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Compressor Efficiency

 - Compressing air adds heat.
 - The less heat added during boost, the more efficient the compressor.
 - The higher the boost, the more heat is injected, the lower the efficiency.

Compressing air to increase charge density adds heat to it. The more compressed the air, the greater the heat increase. The more efficient a compressor is the less heat it injects into the compressed air. But all compressors inevitable inject some heat into the compressed air.

This is why doubling the inlet air pressure does not necessarily double the power output.

Because of the above, more boost will not always lead to more power. Just because the engine is rugged enough to accept more boost doesn’t mean you should run it at the max boost possible.

As already mentioned, increased boost means increased air temp. a point will be reached where the ignition needs to be retarded so much that the net result of raising boost is actually a drop in overall brake horsepower.

The problem is compounded because at higher boost, the turbos efficiency drops below the point where the increased boost produces more power.