Megasquirt Part 2: Bringing It Home

With most of the sensors hooked up, all that's left are the injectors and a couple of wires for ignition control. At this point, we were able to get the MegaSquirt to connect with the MegaTune on our PC. While a few of the readings were initially off, we rectified the situation by using separate coolant and air temp sensors for the MegaSquirt. Don't mind the mess, we'll tidy it all up later

With most of the sensors hooked up, all that's left are the injectors and a couple of wire

After spending a few hours tracing, re-tracing and spicing wires, we were confident that we had all the wires coming out of the MegaSquirt connected to the right wires on the car's harness. We then busted out the laptop, plugged it into the MegaSquirt and powered up the car. After a little fiddling with the settings via our laptop, we had the PC and the Megasquirt talking to each other via serial link. But, it quickly became obvious that a few things were amiss.

After sorting out the temp sensor issues, we realized that we had no tach signal. A quick search at MSEFI.com yielded us an answer. Because of the type of cam angle sensor the Miata uses, it needs a "pull-up" to make an RPM signal the MegaSquirt can understand. The pull-up was just a 400-ohm resistor that we connected to a 12-volt supply and the cam angle sensor signal wire at the ECU.

Getting It Running
With the MegaSquirt installed and communicating with the tuning software installed on our laptop, we were ready to set up a rough tune. There are steps you can follow to make setting up and tuning the ECU a less stressful experience. The MegaSquirt manual lists the 8-steps to tuning:

Here's the GM air temp sensor that we added for the MegaSquirt. We removed the intake manifold before drilling and tapping it so we could screw it into the stock manifold. Some people prefer to put the air temp sensor before the throttle body, but we are more interested in the temps closer to the intake valves... to each his own. The stock ECU is still using the stock air temp sensor, placed elsewhere in the intake tract. The GM temp sensor was much easier to install and threaded into an existing and unused coolant port.

Here's the GM air temp sensor that we added for the MegaSquirt. We removed the intake mani

1.Learn to use the tuning software (MegaTune)
2.Set the constraints
3.Get the engine started and idling
4.Set the injector parameters
5.Set the cold and hot start enrichments
6.Set the VE table
7.Set the acceleration enrichment
8.Check to make sure the resistors aren't getting too hot

We cannot stress enough how important it is to read the MegaTune manual front to back before attempting to tune. There are many parameters and settings that are crucial to the safety of you and your car. While we are doing our best to provide all the info you need to get started, there is no substitute for doing your homework. You've been warned.

Once we set the constraints to match our engine, we crossed our fingers and cranked the engine over. To our delight, the car jumped to life and started idling after only a few turns. The fuel map generator that comes with the tuning software provided us with a surprisingly accurate fuel map, idling with a 13:1 AFR like a champ. Pleased, we went for a spin.

The car not only idled well, but also drove amazingly with the MegaTune-generated fuel map. Even the default settings for things like closed-loop o2 feedback and accel enrichment were surprisingly close. So close, we made no further changes to the fuel settings until we hit the dyno later that week.

In this window, we are generating a base fuel map. By entering info like engine size and torque/horsepower characteristics, the MegaTune can calculate how much fuel the car needs. In our case, the fuel map that MegaTune generated was surprisingly close and required only minor changes to get dialed in.

In this window, we are generating a base fuel map. By entering info like engine size and t

With the fuel control working, we turned to setting up the ignition timing. Tuning ignition timing isn't as simple as tuning fuel, for example, which can be done with a "wideband" O2 sensor. Proper ignition tuning takes a lot more work and involves holding the engine at a specific load on a loading dyno and adjusting the timing for each and every cell (144 individual ones in the case of the MegaSquirt) until peak torque is achieved. This tuning method is based on MBT, or mean best torque. This is a time consuming and expensive process, so we took the easy way out and "borrowed" the ignition map that Flyin' Miata uses with their tried and true turbo kit (which happens to use the same turbo as our DIY kit). Flyin' Miata has spent years developing this ignition map and we doubt we could do any better even if we had the time and money to try. We simply plugged the values into MegaTune and downloaded it to the MegaSquirt.

After making a few minor changes internally, as per the instructions on the MSnS-E web site, we were ready to control spark. We ran two wires from the MegaSquirt to the stock coils and cranked the little engine over. Once again, it fired right up, running just like stock. We took the car for a spin around town, going into boost more than a few times and decided it was good enough to drive to the dyno as it was.

At The Dyno
After a pleasantly uneventful 40-mile drive on rush-hour Los Angeles freeways, we arrived at MD Automotive in Westminster, Calif., to do some additional tuning.

The base fuel map was close enough that after only a handful of passes, we had an AFR of around 11.8:1 under boost with only a few changes to the fuel map. The AFR is ideal for this car driven on 91-octane gas in 110+ degree weather.

The spark map proved to be very good, too, although it may be a tad conservative. So, in the end, "tuning" the ignition timing entailed nothing more than converting and copying Flyin' Miata's map (which took about 30 minutes).

After everything is wired up, there are still a few parameters to set up before you can start the car. The first things to set up are the constraints. If you get any of these wrong your car might not run at all. If you aren't sure what to put in the boxes, search around on the MegaSquirt forum at MSEFI.com.

After everything is wired up, there are still a few parameters to set up before you can st

Happy with the fuel and spark, it was time to get some numbers. After dialing in the fuel, the little Mazda put down about 245 wheel hp at 11-14 PSI of boost. There was a boost fluctuation problem that was later fixed by changing the spring in the wastegate. With a steady 15 PSI of boost, the car should make about 260 wheel hp, or so. Not too shabby for $200 engine management on a ghetto DIY turbo kit.

Yay Or Nay?
All said, it was quite an adventure. It was a lot more work than we ever imagined it would be, but it was worth it. In the right hands, this $200 dollar ECU really can do anything the ECUs that cost 10x as much can do. In less skilled hands, that might not be true. While it doesn't take an IQ of 140 to build, tune and install, it does take the ability to do a little research. For the price, nothing can come even close.

The MegaSquirt does have some disadvantages. While it may have tons of features, many are relatively new and not well documented. Also, with more heavily supported platforms like Honda, having options like Hondata, MegaSquirt probably isn't the best option. But for the tinkerer or person with a car with less aftermarket support, it may be a great option. Just make sure you can follow through with the commitment. This isn't something that you can expect to finish in a weekend. Allow more like a month, working on it 10 hours a week, minimum. As such, it may not be a good idea to install it on your daily driver just yet. Just don't slit your wrists when you can't figure it out.

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