What the inputs mean
- Target power, air-fuel ratio and BSFC: set how much air the target takes. BSFC (brake specific fuel consumption) is pounds of fuel per horsepower per hour. The defaults, 12.0:1 and 0.60, are assumptions, not measurements.
- Boost, altitude and pressure lost: set the pressure ratio the compressor must make. Outside pressure comes from the standard atmosphere.
- Displacement, VE, manifold temperature and rpm range: set how much air the engine actually moves at that boost.
- Map reference: corrects the airflow to the conditions your compressor map is drawn for.
The formulas
airflow (lb/min) = hp × BSFC × AFR ÷ 60
pressure ratio = (boost + pressure lost + outside) ÷ outside
density (lb/ft³) = psia × 144 ÷ (53.35 × (°F + 459.67))
engine airflow = CID ÷ 1728 × rpm ÷ 2 × VE × density
corrected flow = airflow × √(T inlet ÷ T ref) ÷ (p inlet ÷ p ref)53.35 ft-lbf/(lb °R) is the gas constant of air, the same as 287.05 J/(kg K) in metric. Temperatures in the correction are absolute.
Worked example
250 hp target, 12.0:1, BSFC 0.60: 250 × 0.60 × 12.0 ÷ 60 = 30.0 lb/min (0.227 kg/s).
20 psi at sea level, no pressure lost: (20 + 14.70) ÷ 14.70 = 2.36 pressure ratio.
A 2.0 L engine at 90% VE with 120 °F in the manifold: density 34.70 × 144 ÷ (53.35 × 579.67) = 0.1616 lb/ft³. At 6,500 rpm it moves 33.4 lb/min, about 278 hp by the same assumptions. At 5,500 rpm it moves 28.2 lb/min and at 6,000 rpm 30.8 lb/min, so it reaches the 30.0 lb/min target between the two. With the map reference at 59 °F and 14.70 psi, corrected and actual flow are the same at sea level.
How to read a compressor map
The horizontal axis is corrected airflow, in lb/min or kg/s. The vertical axis is pressure ratio. The line on the left edge is the surge line: to its left the flow is unstable. The right edge is where the compressor runs out of flow (choke). The closed loops are efficiency islands, and the curves across the map are shaft speed lines. Plot every row of the table. Points to the left of the surge line or beyond the right edge of the map are outside what that map covers. The map itself tells you the rest; this page does not rate any compressor.
Altitude moves the points: at 5,000 ft the outside pressure is 12.23 psi, so 20 psi of boost is a 2.64 ratio, and 30 lb/min corrects to 36.1 lb/min.
Related calculators
Work out the static ratio first with the compression ratio calculator, and the pressure ratio of a boosted engine with the boost calculator. The engine set also covers dynamic compression, rod ratio, volumetric efficiency and BMEP.
Questions
How much airflow do I need for my horsepower?
Multiply horsepower by BSFC and air-fuel ratio and divide by 60. With the assumptions 0.60 and 12.0:1 that is 0.12 lb/min per hp, so 250 hp needs 30.0 lb/min.
How do I calculate compressor pressure ratio?
Add the boost, any pressure lost in the intercooler and piping, and the outside pressure, then divide by the outside pressure. 20 psi at sea level is (20 + 14.70) ÷ 14.70 = 2.36.
What is corrected airflow on a compressor map?
The airflow referred to the map's reference inlet pressure and temperature, so one map works in any weather. Hotter or thinner inlet air raises it: 30 lb/min at 5,000 ft with 59 °F air is 36.1 lb/min corrected to 14.70 psi and 59 °F.
Does intercooler pressure drop change the pressure ratio?
Yes. The compressor has to make the lost pressure on top of the boost you see in the manifold. With 2 psi lost, 20 psi at sea level needs a 2.50 ratio instead of 2.36.