TorqTune MAP Sensor Calibration

Every CM5 and CM5-LT kit includes a 4-Bar Manifold Absolute Pressure (MAP) sensor which can be used in turbocharged set-ups that generate up to 40 PSI of boost.

CM5 uses the output of the MAP sensor to determine the boost level, which is then used to calculate the pump duty the unit will output.

Each MAP sensor is slightly different, so Torqbyte characterizes each and every sensor we ship out and provides the user with their sensor's specific calibration parameters. These parameters are obtained using high-accuracy automated test equipment and are shown on a label applied to each Torqbyte MAP sensor.

The 3 parameters provided on each MAP sensor's label are:

  • Slope of the sensor's output vs. boost pressure
  • Intercept point on the vertical axis, which represents the sensor's theoretical output at 0 PSI of boost
  • Atmospheric pressure reading at the time of the factory test

The figure below shows the first two parameters (Slope and Intercept) graphically:

The slope and the vertical intercept values define the MAP sensor's transfer function (aka line equation aka sensor's output vs. input relationship), which defines its output as a function of the applied boost pressure (PSI). 

You will notice that the output of the sensor is not expressed in Volts although it's typical for MAP sensors to be specified in terms of their Pressure vs. Voltage relationship. The reason is that the CM5 internal processor doesn't work with Volts.

CM5's electronics use an Analog to Digital Converter (ADC) to convert all incoming voltages (including the MAP sensor voltage) into a digital representation of that voltage, which we will refer to as ADC Counts throughout this text.

The third parameter on the sensor's label is used to capture the atmospheric reading (again expressed in ADC Counts) that was recorded at the time of the initial factory test. This is provided so that other atmospheric values can be calibrated out by applying a correction factor based on the difference between the atmospheric pressure at our factory at the time of test and the user's current local atmospheric pressure. This correction factor is then applied to all active boost calculations performed in TorqTune.

The reason that atmospheric pressure is so important in this case is that Torqbyte uses an absolute-type pressure sensor whose output is referenced to an internal vacuum source. However, boost is a relative quantity referenced to the local and current atmospheric pressure, which varies based on geographic location, altitude, season and general weather conditions.This means that the same absolute MAP sensor, installed in a vehicle that's making 20 PSI of boost will generate a higher output at a location with a low geographical elevation and a lower output at a place with a high geographical elevation. Without a way to compensate for different atmospheric pressures the unit would read boost incorrectly which is not ideal for precise boost-based pump control.

Having said that, the CM5 can be used with both absolute type and gauge type pressure sensors. The gauge type pressure sensors base their output on their local atmospheric pressure, so 15 PSI is 15 PSI no matter at what elevation the reading is made. These types of sensors are generally not used in automotive applications, but they are supported by the CM5 and by the TorqTune software (more on this towards the end of this post).

Q: Why do these numbers matter?

A: These numbers are most important to TorqTune. They allow TorqTune to apply proper calibration corrections to its boost calculations that are sent to and received from the CM5. The unit itself, stores these values in its non-volatile memory so they can "travel" with the unit and its MAP sensor, but it doesn't actually use them for anything. This is because CM5's on-board processor doesn't really know anything about pressures or voltages. It relies on TorqTune to program it with correct ADC Count values and it's up to TorqTune to correctly relate the ADC Counts generated by a particular MAP sensor to the actual boost levels that will be seen in operation.

Q: What should be done with these numbers?

A: If you purchased a regular CM5 or a CM5-LT kit your unit will come pre-programmed with the unique parameters for the MAP sensor that was shipped with your kit. The only thing you will need to do is Read these numbers out of the unit so they can be applied to TorqTune's internal boost calculations.

Let's assume that the label on your MAP Sensor said the following:

Slope: 16.848
Intcp: 251.21
Atmos: 254

Assuming you have previously installed TorqTune, connect the unit to your laptop/tablet/PC using the provided USB cable.

