Re-scaling the Manifold Absolute Pressure Sensor Offset and Multiplier for Aftermarket Sensors

The factory MAP sensor of the Subaru/Scion/Toyota can read to around 1.37 bar absolute, so we recommend that the MAP sensor is replaced on all Forced Induction models. The factory ECU uses an offset and multiplier function to calculate manifold pressure from sensor voltage, when the sensor is changed the output voltage will calculate the measured manifold pressure incorrectly. This may cause non start situations as well as DTC's and poor SD tuning.

Below are the two maps that determine the Manifold Absolute Pressure (MAP) measured output value.

You can calculate the multiplier and offset for a sensor using the manufacturers data and some maths if required, however the popular plug-and-play replacement items are OMNI 3 and 4 bar sensors, the required offset and multiplier are shown below.

The Preferred method to re-scaling the MAP sensor is to start with the standard MAP sensor installed and using Live Data, check the Manifold Absolute Pressure with Ignition ON (engine not running) and at stable Idle, you may want to make a quick log file to use as a reference.

The next step is to fit new 3 or 4 bar MAP sensor and enter new scaling as above then repeating the test with the new MAP sensor to ensure it’s the same measured manifold pressure with the Ignition ON (engine not running) and at Idle.

The log file below shows two log files that have been added together in excel. The first log is with standard MAP sensor that flat-lines at 1.37 bar. The second log is a 3 bar sensor that has been correctly re-scaled.

The two lines are identical up until 5400rpm, where the standard sensor flat-lines while the 3 bar sensor goes on reading the true manifold pressure until 7600rpm.

You may need to raise and lower the MAP sensor voltage Min and Max to avoid the P0107 and P0108 on model fitted with a 2 bar or 3 bar sensor (as the voltage generated at idle will be considerably lower).

If you wish to calculate the sensor scaling factors mathematically using the data sheet follow the worked example below, the first step is to locate the appropriate data in the data sheet

Then use the formula to calculate both the multiplier and the offset values to go into the ROM.

Hopefully this will give you enough information regarding knock detection systems in modern ECU's. If you have any questions, please do not hesitate to contact us using the information below.