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Fluctuations in AFR (Air Temp Related)


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Hi all.

So Ive spent a lot of time refining my map (all road tuning) and making the AFRS nice and precise.  I never have enough time to do the entire map in one shot. I always do bits and pieces - Some cruising, some low down, some foot flat, some high RPM etc.  But I break it down so I can make sure things are perfect in those particular areas until I move on.

Ive been using the Modelled Fueling equation and I find with my IAT varying by around 10-15 degrees I see as much as 1 full AFR off.  In the early morning I will dial in a particular part of the map for lets say 14.5 AFR (IAT's around 30degC).  That afternoon when things have heated up (IAT's around 40degC) the areas that I tuned in the morning around almost 1AFR leaner - so around 15.5 now.  If i correct the fueling when the IAT's are around 40degC - to 14.5AFR - the next morning Im at around 13.5AFR. I do find that certain areas are more heavily affected than others obviously.   

SO last night I watched the GREAT webinar on the Charge Temp Corrections done by HPAcademy.  Long story short there is no way I will dial in my table as accurately as Andre.  I dont have access to a dyno and to simulate the different conditions is difficult on a Turbo Charged Vehicle.

During all of these tests there is no other Compensations taking place (With regards to warm up / coolant etc).  The only hidden ones would be the actual corrections taking place in the background to deliver the right amount of fuel as per the Modelled Fueling equation.

So my question is - how much extra fuel is added /subtracted for every 10degC change in IAT/Charge Temps?  I know Andre says that for every 10degC below 20degC one adds 2.5% of fuel, and we take away 2.5% for every 10degC above 20DegC.  Im loosing/gaining around 7% of fuel for the give or take 10-15degC changes.

Will things pan out any better with the traditional fueling?  Can I not just zero out the Charge temp comps and use the IAT as 'normal' even though im in the modelled fueling? OR do i add a decent bit of Charge correction and try and dial in the charge temp correction bit by bit?  But it just seems like im getting drastic fluctuations! :(

Difficult for me to chose the right path!

Hope someone can advise?




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Hi- Im not quiet as far into my tuning as you but the way you are working through your tuning sounds very similar to what im doing . Im only posting a reply because i have the same thing happening to me . I just haven't logged enough to to accurately state how much it is affecting my afrs . But it certainly is . So im hoping someone can answer you . Cheers - Ken  

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Hi Ken.  I did a lot more fiddling with the Charge Temp Compensation table....seems like I have found a happy place more or less.  It has sorted out the low down manifold pressure/minimal throttle fueling to within 5% or so.  This is more than good enough considering that I will be running with Closed Loop Fueling once the tune is done.

Its not easy to get right - but you have to find a happy place in between the numbers in your fuel map, and the charge temp comps.


I do still have some areas that need more refining - but just takes time.


EDIT: I do know that on OEM subarus that I have tuned in the past - running them in pure open loop will always keep the AFR a decent way out based on pure air temp compensations.  One has to run in closed loop to keep the fueling within a few percent - even with changes in the air temp.  




Edited by Jareds
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