Launch TorqTune and ensure it detects the attached CM5. A green LED should appear in the bottom right corner of TorqTune's interface saying "Connected fw" and displaying the CM5's firmware version and unit type (CM5 or CM5-LT):

Next, focus on the Map Calibration section shown on the main Settings tab:

You will see the default TorqTune MAP Calibration values:

Slope: 16.122
Intcp: 222.312
Atmos: 223

These values will almost certainly not match the parameters of your MAP sensor and if you were to proceed with these default values all your boost readings would be wrong. Worse yet, if you were to hit Write or Write All buttons from the top menu, these default numbers would be sent to the unit overwriting the correct values that were stored there at the factory when your unit was shipped. If you ever do this, you can fix things by manually re-entering all the values from the label on your sensor. (more on this later).

Assuming you haven't accidentally erased the factory programmed settings, the next step should be to click the Read button from the top menu.

Now you will see the Map Calibration section get updated with the numbers for your actual MAP sensor that were stored in your CM5 at the factory:

The grayed out Atmosphere parameter in the Factory Defaults section on the left side shows the atmospheric pressure reading measured at our factory when the sensor was tested. However, your current atmospheric pressure may be different, so you can use the Calibrate button to account for the difference.

Note: You should only click the Calibrate button when the CM5 is connected to a MAP sensor. Never perform this step if the unit is out of the car without a sensor connected (for example if you are configuring the unit at your desk without any sensors plugged in). If no sensor is present the MAP analog input will "float" and any random value could get stored as active atmosphere which will prevent the unit from operating correctly.

Note: You should only click the Calibrate button when the engine is OFF. Never perform this step if the engine is running (either idling or making boost). If you hit the Calibrate button with the engine running a wrong value will be stored as atmosphere which will prevent the unit from operating correctly.

Assuming the MAP sensor is connected and the engine is not running, click the Calibrate button.

A warning prompt will appear once again telling you not to proceed if the engine is running:

If you click OK TorqTune will grab a MAP sensor measurement and drop it into the Current Active Atmosphere field on the right. In this example the new Atmosphere reading is 264 ADC Counts.

 

You should now commit the calibrated values to your CM5 by clicking the Write All button.

Now your TorqTune is calibrated to your unique MAP sensor and also to your local and current atmospheric pressure and all your boost values should be accurate from this point on. If you want to, you can periodically repeat the calibration procedure to readjust everything from time to time and account for changing atmospheric conditions.

At this point you should go to File -> SaveAs to create and save a TorqTune .ttini calibration file with all these settings stored inside. The next time you open TorqTune you should start your tuning session by first opening that .ttini file so that all your MAP settings will be automatically imported instead of the default values that TorqTune will use otherwise.

The last field labeled Deadband is important enough that it requires its own separate blog post.

Q: What if you got a new MAP sensor or what if you accidentally overwrote your MAP sensor's parameters with say the TorqTune default ones?

A: No problem. The solution is very simple. Just as before, make sure that TorqTune is detecting your CM5.

If everything is OK, click the Read button from the top menu:

Focus on the Map Calibration section shown on the main Settings tab:

Click the Edit Defaults button.

A warning window will pop up. Click OK to proceed.

Now the 3 fields in the Factory Default section shown on the left side will become "editable"

 Looking at the label on your MAP Sensor, type in the 3 values from the label to their corresponding fields.

Once all the values are entered, verify they are correct and click the Edit Defaults button one more time

The 3  fields should get grayed out again.

Now click the Set to Defaults button in the Current Active section on the right side.

All the numbers from the Factory Defaults section on the left should get copied over to the Current Active section on the right.

You should now upload the calibrated values to your CM5 by clicking the Write All button.

Finish the procedure by performing a Calibration to the current atmospheric pressure as described above.

What if you want to use your own pressure sensor?

Maybe your set-up makes more boost than the supplied 40PSI sensor can measure or maybe 40 PSI is too wide and you want to narrow the measurements to lower boost values. Maybe you need something that can read up to 100 PSI or even 150 PSI. Once you have the appropriate line equation for your custom sensor, using it with your CM5 or CM5-LT is very easy to do.

Say you are using a CM5-LT in a diesel application and you require a pressure sensor that can measure up to 100 PSI or even 150 PSI of boost. Say also, that you don't want to constantly calibrate for atmospheric pressure and you decide to use a vented gauge-type sensor with a 0-5V output that always represents the actual boost without regard for variations in atmospheric pressure.

