Boost is the amount of air pressure created by the supercharger. Boost pressure is largely misunderstood, and most believe more boost = more power. This is true to a limit. Although air is constantly passing through the supercharger while running, this does not mean it is constantly creating boost. This is because at lower blower speeds, the clearances between the case and rotors allow a minimal amount of leakage, resulting in some loss of boost efficiency. If your engine is not flowing properly due to poor cylinder heads, cam, valves, etc, boost readings will typically increase in the upper RPM range. This occurs because the boost the blower is creating cannot fully enter the cylinder due to restrictions, and the boost is now backing up in the intake tract, resulting in a false boost reading.
Boost is a function of three things. 1- Engine Displacement 2- Blower Displacement 3- Blower RPM. Assuming a constant speed ratio between engine and blower, a large blower will make more boost than a smaller one on the same size engine. As engine size increases, boost goes down if the blower speed and size stay the same. On the flipped side, as engine size goes down, boost increases. On a specific size blower, and engine, boost can be increased by running the blower faster than the engine speed (Over driving), or it can be decreased by running it slower (Under driving). This is when things can go wrong… Some people may believe that over driving to create more boost will create more power….again, this is true to a limit. There comes a point of diminishing returns. When roots superchargers are run at extremely high speeds, they can actually heat up the inlet air to such an extent that the air expands. This overheated air loses so much of its density that although your boost gauge is showing boost, in reality you aren’t putting anymore air into the engine than an unblown engine could get. The problems go the other way as well. Running the blower to slowly compared to engine speed (under driving) would result in inefficiencies at lower engine speeds. Low speed leakage will reduce low-speed boost pressure, causing a decrease in the amount of additional power produced. These are a few reasons why it is important to have a supercharger that is specifically sized for the application, such as a “Thomson Supercharger“.
In the world of forced induction there are many options, all with benefits and downfalls. A highly detailed comparison would not only be lengthy, but somewhat confusing, and most likely boring to most readers. A much more condensed, and simplified version is as follows.
There are basically two categories of supercharger, fixed displacement, and non-fixed displacement. Fixed displacement (Roots and Twin Screw) superchargers pump a specific amount of air every revolution, and do not allow air to reverse flow. Non-Fixed displacement (Centrifugal) superchargers push an unspecified amount of air along, similar to a fan blade, so airflow could be reversed if conditions warrant. Whether a fixed or non-fixed displacement blower is used for a specific application depends at which rpm range gains are looking to be made. The fixed displacement blower (Thomson Supercharger) finds it’s best use at low and mid range rpm. The non-fixed displacement offers benefits more in the high rpm ranges.