Relative pressure is a measurement that disregards ambient atmospheric pressure (approximately 14.5 psi). When a container is described as having 10 psi of relative pressure, the absolute pressure is calculated as:

Absolute Pressure = Relative Pressure + Atmospheric Pressure 10 psi + 14.5 psi = 24.5 psi absolute

Application in Tuning Software#

Tuning software often utilizes relative pressure scales for fuel and ignition mapping.

Example of a fuel map utilizing relative pressure scaling

Note

Column 10 typically represents atmospheric pressure. Values may appear slightly lower than 14.5 psi due to interpolation or minor vacuum losses occurring even at Wide Open Throttle (WOT).

Important

Column 1 is an arbitrary reference point and does not represent an absolute vacuum (0 psi absolute).

Sensor Limitations#

When configuring your ECU, ensure the selected MAP sensor is capable of reading the required pressure range. Refer to the specific sensor data sheets for the maximum absolute pressure limits of your OEM or aftermarket MAP sensor.

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