Compressor maps

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Understanding Compressor maps

A long time ago blown engines were a normal part of turbocharging things and broken pistons, split heads, broken rods and fried bearings were just part of the process. These days people think they can get away with it, but it still isn't going to happen.

It's all about airflow whether it is expressed in cubic feet per minute (cfm) or pounds per minute (lb/min). In any case it takes about 150 cfm per 100hp....so for 200 horsepower you need a turbo that puts out about 300cfm. The real question is what pressure ratio or boost in psi is this airflow possible with your engine. You need to start thinking airflow and not "boost".

Operating characteristics: The compressor operating behavior is usually defined by maps showing the relationship between pressure ratio and volume or mass flow rate. The useable section of the map relating to centrifugal compressors is limited by the surge and choke lines and the maximum permissible compressor speed.

Surge line: The map width is limited on the left by the surge line. This is basically "stalling" of the air flow at the compressor inlet. With too small a volume flow and too high a pressure ratio, the flow can no longer adhere to the suction side of the blades, with the result that the discharge process is interrupted. The air flow through the compressor is reversed until a stable pressure ratio with positive volume flow rate is reached, the pressure builds up again and the cycle repeats. This flow instability continues at a fixed frequency and the resultant noise is known as "surging".

Choke line: The maximum centrifugal compressor volume flow rate is normally limited by the cross-section at the compressor inlet. When the flow at the wheel inlet reaches sonic velocity, no further flow rate increase is possible. The choke line can be recognized by the steeply descending speed lines at the right on the compressor map.

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