The twin-screw supercharger is the other commonly used positive-displacement model. It is similar in appearance to a Roots blower but different on the inside. The design was patented in the '30s by Swedish engineer Alf Lysholm, who-like the Roots brothers-was designing a pump for industrial applications. Decades later, his design would be put to use making cars go faster. The main difference between a Roots supercharger and a twin-screw or Lysholm-style pump is that the twin-screw supercharger is a true compressor becasue it compresses and pressurizes air inside its case.
Like a Roots supercharger, the twin-screw design also relies on spinning rotors, but its shape is much different. There is a male and female rotor, and the lobe count is different between the two, usually three lobes on the male rotor and five on the female. Like in the Roots superchargers, the rotors never touch each other or their housing, but unlike in the Roots, the rotors turn toward each other. Air enters the case from the rear, filling up an opening between the rotors. As the rotors turn, their meshing action pushes the air through a cavity that gets smaller toward the end, compressing the air along the way.
Here is a Whipple twin-screw supercharger installed on an '06 Ford Mustang.
Boost is Not Always Good
We car guys usually equate boost with horsepower: The more boost the better. This is not always true, though. We are going to demonstrate that we should be concerned with the density of the air the supercharger delivers, rather than how much boost it makes.
The purpose of a supercharger is to force more air into an engine than it would otherwise be able to ingest naturally aspirated. Instead of operating with vacuum in the intake manifold, this extra air pressurizes the intake; the extra air pressure is referred to as boost. In basic terms, boost represents an increase in air pressure that is above ambient atmospheric pressure. Ambient pressure is around 14.7 pounds per square inch at sea level but varies slightly due to elevation and barometric conditions.
Extra pressure does not always mean there is actually more air in the intake, since air expands at high temperatures and contracts at low temperatures. In a closed system, heating the air in a fixed volume of space will cause an increase in pressure because the air molecules are trying to expand, but the amount of air or oxygen remains the same.
To illustrate this point, we performed an easy experiment using the air in our tires. We wanted to see how much the tire pressure changed after a few minutes of hard cornering (OK, more like 45 minutes of hard cornering). Before we started driving, we had 34 pounds of air in the tires. After a blast through one of our favorite canyon roads, the tire gauge read about 39 psi. Looking at it another way, you could call the original reading of 34 psi our ambient air pressure. Doing nothing but heating the air in the tires increased the pressure 5 psi. If we were to attach a boost gauge (calibrated to be zeroed out at our ambient pressure of 34 psi), it would read 5 pounds of boost in our tires.
See what we're getting at? Within the confines of your intake manifold, 200-degree air takes up more space than air at 80 degrees. But the 200-degree air has less oxygen at a given volume and mixes with less fuel than the 80-degree air does. At the same boost levels, the 80-degree air will make more power than the 200-degree air because there is more air to burn. The point? The best supercharger is the one that can increase pressure with the least amount of temperature rise.