Quote:
Originally Posted by JRMichler
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.
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Reading turbo compressor maps for dummies:
1) Divide your HP by 10.
2) Find that number on the bottom of the map.
3) Mark the map on the 1 BAR line.
4) Find twice that number on the bottom.
5) Mark the map at the 2 BAR line.
6) Draw a straight line between those two marks.
7) You want that line to intersect the 70% efficiency line at the maximum safe boost pressure for your engine.
8) Now calculate your HP at your engine's TQ peak.
Repeat steps 1-6.
9) You want that to fall on the peak efficiency island. More to the right for street, more to the left for race.
When the above works, then you've found the correct compressor.
For a mildly modded 5.3L V8 or a stock 6.0L V8, you'd want a T76. For a modded 4.8L V8 or a stock 5.3L V8 you'd want a T70. For a stock 4.8L V8 you'd want a T66.