TURBO POWER GAIN CALCULATION

There is a very simple equation for calculating the approximate power output of a normally aspirated engine after a forced induction mechanism is applied.  There is no need to solve anything here, but it is quite revealing to understand what the equation says.  Five factors are involved:

Power  =  Po  x  PR  x  DC  x  Evol Ratio  x  PL

Where:
Po is the original rated horsepower: say 240 bhp

                                                    Boost + 14.7
PR is the pressure ratio, or   ---------------------, at 7 psi this is 1.49
                                                          14.7

DC is the density correction due to heating of the air charge.  This is directly proportional to the absolute temperature of the ambient air to the boost air entering the engine.  At 7 psi these corrections are about .85 without an intercooler, and about .96 with the IC. Our systems operate at 8.5-9.0 psi, however we chose the lower boost number for comparison purposes.

Evol is the volumetric efficiency ratio of the blower to the engine.  Since the CS and the turbo have the same Vol Eff, and the engine is the same, this factor can drop out. Assuming comparable compressors are available.

PL is the power loss correction due to the necessary power taken from the crankshaft to drive the blower.  Here the CS takes about 5% of the engine power, whereas the turbo only robs about 1.5%.  The reason for the difference is that the turbo is largely powered by the heat energy in the exhaust gas.  Keep in mind that the heat energy lost out the tailpipe is about the same number of horses as the engine makes. Remember, of the fuel burned, 1/3 goes to power, 1/3 to heat in the cooling system, and 1/3 out the exhaust. Therefore the lost exhaust energy and the engine power are about the same.  When was the last time you saw a 240 hp fan? That is what is made available to the turbo for a driving force without taping off the crank. We don’t need it all, but that’s what’s theoretically available. Enormous, eh?

 So, plug the numbers in and see what they yield:

 New Power:
                          Non-intercooled Centrifugal:

                                   P = 240 x 1.49 x .85 x (1-.05) = 289 bhp

                          Intercooled Turbo:

                                   P = 240 x 1.49 x .96 x (1 - .015) = 338 bhp