Quote:
Originally Posted by dustyfirewalker
since a turbo map has a pressure ratio on the Y axis and a volume/mass flow rate on the X axis, does a draw through turbo set up cause the pressure ratio to sky rocket? there is a vacuum on the inlet of the turbo so the pressure ratio isnt on the map, right?
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Well, no. Pressure ratio is pressure ratio and compressor RPM is compressor RPM. The pressure ratio is figured from the inlet absolute pressure. The inlet absolute pressure can be anything from below atmospheric (vacuum) to above atmospheric. Atmospheric pressure at sea level is 14.7 PSIA, which would be zero on a vacuum/boost gauge.
Sample calculation 1: Assume a pressure ratio of 2.00 and inlet pressure is atmospheric at sea level, or 14.7 PSIA (PSI absolute). Then the discharge (outlet) pressure is 2.0 X 14.7 = 29.4 PSIA, or 29.4 - 14.7 = 14.7 PSI of boost.
Sample calculation 2: Assume a pressure ratio of 2.00 and inlet pressure is 5.0 PSIA (about 20 In Hg vacuum). Then the discharge pressure is 2.0 X 5 = 10 PSIA. This would still be a vacuum of 14.7 - 10 = 4.7 PSI, or about 9.4 In Hg.
With 1/3 the inlet pressure, it will only need 1/3 the power to drive the compressor to the same RPM. Sizing the hot side needs to be done very carefully.