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BOOST

Turbochargers and superchargers are called "blowers," "boost devices," or "power adding devices." These devices work by compressing and forcing more air into the intake system. This allows more fuel to be added and effectively raises the compression and power of the engine. Adding a boost device can easily increase the power output by 50 percent.

Superchargers are driven off the crankshaft by a series of belts and pulleys or gears. Boost is instantaneous and available through the RPM range of the engine. However, superchargers require a significant amount of horsepower from the engine (sometimes up to 40 percent of the engine's power). Superchargers are also very susceptible to damage from an engine backfire. Turbochargers, on the other hand, are driven by the high-energy exhaust gas that is normally wasted. Because turbos are driven from exhaust gas and must be spun up to an RPM that allows sufficient boost pressure, there is a lag before the horsepower builds. Either device is a quick and fairly inexpensive way to build horsepower.

Turbochargers and centrifugal superchargers are rated by their output boost pressure (or pressure ratio), their CFM (cubic feet per minute) flow, and the efficiency of the device. Roots-type compressors are typically rated in volume (cubic inches) per revolution. Roots type devices are usually less efficient than centrifugal compressors but require lower RPMs to build pressure.

For turbochargers the following apply:

Island Press. Ratio: The pressure ratio at the peak of the island. The value represents a multiple of the ambient pressure. For example a pressure ratio of 1.5 at sea level (14.7 PSI ambient pressure) would mean 22.05 PSI or 7.35 boost pressure.

Island Efficiency: The peak efficiency of the turbo expressed as a percentage.

Island RPM: The RPM of the turbo at the peak.

Island CFM: The flow of the turbo at the island.

Island Surge CFM: The surge flow of the turbo at peak. Surge is a phenomenon that occurs when the turbo's flow and momentum conflict and cause the turbo to pulse or surge. This is not an optimal condition and can actually damage the turbine.

For centrifugal superchargers the following settings apply:

Island Press. Ratio: The pressure ratio at the peak of the island. The value represents a multiple of the ambient pressure. For example a pressure ratio of 1.5 at sea level (14.7 PSI ambient pressure) would mean 22.05 PSI or 7.35 boost pressure.

Island Efficiency: The peak efficiency of the supercharger expressed as a percentage.

Island RPM: The RPM of the supercharger at the peak.

Island CFM: The flow of the supercharger at the island.

Int. Gear: The gear ratio of the supercharger's internal gearing. This value multiplied by the belt gear ratio will give the total gear ratio.

Belt Gear-The gear or pulley ratio of the belt and pulley system used to drive the supercharger from the crankshaft. This value multiplied with the internal gear ratio is the total gearing.

For roots-type superchargers the following settings apply:

Peak Efficiency: The peak efficiency of the supercharger expressed as a percentage.

Volume / Rev.: The amount of air that is moved by the supercharger for each of its revolutions. The value is expressed in cubic inches.

Belt Gear: The gear or pulley ratio of the belt and pulley system used to drive the supercharger from the crankshaft.

For all boost devices the following settings apply:

Max. Boost: The maximum allowed boost from the blower. The blower may actually be creating more boost, but it is wasted. The device that accomplishes this is usually called a "wastegate." The value is expressed in PSI.

Intercooler Eff.: The efficiency of the intercooler. When the blower compresses air, the temperature of the air mass rises dramatically (from 80 to 200 or so degrees).

An intercooler is a device that the "charged" air is passed through to help it cool before entering the engine.

Typical intercooler Settings are as follows:
Air-to-air			  25%
Air-to-cooled water                     100%
Air-to-forced air		  50%
Air-to-ice water                           120%
Air-to-water		  75%
