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TitleTurbo Guide
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Total Pages62
Document Text Contents
Page 31

XXX:XXX | 14:13 | 2/21/03

The engine will flow 524.3 CFM of air assuming a 100% volumetric efficiency. Most
street engines will have an 80-90% VE, so the CFM will need to be adjusted. Lets
assume our 302 has an 85% VE.

524.3 × 0.85 = 445.7 CFM

Our 302 will actually flow 445.7 CFM with an 85% VE.
Pressure Ratio

The pressure ratio is simply the pressure in, compared to the pressure out of the
turbocharger. The pressure in is usually atmospheric pressure, but may be slightly
lower if the intake system before the turbo is restrictive, the inlet pressure could be
higher than atmospheric if there is more than 1 turbocharger in series. In that case
the inlet let pressure will be the outlet pressure of the turbo before it. If we want 10
psi of boost with atmospheric pressure as the inlet pressure, the formula would
look like this:

(10 + 14.7) ÷ 14.7 = 1.68:1 pressure ratio

Temperature Rise

A compressor will raise the temperature of air as it compresses it. As temperature
increases, the volume of air also increases. There is an ideal temperature rise
which is a temperature rise equivalent to the amount of work that it takes to
compress the air. The formula to figure the ideal outlet temperature is:

T2 = T1 (P2 ÷ P1)

T2 = Outlet Temperature °R
T1 = Inlet Temperature °R
°R = °F + 460
P1 = Inlet Pressure Absolute
P2 = Outlet Pressure Absolute

Lets assume that the inlet temperature is 75° F and we're going to want 10 psi of
boost pressure. To figure T1 in °R, you will do this:

T1 = 75 + 460 = 535°R

The P1 inlet pressure will be atmospheric in our case and the P2 outlet pressure will
be 10 psi above atmospheric. Atmospheric pressure is 14.7 psi, so the inlet
pressure will be 14.7 psi, to figure the outlet pressure add the boost pressure to the
inlet pressure.

P2 = 14.7 + 10 = 24.7 psi

For our example, we now have everything we need to figure out the ideal outlet
temperature. We must plug this info into out formula to figure out T2:

T1 = 75

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