Supercharger (also known as a blower) is an air compressor used to force more air (and hence more oxygen) into the combustion chambers of an  internal combustion engine than can be achieved with ambient conditions or atmospheric pressure. They are general  powered mechanically by belt, gear, or chain-drive from the engine's crankshaft.

Turbo chargers do the same thing that a supercharger does but they are driven off of the exhaust gases of the engine rather than the crankshaft.

So what is the principle difference behind a positive displacement blower & a centrifugal: The manor in which they produce boost?

Any supercharger is belt driven off of your motors crankshaft, so its driven speed is directly connected to engine speed. If you double the engine speed the superchargers driven speed doubles.

The amount of airflow a centrifugal blower delivers increases roughly at a square of its driven speed, it is NOT linear. This means that the airflow of the blower will increase faster then what the engines airflow requires, meaning that the boost level will increase with an increase in RPM.  As a result you should set the blower so that its max boost occurs at redline.

Now a positive displacement blower produces airflow in a totally different way.  For each RPM the amount of airflow it pumps is fixed where as centrifugal will vary depending on variables. With the boost level of a positive displacement supercharger being fixed it will deliver airflow linearly so it will generally achieve fully boost around 2,500 RPM & maintain full boost throughout the rpm band.

If you take a centrifugal blower and a positive displacement blower and set them to have the same max boost then dyno them you will find their peak HP will probably be pretty similar but the power & torque under the curve will be higher on the positive displacement blower. When driving them the centrifugally supercharged cars power will start to feel considerably stronger as the RPM rises where the positive displacement supercharged car is going to feel like a much larger engine.

Centrifugal blowers are popular in the aftermarket street market because they are physically smaller in size and are much easier to fit under the hood where as a positive displacement blower will generally require an aftermarket hood.

A major thing to keep in mind when trying to decide between a positive displacement blower vs an centrifugal blower is this, do not over power the cars chassis especially at lower RPM’s.  One of the advantages in a centrifugal supercharger on a street car is the fact that you can control the boost level based on RPM. This means that you can increase boost (power) as the cars speed increases which helps control traction. Now if a car was to have NO traction issues which we all know is almost impossible on a street car then you would want all your boost instantly (positive displacement blower).  Where we generally see better results on a street vehicle using a positive displacement type blower over a centrifugal is on a heavy sport utility vehicle or SUV. These type vehicles generally are very heavy & have lower operating RPM’s so it is generally better to get max boost by 2500rpm because traction is not as much of an issue.

How does a blower make such huge power gains?  A blower compresses air into the cylinder at a rate which is greater than what the motor alone could achieve. By forcing larger amounts of air into the cylinder it is putting more oxygen into the cylinder.  If one was to break down horsepower to its most basic element it would be OXYGEN.  One would then ask why not inject pure oxygen into the motor for optimum power? Pure oxygen is unstable and the burn rate cannot be controlled.  Look at nitrous oxide (N2O), this stands for 2 particles of nitrogen & one particle of oxygen. The 2 nitrogen molecules help stabilize the burn rate of the oxygen molecule as well as keep it cool so that it will stay as an oxygen molecule.