Compressor map

Pressure ratio (y-axis)

The absolute outlet pressure divided by the absolute inlet pressure.

Mass air flow rate (x-axis)

The mass of air flowing through the intake/compressor over time. Usually lbs/min or CFM.

Surge line

The left hand boundary of the map, where the air flow is too low for the current pressure ratio. Blow off valves or Bypass valves are often used to relieve excess pressure to prevent sudden off throttle induced pressure surges.

Choke line

The right hand boundary of the map, where the turbo is approaching its flow limit

Overspeed line

The top boundary of the map, where the turbo is approaching its maximum shaft speed. Past this point, the turbo is liable to "overspin" and cause catastrophic bearing failure.

Turbo speed lines

The lines of constant turbo shaft speed.

Efficiency islands

The concentric regions on the map representing compressor efficiency. The smallest island in the center of the map is the "sweet spot", or where turbo is the most efficient.

Pressure Ratio

Expressed as ${\frac {P_{outlet}}{P_{inlet}}}$ or, more commonly, ${\frac {P_{MAP}+P_{ICdrop}}{P_{ambient}-P_{id}}}$

Where

• $P_{outlet}$ is the absolute pressure at the turbo compressor outlet
• $P_{inlet}$ is the absolute pressure at the turbo compressor inlet
• $P_{MAP}$ is MAP (manifold absolute pressure)
• $P_{ambient}$ is ambient air pressure
• $P_{ICdrop}$ is intercooler pressure drop (pressure difference between $P_{outlet}$ and $P_{MAP}$)
• $P_{id}$ is intake pressure drop (pressure difference between $P_{inlet}$ and $P_{ambient}$)

Alternately, if boost is known, it is approximated by ${\frac {P_{ambient}+P_{boost}}{P_{ambient}-P_{id}}}$. Intercooler drop is ignored here for simplicity.

Examples

Google calculator provides a handy way to deal with unit issues.

For example, with 20psi boost and 1psi inlet drop, type this into google (assuming you live at sea level):

(1 atm + 20 psi)/(1 atm - 1 psi)=

and you get:

(1 atm + 20 psi) / (1 atm - 1 psi) = 2.53330013 , or a pressure ratio of approximately 2.5

A few others:

• (1 atm + 1 bar) / (1 atm - 1 psi) = 2.13
• (1 atm + 1.3 bar) / (1 atm - 1 psi) = 2.45
• (1 atm + 22 psi) / (1 atm - 1 psi) = 2.68
• (1 atm + 24 psi) / (1 atm - 1 psi) = 2.83

Air Flow

Most turbo maps express air flow in terms of mass per time (commonly grams per second or lbs per minute) or volume per time (commonly cubic feet per second, CFM, or cubic meters per second.)

If you have accurate MAF numbers (i.e. your car is not modded in a way that scales the MAF readings), you can convert this directly to the desired units.

If you do not have accurate MAF numbers, but do have an approximate horsepower target, you can use the rule of thumb that 1 lb/min of air generates 10 horsepower.

Examples

From g/sec

To convert grams per second into pounds per minute, divide by 7.55

300 (grams per second) = 39.6832072 pounds per minute

To convert grams per second into m³/sec, divide by 1225 (density of air at sea level)

300 / 1225 = 0.2449 (300 (grams per second)) / ((1 225 g) per (cubic meter)) = 0.2449 m³ / s

If you have two turbos, divide g/sec by twice that, or 2450

300 / 2450 = 0.1224

(300/2 (grams per second)) / ((1 225 g) per (cubic meter)) = 0.1224 m³ / s

From horsepower

To convert horsepower to pounds per minute, divide by 10.

To convert horsepower to m³/sec, divide by 1620

420 / 1620 = 0.259

((420 horsepower) / ((10 horsepower) per (pound per minute))) / ((1 225 g) per (cubic meter)) = 0.259 m³ / s

If you have two turbos, divide horsepower by twice that, or 3240

420 / 3240 = 0.130

((420/2 horsepower) / ((10 horsepower) per (pound per minute))) / ((1 225 g) per (cubic meter)) = 0.130 m³ / s

References

An excellent source of more information can be found here.