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Map based ecu VS altitude change

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  • Map based ecu VS altitude change

    I recently switched to a d-jetro (map based PFC) and I'm having trouble with my tune changing significantly with elevation.

    I live at 7000 ft elevation and dd my car. On weekends I dd my car at 1400 ft elevation and this is also where I race when I get the chance.

    I have my car set-up to run pretty good at low elevation for now, it idles a little lean and light throttle cruises a little lean, but is spot on under load. As I go up in elevation I end up richening the k value to keep my AFR's safe under load, but this makes my low load AFR's rich.

    I don't mind changing the k value with altitude, but constantly changing the map (like I was doing the first couple trips until I realized what was happening) is a pain, and IMHO unsafe to rely on. Is there anything I can do (with or without a datalogit) to make the transition more linear? Aside from having 2 seperate maps saved and switching via datalogit (which I don't have yet.)

  • #2
    What ECU are you using? Can you get a sensor in there for barometric pressure? The AEM EMS claims to have one built in.

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    • #3
      He's using a PowerFC - don't think there's a baro sensor built into it. Maybe with datalogit you could do something?

      It's a pretty simple change, and I had calculated it out for a friend's ECU once from -1000 to +10000 elevation. Actual correction on a turbo car will need some fiddling since your turbine flow is sonic, and your intake flow might be sonic.
      '18 Chevrolet Volt - Electric fun hatch for DD duty!


      DefSport Koni Sleeve and Spring Perch Buy!!!
      http://www.nissanroadracing.com/showthread.php?t=5902

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      • #4
        I had the same ecu and it never worked right with only a 2k change in altitude. The easiest fix might be to change back to maf.

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        • #5
          I thought that might be the case, but I just switched from bikirom so I'll take running seperate tunes for high and low elevation over going back to that... Maybe I'll just have to dish out for datalogit...

          I actually thought about having an altimiter (I guess a barometer would be better, just hadn't thought about that...) and a low/high elevation tune, then just interpolate between the two at what ever percentage to match the alti/baro. I didn't know AEM was already doing that, I thought I was onto something...

          Well, while I'm on the topic, I'm under the impression that it's ok to run a little lean (15-16:1, maybe even 17:1) at low load, light throttle cruising. This will hurt throttle response, but won't run EGT's dangerously high. Is this correct, or is it unsafe?

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          • #6
            You won't hurt anything, but you do want light load AFRs to be about 15.0-15.5:1 for best fuel economy. 17:1 is going to make the car feel like a dog(torque drops off from 13:1 and up give or take), and your gas mileage will drop(more fuel to get the same torque due to the inefficiency and higher percentage of partial misfires).

            Under load you really don't want to go over 12.0:1 unless you're running really low boost like 8-10 psi.
            '18 Chevrolet Volt - Electric fun hatch for DD duty!


            DefSport Koni Sleeve and Spring Perch Buy!!!
            http://www.nissanroadracing.com/showthread.php?t=5902

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            • #7
              Originally posted by Def View Post
              You won't hurt anything, but you do want light load AFRs to be about 15.0-15.5:1 for best fuel economy. 17:1 is going to make the car feel like a dog(torque drops off from 13:1 and up give or take), and your gas mileage will drop(more fuel to get the same torque due to the inefficiency and higher percentage of partial misfires).

              Under load you really don't want to go over 12.0:1 unless you're running really low boost like 8-10 psi.
              I knew it would decrease response, but I was thinking the leaner you are the better your fuel economy. That makes a lot of sense though, thanks for clearing that up.

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              • #8
                Nope, BSFC usually peaks right around 15.3ish. For a cat you'll get best emissions running right at stoichiometric.
                '18 Chevrolet Volt - Electric fun hatch for DD duty!


                DefSport Koni Sleeve and Spring Perch Buy!!!
                http://www.nissanroadracing.com/showthread.php?t=5902

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                • #9
                  If I run leaner then about 15.5:1 afr at cruise (very light load) on my KA, the engine would start to surge and misfire a little bit. I was advised to add more ignition timing to eliminate the surge. I dialed in 5 degs more lead and was able to run 16-17 afr with no surge and no noticable misfires. The flame front travels much slower as you lean out the fuel mixture past a certain point, so you need to advance the timing to get the peak pressure point near the right spot.

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