Note: You should only use sensors whose output is approximately 0-5V. Never use sensors that are capable of outputting more than 5V as that could damage the CM5's MAP Sensor analog input.

Say that you've narrowed your choices down to something like the Honeywell MLH100PGB06A (100PSI) or MLH150PGB06A (150PSI).

Assume that you finally decide on the 100 PSI sensor and that it has the following output characteristics:

0.5V at 0 PSI
2.5V at 50 PSI
4.5V at 100 PSI

The first thing to note is that this pressure sensor, just like all the others, is defined in terms of its voltage output with respect to applied pressure, not in terms of ADC counts that TorqTune requires.

The conversion between Volts and ADC Counts is quite simple:

To convert Volts to ADC Counts multiply Volts by 204.6
To convert ADC Counts to Volts multiply ADC Counts by 0.00488759

The output characteristics of the 100 PSI sensor above can be rewritten as:

0.5V * 204.6  = 102.3 ADC Counts at 0 PSI
2.5V * 204.6 = 511.5 ADC Counts at 50 PSI
4.5V * 204.6 = 920.7 ADC Counts at 100 PSI

Although you can do this by hand using very simple math, the quickest way to obtain this sensor's line equation is with Excel.

Open a new Excel sheet and enter the boost values in the column A and enter the ADC Count values in the column B. Don't reverse this order or the end result will be wrong.

Select the values entered and go to Insert -> Scatter Chart.

A scatter chart should appear that has the boost values on the horizontal (X) axis and ADC Count values on the vertical (Y) axis.

Right click on one of the points and go to Add Trendline

 In the Format Trendline window that pops up ensure that the following options are selected:

  • Linear
  • Display Equation on chart

Now the chart should now be showing a straight line and an equation.

The number beside the x is the Slope, the number right of the + sign is the Intercept. Since this sensor is a gauge-type sensor we can assume that at 0 PSI it will generate an output equal to 102.3 ADC Counts, as we have calculated before. Since TorqTune only accepts whole numbers in its Atmosphere fields, let's round down to the nearest whole number which is 102.

So for this particular sensor we would use the following parameters:

Slope: 8.184
Intcp: 102.3
Atmos: 102

Then we would follow the same procedure described above under "What if you got a new MAP sensor or what if you accidentally overwrote your MAP sensor's parameters with say the TorqTune default ones?" to apply the new values to TorqTune and upload them to the unit.

If you are working with a sensor that you know puts out 0-5V, but are you are not sure what its line looks like and/or you have absolutely no technical information about its characteristics, there is still hope.

You will need a set-up where you can vary the pressure to the sensor and read that pressure from a known-good pressure gauge (probably mechanical). This gauge should be "trusted" as being accurate. If this gauge is not accurate this procedure will produce unusable results.

You will have to connect the sensor to your CM5 and launch TorqTune.

Enable the data streaming by clicking the Live Updates button ensuring the LED beside it turns green.

On the Live View tab change the way the data is displayed in the drop down menu from Physical Units to RAW Decimal.

This will force TorqTune to display ADC Counts instead of PSI in the field where the MAP sensor pressure is normally shown.

Now that you can view ADC Counts for your sensor, you should first write down the ADC Count value that is displayed when the sensor is just seeing atmospheric pressure (i.e. no boost). This will be your third parameter -> Atmosphere.

After that's done start to slowly vary the pressure applied to the sensor in small increments. This is best done with a decent manual pressure regulator. Pause every so often and write down the pressure reading (in PSI) from your mechanical gauge as well as the corresponding ADC Count value from TorqTune's live view. The more points you collect the more accurate your sensor line equation will be.

When the ADC counts get to about 900 or so you should stop the test. Note the mechanical gauge's reading in PSI. That is around the upper end of the pressure range that this sensor is able to measure.

When you have collected enough points, enter them into Excel and have it generate a line equation as described above. From that line equation you can extract the other two parameters you'll need - the Slope and the Intercept.

Use the procedure above and upload the new values to the unit and to apply them to TorqTune. Don't forget to File -> SaveAs to create a .ttini configuration file.

You're done.

Posted by Torq Byter_ on 24 October, 2015
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