Discussion and questions related to the course Understand AFR
could someone please explain the have over want formula in respect to adding and subtracting fuel
i havent practiced this for over 2 year since doing the efi fundamental now i have access to a dyno 24/7 and i am practicing just need a little help thanks .it my be simple but i just need clarification
You simply take the measured air fuel ratio as shown by your wideband. Let's say this is 12.5:1. If however your actual AFR target at this point is 13.0:1 you can calculate a correction factor to get to your target. Divide 12.5 by 13.0 and you get 0.96. Multiply the value in your fuel table at that particular pint by 0.96 and you will find your AFR now reads on target.
A quick point for a couple of specific application, namely reflashing Evos and Subarus, maybe the same for other OEM applictions also. The fuel tables are displayed in AFR. Most tuners ignore the actual values in the table when tuning the cars, i.e. they don't try to make the actual afr equal to the afr in the table (it never corresponds on a stock car anyway). If you are going to tune it this way the formula needs to be flipped, i.e. the correction factor needs to be want afr/have afr as larger numbers in the table equate to leaner mixtures and smaller numbers are richer mixtures.
(This is the quick way of tuning these cars. The right way is to re-scale the MAF so the AFR table matches the actual AFR and then you can just punch in the AFR values you want into the table. When rescaling the MAF, the formula would be have/want, i.e. smaller numbers = leaner, larger = richer).
You're correct HS and this is covered in detail in the Practical Reflashing course. Provided you understand the concept (ie to richen the air fuel ratio you need to ask for a smaller AFR target) you can manipulate the formula to suit.