Originally Posted by Joe Perez

This question seems to come up a lot, so I'm creating a sticky that I can point people to in the future. This is current as of Oct 2014, and will (hopefully) be updated as time goes on.

Note that I am focusing only on offerings from DIYAutoTune, which is considered by most to comprise the "mainstream" MS product line. There are also some excellent Miata-specific MS-based ECUs available from MSLabs, however I am not particularly knowledgeable about those.

Also, since the question comes up a lot: no aftermarket ECU will return OBD-II codes, and this includes the MS family. Those of you who live in areas where plug-in emissions testing is performed on '96 and newer vehicles will need to deal with that. Many people swap ECUs (and injectors, etc) once ever year or two, a few die-hards come up with custom "parallel" solutions that place the MS beside the stock ECU, letting the former run the engine and the latter talk to the OBD-II test station. This is very hard to do right.



Now, the breakdown:


Absolute cheapest: MS1, the original:


MegaSquirt I Kits Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products - DIYAutoTune.com

For $219, you get a box of parts that you solder together yourself, and is then useless until you do a bunch of additional custom and poorly-documented modifications and also construct a new wiring harness from scratch. This is how we all used to do it, and none of us do anymore. This unit uses a 30 year old CPU running a software package that leaves much to be desired, on a circuit board that was originally designed to run throttle-body injection with a distributor and uses some really lousy, sub-optimal circuit designs. So don't do it.

Just don't.




Also ran: MS2, the original:


MegaSquirt II Kits Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products - DIYAutoTune.com

MS2 places a new CPU onto the same crappy old circuit board as MS1. The software sucks somewhat less, but it's otherwise the same old ---- where you've gotta do a bunch of custom modifications to the board and live with crappy circuit design.




DIYPnP:


DIYPNP Nippon Denso 76pin Unassembled Kit DIYAutoTune.com

Now we're getting somewhere.

The same CPU as the MS2, on a new board, with less-bad circuit design. Decent software, and once it's assembled you have a plug-and-play solution with no harness fab needed. A good choice if you're skilled with a soldering iron, a terrible idea if you're not.





MSIII:


MegaSquirt III Kits and Assembled ECUs Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products - DIYAutoTune.com

Yet another new CPU tacked onto the same old circuit board from the MS1. This also adds the "X" expansion board, which finally adds native support for multiple ignition coils and injectors, PWM-based idle valves, etc., meaning you no longer have to do a bunch of custom modifications internally. Sadly, it still retains some of the crappy circuit topology from the original 1990s vintage MS1 design and you still have to build your own harness. This is an intermediate step in the right direction; cutting-edge software married to trailing-edge hardware.

To be avoided by all but the most hard-core techie types.





MSPnP2:


MegaSquirtPNP Plug-and-Play MegaSquirt Engine Management Systems by DIYAutoTune Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products - DIYAutoTune.com

This is essentially a fully pre-built version of the DIYPnP. Based on the MS2 CPU, it's an excellent choice for the '90-'00 Miatas.





MS3 Pro:


MS3-Pro Megasquirt Kits / Assembled Engine Management Systems, Wideband o2 Sensor Systems and tuning products - DIYAutoTune.com

****.

A generic, non plug-n-play ECU based on the MS3 CPU, but with completely new circuit board from the ground up which carries over none of the shitty circuit design from the original MS1. (They even "borrowed" the crank decoder circuit design that we came up with on this forum a couple of years earlier.) This will fit any car, but requires harness-building like the old ones. Expensive and worth every penny, easily the best universal ECU on the market today. I had the privilege of running a beta version of this on my old 1990 car, and loved it.




MSPnP Pro:


MegaSquirtPNP Pro MM0105 for the 01-05 Mazda Miata, manual trans DIYAutoTune.com

****, in plug-n-play form.

Only available (for now) for the '01-'05 cars (a separate version exists for the '04-'05 Mazdaspeed). Based on the MS3 Pro design and software. The best plug-n-play available for the later-model cars.