- Pressure Ratio ( ) is defined as the Absolute outlet pressure divided by the Absolute inlet pressure.Where:
- = Pressure Ratio
- P2c = Compressor Discharge Pressure
- P1c = Compressor Inlet Pressure
- It is important to use units of Absolute Pressure for both P1c and P2c. Remember that Absolute Pressure at sea level is 14.7 psia (in units of psia, the a refers to “absolute”). This is referred to as standard atmospheric pressure at standard conditions.
- Gauge Pressure (in units of psig, the g refers to “gauge”) measures the pressure above atmospheric, so a gauge pressure reading at atmospheric conditions will read zero. Boost gauges measure the manifold pressure relative to atmospheric pressure, and thus are measuring Gauge Pressure. This is important when determining P2c. For example, a reading of 12 psig on a boost gauge means that the air pressure in the manifold is 12 psi above atmospheric pressure. For a day at standard atmospheric conditions,
12 psig + 14.7 psia = 26.7 psi absolute pressure in the manifold
- The pressure ratio at this condition can now be calculated:
26.7 psia / 14.7 psia = 1.82
- However, this assumes there is no adverse impact of the air filter assembly at the compressor inlet.
- In determining pressure ratio, the absolute pressure at the compressor inlet (P2c) is often LESS than the ambient pressure, especially at high load. Why is this? Any restriction (caused by the air filter or restrictive ducting) will result in a “depression,” or pressure loss, upstream of the compressor that needs to be accounted for when determining pressure ratio. This depression can be 1 psig or more on some intake systems. In this case P1c on a standard day is:
14.7psia – 1 psig = 13.7 psia at compressor inlet
- Taking into account the 1 psig intake depression, the pressure ratio is now:
(12 psig + 14.7 psia) / 13.7 psia = 1.95.
- That’s great, but what if you’re not at sea level? In this case, simply substitute the actual atmospheric pressure in place of the 14.7 psi in the equations above to give a more accurate calculation. At higher elevations, this can have a significant effect on pressure ratio.
For example, at Denver’s 5000 feet elevation, the atmospheric pressure is typically around 12.4 psia. In this case, the pressure ratiocalculation, taking into account the intake depression, is:
(12 psig + 12.4 psia) / (12.4 psia – 1 psig) = 2.14
Compared to the 1.82 pressure ratio calculated originally, this is a big difference.
- As you can see in the above examples, pressure ratio depends on a lot more than just boost.