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Stoichiometric AFR and Lambda in MS43

Understanding AFR

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Discussion and questions related to the course Understanding AFR

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I've been using Lambda as my tuning default on a past project, but have just started using MS3 in my newest project. One of the settings asked what the Stochiometric AFR is of the fuel I'm using. I put down 14:1 because all of the pump gasoline near me is 10% ethanol, but I'm also using a Flex Fuel sensor and running primarily E85. Should I put 14.7:1 in the setting for gas and let the flex fuel sensor make the adjustment for the alcohol? I've been wondering why my wideband displays are showing a different reading from what MS 3 is showing, and I think maybe I outclevered myself.

Your last comment may be correct, but I may not be quite following you.

"AFR" , can be rather confusing, as it not only varies with different fuel mixture, but with the correction factor used to convert the lambda which is the base value, to the displayed value. This seems to be a bit of a poser for many, at present, as there have been several posts on this.

You mention two very different fuel blends, that will have very different stoich' AFRs? you also say you are using two different monitoring methods?

Depending how it is set up, the ECU may be monitoring the alcohol content and lambda, as mentioned, the different fuel will have a different AFR for a given lambda value, but it should be calculating the AFR that would give that value of lambda with the fuel it is detecting.

However, as the guage is more likely to be a 'dumb' device that only measures the lambda, it has to be given a calibration value to display the AFR that 'should' give the lambda with the fuel it is told is being used. It cannot read the AFR accurately for any fuel blend that isn't exactly what it is told is being used. Change the fuel and for the same lambda the gauge will read the same, even though the actual AFR is different.

The difference in the way the two AFR displayed values are produced from the data they are given means they will be different except when the 'dumb' gauge correction exactly matches the fuel.

This is why it's a good practice to learn to use lambda values for fuelling as, like in this case, if that was displayed they'd be in agreement.

I'm not quite sure what you mean about the settings, if there is an option, use lambda as that takes the actual AFR headache out of it.

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