To determine Absolute Pressure:
P1c + P2c = Absolute Pressure
P1c = Compressor Inlet Pressure
P2c = Compressor Discharge Pressure
Example: 14.7psia + 10psig = 24.7psi Absolute Pressure
To determine Pressure Ratio:
(P1c + P2c) / (P1c – 1) = Pressure Ratio
(Note: Any restriction in the air induction system caused by an air filter or ducting to the inducer side of a compressor creates a ‘depression’. So, calculate for an approximate 1psig loss in the formula. This is why ‘P1c – 1’ is used in the equation.)
Example: (14.7psia + 10psig) / (14.7psia – 1psig) = 1.80 Pressure Ratio
To determine the actual airflow needed to achieve a certain horsepower goal:
HP * A/F * (BSFC/60) = Wa
Wa = Airflow Actual (lbs/min)
HP = Desired Flywheel Horsepower
A/F = Desired Air/Fuel Ratio
BSFC/60 = Brake Specific Fuel Consumption divided by 60 to convert lb per hp/hr converted to minutes.
Example: 400 * 12 * (0.55/60) = 44.0 lb/min of air mass.
To determine Manifold Pressure required to meet flow target:
Wa * R * (460 + Tm) / VE * (N/2) * Vd = MAPreq
MAPreq = Manifold Absolute Pressure required (psia) to meet airflow/horsepower target.
Tm = Intake Manifold Temperature (Degrees F)
VE = Volumetric Efficiency
N = Engine Rpm/ Speed
Vd = Engine Displacement in CID (cubic inch displacement)
Example: 44 * 639.6 * (460 + 130) / .92 * (7200/2) * 122 = 41.1 psi Absolute Pressure
So, 41.1 psi Absolute Pressure – 14.7 psia = 26.4 psig boost pressure
To determine Compressor Discharge pressure needed to compensate for pressure drop through induction system:
MAP + ∆Ploss = P2c
MAP = Manifold Absolute Pressure (psia)
∆Ploss = Pressure loss between the compressor outlet and the intake manifold (ex:2 psi)
P2c = Compressor Discharge Pressure
Example: 41.1 psia + 2 psi = 43.1 psia
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Thank you, sir, for your comment. Hope that you enjoy the articles.