Part 5 – Tuning
Tuning was more difficult than I expected it to be in some ways, while easier in others. It was more difficult because this is when any mistakes you made along the way will start to show, or any problems with the hardware will show up. It was easier because the actual tuning (once you’ve got the glitches worked out) went smoother than I expected. I ran into 2 minor glitches that ended up taking a lot of time to figure out. The 1st was that my aftermarket performance ignition coil was drawing the voltage down too low at the coil. It still produced a spark, but the computer had problems detecting RPM properly. This was fixed by removing the ballast resistor (not needed anymore anyway). The 2nd was that I used improper tubing for my vacuum line. This tubing worked fine while cold, but once warm it got soft and “sucked together” like a straw in a thick milkshake. Switching to proper automotive grade vacuum line fixed this.

Now, on to the actual tuning!
To tune the Megasquirt ECU we use a free program called “MegaTune”. When you 1st start megatune, you are brought into the main dashboard view. This view is a great overall view of what’s going on with the engine.

The 1st step in tuning the megasquirt ECU is to tell the ECU what kind of engine you have and what type of fuel injection equipment is being used. Pictured below is the main settings window for fuel delivery. The key setting in Megasquirt is the Req_Fuel setting. This setting is a base number in which the computer makes all of it’s adjustments around. Req_Fuel is the amount of fuel needed for 1 piston at 100% volumetric efficiency. The megatune software comes with a built in calculator that lets you calculate the proper value for your engine. The other settings tell the ECU about your engine and the fuel injection equipment used.

• O2 sensor is allowed to alter amount of fuel injected by a maximum of 15%
• O2 sensor is ignored until engine is above 160*F
• O2 sensor is ignored when RPM is less than 1300
• O2 sensor is ignored when throttle position sensor is above 85%
• O2 sensor is ignored when vacuum is less than 85 Kpa.

Once you tell the computer about your engine and set some basic settings, the next step is to get the car idling. Generally this is done by playing with the warmup wizard until you can get the engine started.

From there it’s a matter is adjusting your VE table. There is 2 ways to do this, by looking at a table view or a chart view. Pictured below is the chart view. You can see the 3D graph of the volumetric efficiency table. The axis’s are as follows: RPM is left to right, manifold vacuum is front to back, and volumetric efficiency is up and down. The green lines that connect to the green dot is where the engine is currently running and thus where the computer is looking on the chart to figure out how much fuel to inject. The red lines with the red X is where my tuning cursor was sitting. To tune the VE table, you use the gages to the left (mostly the O2 sensor) to decide if enough fuel is getting injected. To change the amount of fuel you put your tuning cursor (red X) to wherever the green dot is sitting and then use the arrows to increase/decrease the volumetric efficiency.

Here’s the VE table in the more standard view.

Having a Wideband O2 sensor allows me to use the Megasquirt to set target Air/Fuel ratio’s. What this lets me do is program the engine to run lean when at cruise for best fuel economy, but then run rich when racing for best power. Pictured below is my target air/fuel ratio table.

Once the VE table and target AFR table are set, then it’s time to move on to the acceleration wizard. The acceleration wizard is used to inject extra fuel under acceleration. Much like an accelerator pump in a carburetor. The Meqasquirt ECU allows you to base acceleration enrichment on throttle position, manifold vacuum, or a percentage of both.

Tuning took me about 2 days once I had the other bugs worked out. I should mention that I’ve only shown less than ½ of the various tuning settings and features used in the Megatune software.

The next part is to have the Megasquirt ECU do all of the ignition control. More tuning will be discussed then.