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Choosing an AFR

Understanding AFR

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Discussion and questions related to the course Understanding AFR

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In the video I heard something Ive heard before but never got an explanation that satisfied me. and it was that a richer AFR is used to not only achieve a little more power, but also help manage temperature. how exactly does the extra fuel in the mixture cool the combustion. is it extra mass to absorb the heat? can somebody please give me a good explanation, its hard to even find online.

It works on the basis of the Latent heat of vaporization. Petrol normally is able to reduce charge temps by about 20 degrees C with a stoichiometric mixture, when you run a richer mixture you have extra fuel vaporizing, which in turn cools the charge more.

hey Chris250... so when you say petrol or gasoline as able to reduce charge temps by 20 degrees C at stoich, is this implying that stoich is not standard? what temp and mixture are you comparing the stoich mix to when you say it is lowering the charge temps 20 degrees C from? and just to get the visual, does what your saying about the extra fuel vapor mean that it absorbs the heat? i guess because anything richer than stoich, the extra fuel doesnt actually burn right? because fuel needs air (oxygen) to burn and stoich gives just enough fuel to burn all the available oxygen in the system, so any richer and theres not enough oxygen to burn the leftover fuel so its left in the chamber to absorb heat and in turn cool the charge temps? and does anybody know WHY this extra fuel vapor that doesnt burn help to gain more power? i apologize if i come of a bit redundant, and i especially mean no offense if im coming off as skepticle i think this website has been awesome and very informative so far, i just want to ensure that i have a good grasp on whats actually going on in there, please let me know if i am not right or if i am off about anything. thanks for your time and patience

Stoich is the theoretical perfect burn:


My reference to using a stoich mixture of petrol is the 14.7:1 which is covered in the course.

The 20 degrees C drop is from the air itself, pre mix, to the air mixed with the fuel without any combustion. This is the fuel absorbing the heat as it vaporizes, this is an endothermic reaction. If you add more fuel into the mix it absorbs more heat as the fuel vaporizes.

The extra fuel itself doesn't gain more power as covered in of of Andres webinars (a great source of extra info):


It just allows further tuning without heat causing knock limits to hold you back further.

ahhh... awesome! Thank you

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