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Are AFR affected by the octane ratings?

Understanding AFR

Discussion and questions related to the course Understand AFR


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good day to all HPA members and to us soon to be a good tuners, question i have is... if i put in a pump gasoline with an octane rating of 100 will i still follow the gasoline stoich reading? which is 14.7? i'm confused with e85 which has an octane rating lets say 105-110 but the stoich setting on e85 is 9.8 or something... should i stick to the term gasoline is gasoline no matter what octane rating it has... if it is pump gas it's stoich is 14.7... or there is a corresponding stoichiometric value if the gas to contains a higher octane rating?... hope you could help me out... thanks all and god bless!

G'day Julius.

No, the stoichiometric ratio of gasoline does not change with the octane rating. Higher octane gasolines (petrols) are more finely refined to only contain the longer hydrocarbon chains, or have additives to bring the octane number up, but they're ideal combustion ratio is still 14.7:1. At the absolute limit, there probably is a slight variation in the stoich ratio, but it would be insignificant, and not within the measurement range of a typical wideband o2 sensor.

Once you start moving to other fuel types, ethanol, or an ethanol / gasoline mix, is when your stoich ratio changes, as the new fuel has a different molecular structure, different numbers of hydrogen and carbon atoms per molecule, so require a different number of oxygen atoms to perfectly combust with.

Also, the temp 'pump gas' is a bit tricky these days, as there are plenty of places you can get ethanol blend fuels at the pump, so be careful with that one!

Hope that helps :-).

It really helped a lot... Thanks zac... Thought that higher octane would change or affect the stoich value of pump gas. Wew... This answer removed a fishbone in my neck haha... So higher octane means i can adjust bit advance my ignition timing to 2-3 degrees. Thanks for the fast reply zac.

If you are at 0% ethanol and go from 95 RON to 98 RON for example the stoichiometric AFR doesn't change. If you go from 95 RON E0 to E85 (which has much higher RON incidentally), the stoichiometric AFR does change.

Same goes to 95 octane then 100 octane pump gas right?... As long as it has 0 ethanol or other gas mixtures... Right?... Thanks again... Will be tuning for 14.7 stoich...

Yes, as long as its still 100% gasoline (petrol). You do need to be careful, as some pump fuels are an ethanol blend. I'm not familiar with what is available in the Phillipines though, but it should say on the pump if the fuel is an ethanol blend.