We live in an atmosphere that, at sea level, contains approximately 15 (14.7) pounds per square (PSI) of pressure. This means that an engine's ability to breath is limited by the vacuum or suction, created by the intake stroke of the engine's piston, fed by a maximum pressure of 15 pounds. So, even in the most perfect or miraculously efficient engine design, there can only be a maximum differential between atmosphere and the intake valve of the engine, of about 15-PSI absolute pressure. Not to mention that since few of us live and drive on the beach, we have even less than 15 PSI to work with. This is because as you rise in altitude the atmospheric pressure drops.

Now let's consider turbocharging. In the most basic of definitions a turbocharger is a device that drive more air into an engine such that the 15-PSI of absolute pressure is now boosted to something greater, and thus more air is forced into the engine's cylinders. Horsepower is only produced by the combustion of fuel. Therefore the more fuel that is burned the greater the horsepower produced. But the fuel can only be productively burned if it is burned in the engine's cylinders and there has to be enough air present to do this. When a turbocharger is applied to any engine the fuel delivery rate can be increased and more horsepower developed. In many cases a turbocharger can deliver enough additional airflow such that the same size engine can easily produce 100% or more horsepower than in its "naturally aspirated" or, non-turbo state of being. (Excerpt from book Turbo, by Jay K. Miller, click here to find out where you can purchase this book and gain a greater understanding of turbocharging systems)