Forced Induction (blowers) - an introduction

Q. What is forced induction?

A. Forced induction is any mechanical method to force more air/fuel mixture into an internal combustion engine at pressures above atmospheric. Why do we want to do this? In a word - POWER.

An internal combustion engine is really just an air pump. More air in, more power! To get more air into the pump, increase the displacement of the engine, give it forced induction, or increase the efficiency of the intake and exhaust components.

There are several considerations to adding forced induction to a street driven car : Driveability, available room for the blower and it's associated plumbing, cooling for the extra power, and durability of the drivetrain components being used, to name a few.

If you plan on using a blower, you MUST take into consideration the compression ratio when deciding how much 'boost" to run. Adding forced induction will raises combustion chamber temperatures, which can lead to detonation which saps power or, ESPECIALLY with forced induction motors, severe (and possibly spectacular) component failure(s).

Another issue when building a blower motor is, high compression and low boost or low compression and high boost? Keep in mind a high compression blower motor still has a pretty low compression ratio (hopefully, anyway) compared to a N/A motor. A ratio of 9:1 is pretty high for a motor running a lot of boost (9lbs or more), but pretty low for a high performance N/A motor (10.5:1 or higher is typical for modern knock-sensor equipped motors). Below is a quick comparison of the high CR/low boost low CR/high boost setups

Look at those differences! The 7:1 with 12psi and the 9:1 with 6psi have the same "effective" compression ratio, but look at the power difference! 50HP!

A discussion of the common types of forced induction can be found here