Tag Archives: Air to Fuel Ratio

Rover Mini MPi Inlet Manifold and Throttle Body

I’m embarking on a project to fuel inject my 1987 Austin Mini, which has already had an engine swap from a 998 to a 1098, but currently runs on an HS4 1.5″ single carburettor.

This obviously offers a reasonable level of tunability, with the ability to change needles, dashpot damper springs, dashpot oil and mixture adjustment and is relatively easy to set up for quite a reasonable state of tune, especially now that I have an AFR meter installed.

However, I would like to try fitting an MPI system and develop my own ECU for electronic fuel injection. Due to the ‘Siamese ports’ on the A-Series engine causing┬ácharge robbing, and only having one fuel injector per two ports, this is apparently something that’s difficult to achieve well, which makes me want to do it all the more.

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What does a Manifold Absolute Pressure (MAP) sensor do?

Manifold Absolute Pressure (MAP)/ Manifold Pressure sensors are used to determine the pressure of the air inside the inlet manifold, just before it enters the engine inlet ports. This is generally measured between the Throttle Body and the intake ports on a naturally aspirated engine, and between the turbocharger compressor outlet (after the Intercooler) on turbocharged or supercharged engines.

GM Manifold Absolute Pressure sensor

GM/Bosch Manifold Absolute Pressure sensor

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How Torque varies with Air to Fuel Ratio

For my Stellaris Launchpad engine simulator, I wished to add a calculation of the Air to Fuel Ratio (AFR) or lambda/equivalence ratio for the current engine conditions. I did this crudely by using the throttle position, engine speed (this->s) and current fuel pulse width (this->F) as factors in calculating the AFR:

this->AFR = (0.32666 * this->s * this->throttle)/(this->F);

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