Budget EFI - Megasquirt

We employed the Innovate Motorsports wide-band air-fuel ratio meter to help us quickly dial in the air-fuel ratio on the engine. This took only a matter of minutes and we ended up with a best power air-fuel ratio of around 12.9:1. The MegaSquirt is also set up to run in closed loop with a three-wire heated oxygen sensor that works very well for part-throttle tuning.

We employed the Innovate Motorsports wide-band air-fuel ratio meter to help us quickly dia

Get Stimulated

One of the options for building the MegaSquirt system is to also construct something called a MegaSquirt Stimulator. This slick little tool is a simple signal generator that uses a circuit board and five potentiometers that operate like volume control knobs on your stereo to create signals that simulate engine rpm, MAP, coolant temperature, throttle position, and inlet air temperature voltage output. By connecting the stimulator to the MegaSquirt computer along with a laptop, you can use the MegaTune software to evaluate the signals from each of the five inputs to ensure that the MegaSquirt ECU is performing properly.

We used the stimulator box several times to evaluate the MegaSquirt when diagnosing engine problems. In all but one case, the stimulator box proved the computer was fully functional and our difficulties were caused by outside problems (see sidebar "Installer Errors"). The stimulator turned out to be a wonderful diagnostic aid we used several times to prove that the ECU was not the problem--even when we were convinced it had to be bad!

11This is a reproduction of the first MegaTune input and output screen. The gauges indicate rpm, TPS, pulse width (bank 1), duty cycle (bank 1), MAP, inlet air temp (MAT), pulse width (bank 2), and duty cycle (bank 2). The small circles across the bottom represent the output from the oxygen sensor with a lean-to-rich scale with 14.7:1 air-fuel ratio directly in the middle.

11This is a reproduction of the first MegaTune input and output screen. The gauges indicat

Wide-Band Tuning

The only issue we never fully resolved with our quick evaluation of the MegaSquirt was too-lean readings from our three-wire, heated oxygen sensor. Luckily, we had decided to also use our new Innovate Motorsports wide-band oxygen sensor to help us tune the 355ci small-block. When the narrow-band oxygen sensor didn't agree with the Innovate unit, we ignored the narrow-band sensor completely. Time didn't allow us to go into why that unit failed, but it may have been just a poor sensor or perhaps a wiring problem with the sensor. The MegaSquirt circuit certainly performed as intended.

MegaTune allows you to monitor how the engine is performing on your laptop while a friend drives the car. With the engine on the dyno, we could watch the small green circle move around the map based on load and rpm. Using the arrow keys on the laptop, we can place the red "X" at any location to add or remove fuel. Then, using the laptop's "shift" key along with the "up" arrow we could add fuel, or "shift" and the "down" arrow would reduce fuel. Tuning was literally that easy.

MegaTune allows you to monitor how the engine is performing on your laptop while a friend

The beauty of using the Innovate system is that we were able to quickly establish best power using the air-fuel ratio sensor to hone in on best power. At first we aimed for 12.5:1 and then relied on power output to lean the mixture out to around 12.9:1 for best power. Still using the wide-band sensor, we also partially tuned the midrange cruise to run around 15:1 for best highway fuel economy and also used the feedback from the unit to set the idle air-fuel at 13.5:1 where the engine seemed to be the happiest.

The MegaSquirt is intended to use the narrow-band oxygen sensor to run in closed loop, and if our sensor had cooperated, it's clear the system would have worked just fine. But for wide-open-throttle tuning, the Innovate Motorsports wide-band sensor sure made it easy to get us close to best power with its accurate air-fuel information. After three quick back-to-back full-power runs, we had peak power dialed in with a few keystrokes on the computer.