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General MegaSquirt ECU : Initial Setup


Nov 14, 2013
Independence, Kansas
This was copied from the Bartuneengineering.wordpress web site:
Printable Copy in WORD at the bottom of this thread or to just read, this is the initial ECU setup instructions to get MegaSquirt up and running on your car if you are using MS.


So once you’ve either assembled or received your pre-assembled ECU, you’ll need to load firmware and a base map onto it if it hasn’t already been done. The safest way to do this is to power the ECU up outside the car and connect it to your computer. If this is not possible then it can still be installed into your vehicle to power up but it is recommended that the ignition coils are unplugged.
Firmware can be downloaded from

With the ECU powered up and the serial/usb port connected to your computer, run the firmware file and go through the steps to install the correct firmware or compete the firmware update in TunerStudio and follow the prompts.
Once this is complete you can open up TunerStudio and create a new project file. When completed you should see a dashboard of gauges.

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Next you’ll need to load your base map by clicking File>Open Tune (msq).
You may get some errors loading a base map meant for an older/different firmware version, but it’s usually nothing to worry about. It is still a good idea to look through the errors.

Initial Setting to Change:
The first thing to check is your injector size. When changing injector size the ‘Required Fuel’ value needs to be changed. To work out the new Req.Fuel value you use a simple formula.
(old injector size / new injector size) x Req.Fuel = New Req.Fuel value.
For example, if your stock injectors are 230cc and you’ve installed 440cc ones and your current Req.Fuel value is 10ms you would do:
(230cc / 440cc) x 10ms = 5.2ms

If the base map has been set up with the correct engine and injector sizes then you can use the required fuel calculator instead and just adjust the injector size to the new values. The calculator will determine the correct value to use.

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The Req.Fuel value can be found under Basic/Load Settings>Engine and Sequential Settings. You will also need to alter injector dead time and small pulse-width calibrations to suit your injectors. The dead time setting can be found under Fuel Settings>Injector Dead-Time/PWM.

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MS2 Injector Dead Time Settings

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MS3 Dead Time Settings

You may also want to check that your rev limiter is enabled and set to a suitable value. Settings for this can be found under Basic/Load Settings>Rev Limiter. A dynamic engine speed limit can also be set based on coolant temperature however it is not necessary.

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Sensor Calibration:
Before you can start the engine and start tuning you will have to calibrate your sensors. Firstly go to Tools>Un/Lock Calibrations and change the value to unlocked. This will allow you to set up the calibrations of your sensors.

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Calibrate throttle position sensor (TPS):
To calibrate the TPS press the closed throttle “Get Current” button with no throttle input, and then press the full throttle “Get Current” button with the throttle fully depressed. Next look at the TPS gauge on the dashboard and make sure you get 0% with the throttle closed and 100% with the throttle open. The closed throttle value should be lower than the open throttle value. If it’s the other way around then you’re TPS sensor isn’t wired correctly however it should still work.

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Calibrate manifold absolute pressure sensor (MAP):
Select the MAP sensor you are using. For the DIYPNP and V3.0 and V3.57 boards this is usually MPX4250. With the engine off at sea level the reading should be around 100kPa. At higher altitudes it will be lower.

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Calibrate coolant temp sensor (CLT):
Open the calibrate thermistors menu to calibrate the CLT and IAT sensors. These values will be different depending on if you’re using the OEM IAT sensor or not. The OEM MX-5 CLT sensor can be chosen from the drop down list and is listed as “RX-7_CLT(S4 & S5)”. If you are using a GM style sensor select GM in the drop down list.

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Calibrate manifold air temperature (MAT):
The values you input here will depend on the type of temp sensor you are using. If using a GM temp sensor you can just select it from the drop down menu, otherwise if using the OEM MX-5 MAT sensor select “RX-7_MAT(S4 & S5)”.

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Calibrate wideband AFR sensor:
Select the wideband controller you’re using from the drop down list. If your wideband is not listed you will have to manually enter the voltage points that correspond with a certain AFR, which should be given with your wideband.

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Calibrating Trigger Angle Offset:
To do this you will need a timing light. Go to Ignition Settings>Ignition Options/Wheel Decoder. All the settings in this menu should be correct from the base map, however the trigger angle offset may not be. You need to make sure that the timing the engine sees matches the ECU output. Change the Fixed Advance setting to “Fixed Timing” and set the fixed timing to 10 degrees or a value that matches your engines timing reference point. Hook up your timing light to cylinder 1. You will need to turn the car on to check the timing so read the section below first.
Check to see that you are reading 10 degrees off the crank pulley using the timing light, and if not adjust the “Trigger Angle/Offset(deg) value until it does reach the fixed timing advance output by the ECU. Once that is done you can change the Fixed Advance back to “Use Table”. This process just makes sure that the timing the ECU is outputting is the same timing that the engine sees.

Note: If using non-standard coils such as LS-2 or Toyota coils then the nominal dwell value needs to be adjusted accordingly.

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First Start:
Plug your coil pack(s) back in if you have unplugged them. Next try and start your car. You may need to give it a bit of throttle input to get it started. Once it has and, assuming it idles, check that your sensors are all reading correctly and that your AFR is not a crazy value.

AFR should be somewhere between 13-15 at idle once at operating temperature (should be adjusted to around 13.5-14:1 for batch injection or 14.7 for sequential), but you may have to adjust your fuel VE table if it isn’t. If everything looks good you can start tuning. If it fails to start a good idea is to take a datalog while cranking to try and determine the problem.

Sensor check:
MAP – should be reading around 100kpa if you’re near sea level with the engine off.
TPS – should read 0% with closed throttle and smoothly move to 100% with opened throttle
CLT – with the engine cold it should be within a few degrees of the ambient temperature.
IAT – same as coolant
O2 – the wideband should be reading max lean value with the engine not running


  • MegaSquirt Initial Setup.docx
    734.9 KB · Views: 10


Nov 14, 2013
Independence, Kansas
Yes I thought so too Steve, thanks.
When I stumbled on it and READ IT, it was a very good read and explained alot of things so now we have it here to save on DSMTUNERS!
Who knows when someone will use MS and need some startup advice.
It should have been on here quite a while ago IMO. It was probably just lost on the web and I stumbled onto it. :thumb:


Proven Member
Feb 9, 2019
A lot of useful information can also be obtained from DIYAutotune helpline, but people don't think to check there even though it's where most people purchase the ecu from :idontknow:

Here's the main article on their website, but any questions can be asked to them

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