READING TURBO COMPRESSOR MAPS, PART III

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft forum turbo blog)

"One must strike the right balance between speed and quality."-- The Right Honourable Clare Short (Former MP,Birmingham Ladywood, UK)

The next compressor map elements of note are the series of r.p.m speed lines arcing from the surge limit line on the left over to the right.

Each of these speed lines is a graph of compressor performance at specific compressor r.p.m.

 

Warner (Street Turbocharging) and Hartman (Turbocharging Performance Handbook) both provide detailed explanations of how turbocharger engineers use these speed lines in building a compressor map.

The short version is that the turbos are run up to a particular test r.p.m. on a "test rig" (sort of a turbo dynomometer) and then the outlet flow is restricted with a valve to measure how efficient the compressors are at various mass air flow levels (the bottom axis of the map).

Note that until the compressor becomes a flow restriction (the right side of the map) that pressure ratio and compressor speed are closely linked. The speed lines turn down sharply at the right side of the map because the compressor is simply too small to efficiently supply any more mass air, reducing the pressure ratio.

On this map, the downturn in the speed lines becomes more severe at higher compressor r.p.m. levels. This suggests that increases in mass air flow beyond the efficiency range of the compressor can lead to drastic increases in speed as the compressor struggles to "keep up" with air demands.

The map speed lines also show that for each level of mass air flow, there are many pressure ratios that can supply enough air. Thus, if the pressure ratio increases, and mass air flow does not, then the output is being more restricted. (it works the same way with a garden hose nozzle)

Of course, in the real world, the "restriction" of compressor output isn't a test valve. It's the physical ability of the engine to induct, react, and exhaust air. That means better "breathing" engines require less pressure and compressor speed to obtain a particular level of mass air flow.