Get Stimulated
This 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.
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!
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.
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.
This is a volumetric efficiency (VE) table we generated using the MegaTune software for a 350ci engine idling at 900 rpm with no more than 15 inches of vacuum. We estimated peak torque at normally aspirated 440 lb-ft at 4,000 and peak hp at 375 at 6,000 rpm. We loaded a similar table into our engine, and it fired right up and ran respectably while we fine-tuned it for best power. The numbers in this VE table represent the percentage of cylinder filling. To enrich the mixture, increase the number value in the corresponding box. The vertical scale is load (expressed in kPa or kilopascals--metric for pressure where 1 kPa = 0.145 psi). The horizontal scale is rpm.
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.
Installer Errors
Here's where we come clean. Did this installation go as smoothly as it might appear? Absolutely not. But that's not because the MegaSquirt is a difficult system to master. Granted, this is not a system for the entry-level car crafter who barely knows one end of a screwdriver (or injector) from the other. But neither is this system overly difficult. The difference is that with this budget-based system, the designers are letting you do more of the work. What do you expect for $225?
We did have difficulties, but almost all of it was self-inflicted. The reason the engine was tested on two different dynos is that during our original test at Westech, we had engine-related problems that complicated our attempts with the carbureted baseline. Eventually we discovered that in earlier work on this engine I had placed a solid lifter on the intake lobe of Number 2 cylinder that kept the intake valve open and killed the power. Once we fixed that problem, we ran into trouble getting the injected version to run. We discovered an intermittent connection problem with the chip that showed up when we checked operation with the stimulator. We replaced it with a spare chip Tim had thoughtfully purchased for $10. However, the engine still wouldn't run properly and we ran out of time. Later, Tim discovered that we had forgotten to load the base fuel map onto the second chip!
Dyno-testing at Duttweiler looked like it would be a cakewalk, and at first the engine fired right up and ran well with the base VE map. But suddenly, the engine would run rough, misfire, and quit. Since MegaSquirt doesn't control spark, our setup included an MSD billet distributor and an MSD 6A controller. Since the MSD box does emit electro-magnetic noise, we thought that might be the problem. Eventually, we traced the intermittent problem to the white wire on the MSD box occasionally grounding out to the tray on the dyno! Once we insulated that wire, the engine ran like a champ!
The point of all this is that only once was there a problem with the MegaSquirt and that was quickly rectified. The rest of our problems were self-inflicted, yet we still kept thinking that the source of our frustration was the MegaSquirt computer. The lesson here was that we tend to blame the stuff that we don't understand, when usually it's some dumb mistake that we've made that has nothing to do with the black box. This is a lesson that was hard-earned and worth remembering.
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.
Parts List
The following is a list of the parts we used to build the MegaSquirt system and run it on the engine. It's a little confusing, because you purchase Printed Circuit Boards (PCBs) from MegaSquirt, but the rest of the electronic parts are from a separate company called Digi-log. It's all on the MegaSquirt Web site. Prices for all MSD and SLP products were taken from www.summitracing.com.
Component; Manufacturer; PN; Price
ECM board; MegaSquirt; MSPKIT; $120.00
Digi-key parts; Digi-log; -; 51.40
Relay board kit; MegaSquirt; RELAYPCB; 15.00
Relay board parts; Digi-log; -; 38.91
Stimulator box; MegaSquirt; STIMPCB; 20.00
Stimulator parts; Digi-log; -; 21.92
Injectors, 36-lb/hr; MSD; 2018; 421.00
Injector harness; MSD; 2400; 15.00
Coolant sensor; MSD; 2310; 12.00
Air inlet sensor; MSD; 2320; 17.00
Oxygen sensor, heated three-wire; MSD; 2330; 73.00
Bung for oxygen sensor; MSD; 2335; 19.00
MSD 6A box; MSD; 6200; 150.00
Used TPI manifold; pre-owned; -; 200.00
Misc. wiring and parts; -; -; 50.00
Performance-tuned runners; SLP; 20002; 260.00
Gasket set; SLP; 20030; 20.00
Total: $1,504.23
If you cleaned a set of used injectors, reused original sensors, and passed on the MSD box and SLP runners, you could get this price down to around $600, and that's assuming you're starting with a clean slate.
Dyno Results
A major portion of the evaluation of the MegaSquirt was to assemble the EFI computer and then test it on the dyno. We decided to run the engine first with a carburetor, which is listed as Test 1. The engine was a 355ci small-block Chevy with 9.5:1 compression, forged Childs & Albert pistons, Edelbrock Performer RPM aluminum heads, and a Victor Jr. single-plane intake with a 750-cfm Barry Grant Speed Demon carburetor.
For the EFI test, we purchased a used TPI manifold and plumbed in a new set of MSD 38-lb/hr injectors and SLP runners to ensure sufficient fuel and airflow for this small-block. Since the TPI manifold was designed as a torque piece, the test was not about making horsepower but instead to show that the MegaSquirt could perform as intended. Had we outfitted the Victor Jr. with injector bungs, we feel confident that the power numbers would have been very similar. Overall, the test was a complete success.