What is the difference between a Roots blower and a screw blower?

A supercharger is one of the oldest methods of adding power to an engine, so it’s no surprise that racers around the world have always found ways to harness that power to make their cars faster. The supercharger has been a staple in drag racing since the sport began, from the first hot rods on the salt flats to the beginnings of Top Fuel racing. The two most common types of superchargers in drag racing are the roots and screw blowers that can be seen sticking out of the hoods of different racecars in venues around the world. Today, we’re going to take a look at these superchargers to discuss their internal workings and how they’re so similar yet so very different.

Because an engine acts as a giant air pump, the best way to make more power is to increase the amount of air that flows through it. The supercharger forces more oxygen into the motor, allowing more fuel to burn, and that in turn creates a higher power output. The roots and screw blowers you see at the track may look like they bring air into the motor the same, but there are actually a few differences in how they function.

Blowers have been a part of drag racing since its earliest days and old blowers can still be seen on nostalgia cars like these.

The Roots Blower

The roots supercharger acts like a large positive displacement air pump on the top of a motor and is what the nitro-guzzling NHRA Top Fuel and Funny Cars use to make their power. Inside the blower, there are two rotors that typically have three lobes each, and they’re what move the air inside the case. The air is held inside pockets that are around the lobes and is then moved from the intake to the discharge side of the lobe. As this is done, the rotors move more air than the motor can ingest, creating boost inside the manifold of the supercharger.
Roots Blower Vs The Screw Blower

When it comes down to it, the goal of a roots and screw blower is the same; move as much air into the motor as possible to increase power. How well that’s done between the two is where the big differences arise and show which blower can make more power.

Andy Severyn, John Sears, and Chelsea Clark are the core group of people who run PSI Superchargers, one of the biggest names in the roots and screw blower market. They have spent years perfecting both kinds of blowers and know what it takes to make big power with each.

What is the difference between a Roots blower and a screw blower?

Roots Blower Overdrive

A roots blower can have a certain amount of overdrive based on the class you are in. Janis explains how overdrive in a roots blower works. “While you would think that turning a supercharger at a higher overdrive would make more power, it’s not always the case. It actually will create a lot of heat in the supercharger, which then causes the intake temperature to rise, thus making your turn up much more difficult. In our particular case, the NHRA mandates that our supercharger has an overdrive limit of 16.5 percent overdrive. This means that with the pulley combination of the top pulley and the bottom pulley, the blower is turning 16.5 percent faster than the actual engine RPM. It goes hand in hand no matter what overdrive combination you are using.”