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    DME Fuel calculations (Flash Tunes)

    So how does the DME perform its fuel calculations? I know that the DME fueling is closed loop and pretty much a hands off process in terms of tell it what AFR (Cobb ATR) to target and sit back enjoy the show. Does the DME adjust to reach AFR targets for say, charge air temps, show as a deviation in fuel trims (more fuel needed for denser air) or are these calculations made by the DME itself and the short term fuel trims just show the deviation from its own algorythms?

    Lets take an E85/93 Octane mix for example. If we are running 30% ethanol and short term fuel trims are reaching 25% under high load, what will happen to these fuel trims once the ambient temperatures start dropping?

    Same could be said for larger/more efficient turbo(s) and the DME's calculations for fueling as well.

    Thoughts @dzenno @jhershorin @Terry@BMS

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    Fuel volume is IPW and pressure. The mix is calculated based on load which is MAP, temp, rpm. Closed loop is based on wideband feedback and trims are % of fuel added/subtracted to meet AFR.

    Colder weather, trims should stay fairly similar.

    RBs flow more so you would need more fuel via CL unless you can increase load per MAP OR increase VE (same thing). I assume there is a VE table.

    With E85 you would need to change fuel volume per load. I know others have access to the IPW tables, so likely Cobb does also.

    Of course there are different ways to make these calculations and this would be the specific relationship/calcs between tables… maybe the experts can add something.

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    So fuel trims represent the deviation of fuel required to meet target AFR's, correct? From what i understand the DME will change IPW and pressure as needed to meet target AFR's, but im not positive on this.

    Outside of general interest, one of my concerns is that as temperatures drop, how much will this affect fuel trims. I might be getting tuned on 50:50 E85/93 & Meth which should put me just under the max fuel trims window. Does anyone know if fuel trim per load is typically linear? From what i have seen in my logs, my observations say no. From what i recall, the higher the load, the higher the fuel trims.

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    Yes fuel per load will basically be linear. You will need about 15% more fuel with 50/50. Need to log many different driving situations and review them thoroughly. Make sure trims aren't too high... If there's a couple brief instances of maxing or close to it during transitions i would'nt worry about it.

    Fuel pressure is pretty steady. Volume changes by ipw primarily.

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    Closed loop targeting via fuel trims working on top of an open loop table. The open loop/IPW tables are what we really need access to for E85 fueling with flash tunes.
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    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    Fuel volume is IPW and pressure. The mix is calculated based on load which is MAP, temp, rpm. Closed loop is based on wideband feedback and trims are % of fuel added/subtracted to meet AFR.

    Colder weather, trims should stay fairly similar.

    RBs flow more so you would need more fuel via CL unless you can increase load per MAP OR increase VE (same thing). I assume there is a VE table.

    With E85 you would need to change fuel volume per load. I know others have access to the IPW tables, so likely Cobb does also.

    Of course there are different ways to make these calculations and this would be the specific relationship/calcs between tables… maybe the experts can add something.
    Great explanation, repped. Only thing to add is that mounting larger turbos on the car will cause fuel trims to rise as this car uses a MAP sensor instead of MAF. As it doesn't "measure" MAF, it calculates it based on stock turbo air mass and uses that along with the IAT sensor values to calculate MAF. Luckily the car is 100% closed loop so any/all the changes are automatically dealt with by the DME through IPW, fuel pressure and trims like @JoshBoody mentioned.

    One thing that should change the tuning picture with respect to fueling significantly for the N54 is upcoming alternative fuel R&D that we're working with Cobb on. Stay tuned Click here to enlarge
    Click here to enlarge

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