[Tom Vaught]

The most difficult part of Supercharging/Turbocharging an engine is to select the right device for the application.

Each device has a long list of pros and cons. It is the task of the person(s) involved in the installation to balance the various compromises with the actual needs of the engine in order to meet the targets for that given application.

The first step is to determine the needs for the given application being evaluated.

This requires that the performance characteristics (HP and Torque curves) for the base Naturally Aspirated engine to be compared to the performance targets for the boosted engine (HP and Torque curves, Airflow, and Pressure Ratio requirements.

This will identify the areas in which Boosting will be required and what level of Boost is needed. From this analysis, the type of boosting device can be determined.
This is assuming that you are after the best unit for the application and not trying to emulate a specific boosted engine from a different time period. An example would be installing a 1960s Paxton Supercharger on your favorite 1960s Ford Collector Car.

Sometimes there are several Boosting Strategies that can accomplish the same task.

When this occurs, then other factors (cost, complexity, reliability, packaging, etc) must be considered in order to make the best choice.

Major Factors that can influence the decision:

If the engine need performance enhancement across all RPM points then the Roots or the Lysholm Supercharger might be the best choice if the given HP target is within the range of the device.

If the engine need performance enhancement at High RPM only, the Turbocharger or the Centrifugal Supercharger might be the best choice if the given HP target is within the range of the device.

If the engine Pressure Ratio requirements are 1.8 or lower, then the Roots Supercharger would be preferred over the Lysholm Supercharger.

If the engine Pressure Ratio requirements are 2.0 or higher, then the Roots Supercharger is beyond its capability and should not be considered. 2.0 Pressure
Ratio is 15 psi of boost pressure. A Lysholm Supercharger is more efficient in the above 2.0 Pressure Ratio range.

Higher Pressure Ratios raise the air charge temperature and can require intercooling or an alternate fuel like methanol or a form of water injection to cool the air charge.

Low Boost 5-7 psi or 1.5 Pressure Ratio can get by without intercooling.

Note, the above are general considerations and are not always correct for every application. A engine with E-85 fuel, a lower compression ratio, fuel injection, and intercooling may work well at 35 psi of boost. It depends.

Once this work is done then the next step is looking at the Turbocharger/ Supercharger "Maps" and determining the Airflow and Pressure Ratio requirements.

There are TWO MAJOR TYPES of MAPS: Compressor Maps and Turbine Maps.

Compressor Maps are used for both types of Boosting Devices and Turbine Maps ( far right map) are used only for Turbochargers.

Tom V

Do a search on Compressor Maps and also images of compressor maps and you will see a lot of examples.