiority of the screw type.