I've been planning on installing a Megasquirt engine management system in my car. Once installed, I'll be able to use a laptop to modify the tuning of the car. This is required when modifying the engine (in my case, I'm planning on going with a turbo).

My initial goal is to install the MS but only connect the inputs, so I can see if my wiring is correct and I can monitor and log data with the laptop. Once I have that working, I'll cut over the spark and fuel outputs from the stock ECU to the Megasquirt, and try my hand at tuning. This will be good practice prior to installing, as the engine is much more forgiving when it's still stock.

Parallel or Standalone?

In a parallel install, the stock ECU is kept in place and the MS is installed and wired to share responsibility for managing the car's electronics. In a standalone install, the MS has full responsibility and the stock ECU is bypassed or removed entirely.

I'm planning a parallel install.

The Megasquirt will then be responsible for controlling fuel and spark, and the stock ECU for managing idle, the a/c, and the alternator.

This has two advantages. For one, it's simpler to let the stock ECU do what it's good at. Setting up the Megasquirt run as a standalone ECU on a Miata requires additional custom circuitry. Even more important, it will allow the stock ECU to communicate through the OBD-II port, which is required for emissions testing here.

Keep in mind that this is not a "piggyback" solution - the Megasquirt will be solely responsible for firing the ignition coils and the injectors.

Megasquirt and Sensors

There are two basic families of Megasquirt - the older MS I and the newer MS II. The MS II has the advantage of being able to support the stock sensors on a 2nd generation Miata (aka NB, or model years 1998-2005). Fewer people are running the MS II on a Miata, however, so this is fairly cutting edge. It also requires some additional custom circuitry. You can read all about it over at the Megasquirt forum on

I decided to go with the MS I, mainly because I don't want to be an early adopter when it comes to my daily driver, and because I didn't want to spend anymore time on this project. Even buying a pre-assembled MS I from DIY Autotune with the custom circuitry (Miata mods) already built, this was going to be a considerable project.

One major downside of going with the MS I over the MS II is that the MS I can't interpret the somewhat odd cam sensors in NBs. There are two ways to solve this problem. One is to modify the crankwheel by grinding off some teeth so that it sends signals that the MS I can interpret. This, however, makes it difficult to revert back to the stock ECU. The other option is to install a CAS sensor from a 1st generation Miata (aka NA, or model years 1989-1997). Because I want to easily revert back to stock, and I want to do the install in stages, I plan on installing the older sensor.

In addition to the CAS sensor, I will be installing a GM IAT (intake air temp) sensor, and a wideband O2 sensor.

Useful Links

Wikipedia Article on Megasquirt
How to make and install your own DIY MS
The Megamanual

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