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by rainbowzootsuit 1119 days ago
Not sure on speeduino, but for Megasquirt, you would start with a best guess fuel map based on engine displacement. These are data that can be shared around.

Then, to tune precisely to the engine, you datalog the oxygen sensor or fuel ratio sensor in closed-loop and process that data to refine the fuel map.

I'm harking back to Megasquirt 1 here so things might be a little different now.
1 comments
lubesGordi 1119 days ago
So does the best guess basically just mean estimating a fairly rich mix at first? Then for tuning are you trying to lean it out to the point you start pinging then back off?
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rainbowzootsuit 1119 days ago
Yes, sorry to be obtuse.

The MegaSquirt measures as its most basic inputs Manifold Absolute Pressure (MAP) and engine speed (RPM) and uses a Volumetric Efficiency (VE) [1] in a lookup table to calculate the fuel injection pulse width in order to combust at a stoichiometric air:fuel ratio (14.7g air:1g fuel) or maybe a bit rich (<14.7:1). A VE <100% is implied by all engines under vacuum because their displacement is constant while the atmosphere in the manifold is at a reduced density. Also good add an inexpensive intake air temperature sensor to adjust air density.

An oxygen sensor in the exhaust stream will "switch" between a lean signal —approximately 0.1 volts—and rich signal—approximately 0.9 volts. This acts as feedback to the engine management to add a correction to the theoretical VE.

More advanced is a "wide band O2" or "lambda" sensor that measures the air/fuel ratio over, as the name implies, a range of air/fuel ratios rather than just a nearly binary output at stoichiometry.

Many engine management systems use a Mass Airflow Sensor (MAF) to directly measure the mass of air taken into to the engine through a single point in the intake air piping and don't need to know much about the details of the VE of the engine.

[1] https://x-engineer.org/calculate-volumetric-efficiency/

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millzlane 1119 days ago
Yes, it's how the pros do it. After a bunch of cars with the same mods you can tune in the base map pretty much withing 20% error. Copy that map to a car with the same mods. Then it's just a pull on the dyno and refine the fuel maps, rinse and repeat until all cells are stoic or rich but not too rich at WOT. That's a very simple explanation. But there's also things like timing, VVLT, Boost controllers, torque maps, and clutch friction points for DCT's that can all be adjusted.
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