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Thread: Airmass different for same MAP and RPM

  1. #1

    Airmass different for same MAP and RPM

    Hi all,
    I'm just trying to understand why at a specific MAP and RPM there could be a different air mass. My assumption is that the ECT and IAT is about the same.

    Here's a graphical representation of what I'm trying to understand.

    Max Values
    Screenshot 2014-06-13 15.39.44.png

    Min Values
    Screenshot 2014-06-13 15.42.47.png

    For some reason I thought the values for airmass would have been closer because the ECT only varies about 2-3C and the IAT only varies max 8-10C.

  2. #2
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    'Histograms' are aggregative by nature. This makes them useful, but also can be very misleading. The two pics you posted are of max and min values. Of course, every cell is going to have some variance to it (where's my variance button, HPT?) you're looking at a cell at 1200rpm and 30kPa, that can be barely off idle, that can be coming to a stop (throttle shut), it can be a lot of things, thus there's gonna be a lot of variance with wild swings. If you want to know why, you need context. Because 'histograms' aggregate, they hide extra info. You don't get to see things like what IAT it was, or what was the TPS just before this cell. For 'data in context' you want the in-time charts, populated with all the pertinent data. That gives you context, does not hide data, and with proper grouping and scaling, it can give you pretty good clue about which entities are correlated.

  3. #3
    Quote Originally Posted by redhardsupra View Post
    'Histograms' are aggregative by nature. This makes them useful, but also can be very misleading. The two pics you posted are of max and min values. Of course, every cell is going to have some variance to it (where's my variance button, HPT?) you're looking at a cell at 1200rpm and 30kPa, that can be barely off idle, that can be coming to a stop (throttle shut), it can be a lot of things, thus there's gonna be a lot of variance with wild swings. If you want to know why, you need context. Because 'histograms' aggregate, they hide extra info. You don't get to see things like what IAT it was, or what was the TPS just before this cell. For 'data in context' you want the in-time charts, populated with all the pertinent data. That gives you context, does not hide data, and with proper grouping and scaling, it can give you pretty good clue about which entities are correlated.
    Thanks for the explanation. I guess I assumed that since we tune speed density lambda against MAP and RPM that the calculated air mass for each cell would have been a lot closer from the minimum to maximum values.

    I was just scanning this with everything enabled (MAF, SD, dynamic) and then filtered a 10C window for IAT and ECT and filtered transient fueling.

    Do you think if I isolated the SD or MAF calculated cylinder mass that the variance between high and low values in each cell would be smaller?

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    Yes, but it wouldn't be very descriptive of what's going on. You want to look at both histograms and time charts simultaneously.

    Remember that air has its own weight and thus momentum, so just the fact that you slam the TB shut, does not mean you're going to have minimal airflow instantenously. For a moment, you will have some airflow, between max and min, with rpm high, tps 0%, and rapidly falling MAP. depending on scanning frequency you can see the transition, or you might miss it.

    Don't just go filtering things until you see what you think you should see. The numbers should be descriptive, they should give you insight to how things work. They're not there to buff your ego by being forced into reaffirming your expectations. That's just bad science.

  5. #5
    Senior Tuner mbray01's Avatar
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    The theory of how the histogram works is that you want to hit the cells multiple times. Let the program work over an extended period of time I usually run the histogram for about 10 minutes for each tuning session. You use average, and this is going to give you the best representation of your actual setting for that cell. Just like the previous poster said, many thing affect it, is there left over fuel from the previous cell, as it moved. Slow deliberate throttle movements will yeild the best map of where the vehicle is. It's been said on this forum many times, get as much information as you possibly can. On my personal vehicles I will make logs on the ride to and from work(about 1-1/2 hours each way. For customer cars logs should be atleast 10 minutes that way you are hitting cells atleast 20-25 times each
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  6. #6
    Quote Originally Posted by redhardsupra View Post
    Don't just go filtering things until you see what you think you should see. The numbers should be descriptive, they should give you insight to how things work. They're not there to buff your ego by being forced into reaffirming your expectations. That's just bad science.
    Thanks for the dialog! It's very interesting.

    I only intended to use filtering to refine the data to better simulate load cell dyno testing from data gathered driving on the street.

    I though the scanning sample rate on the E67 is 40 cycles per second (2.5ms between). I have a hard time believing that you would miss changes that occur in MAP with that sampling rate, no?

  7. #7
    Quote Originally Posted by mbray01 View Post
    The theory of how the histogram works is that you want to hit the cells multiple times. Let the program work over an extended period of time I usually run the histogram for about 10 minutes for each tuning session. You use average, and this is going to give you the best representation of your actual setting for that cell. Just like the previous poster said, many thing affect it, is there left over fuel from the previous cell, as it moved. Slow deliberate throttle movements will yeild the best map of where the vehicle is. It's been said on this forum many times, get as much information as you possibly can. On my personal vehicles I will make logs on the ride to and from work(about 1-1/2 hours each way. For customer cars logs should be atleast 10 minutes that way you are hitting cells atleast 20-25 times each
    I get that you need to get multiple cell hits to get usable data but I'm really getting the impression that you need to be very specific using filters on your histogram to best emulate stead state lambda error.

    Judging from my initial scans it would seem that loaded dyno testing is far superior to street tuning.

  8. #8
    Senior Tuner mbray01's Avatar
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    definitely load bearing dyno is the way to go, but not everybody has access to one.


    With the multiple cell hits, is going to do the filtering for you, sure there are going to be some really lean percentages, as well as really rich percentages. The more hits you get, the more it is going to essentially filter out the highest and lowest.

    The sample rate of the pcm is high sure, but there is a lot of time between when the air enters the throttle body, goes through all 4 cylinder cycles, exits the exaust valve and goes through the exhaust and reaches the wideband, or the o2's. Which ever you are using. there is where a lot of discrepancies happen when hitting different map/rpm zones. this is why it is absolutely imperative to keep as slow of throttle as possible.

    Like those histo images you posted to have gotten those cells posted like that, you would have had to snap the throttle, which is why you have the discrepancies. Slow pedal movements=less variances in the histo. Gradual pedal movements will yield better results than using filters to screen out the unwanted hits
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  9. #9
    Thanks all! I guess I'll spend some more time driving around and analyzing the data.

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    Quote Originally Posted by mbray01 View Post
    definitely load bearing dyno is the way to go, but not everybody has access to one.


    With the multiple cell hits, is going to do the filtering for you, sure there are going to be some really lean percentages, as well as really rich percentages. The more hits you get, the more it is going to essentially filter out the highest and lowest.

    The sample rate of the pcm is high sure, but there is a lot of time between when the air enters the throttle body, goes through all 4 cylinder cycles, exits the exaust valve and goes through the exhaust and reaches the wideband, or the o2's. Which ever you are using. there is where a lot of discrepancies happen when hitting different map/rpm zones. this is why it is absolutely imperative to keep as slow of throttle as possible.

    Like those histo images you posted to have gotten those cells posted like that, you would have had to snap the throttle, which is why you have the discrepancies. Slow pedal movements=less variances in the histo. Gradual pedal movements will yield better results than using filters to screen out the unwanted hits
    Use filters to ignore data during rapid TPS movement?
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  11. #11
    Senior Tuner mbray01's Avatar
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    The idea was to inform the op that using filters is more of an advanced idea. Better to utilize the histo as they set now, and learn how the histograms work, then start dabbling with the filters and settings. When you are first starting out you don't want to make a bunch of changes and give yourself a false sense of obtaining only good information, and believing that all the information is correct, because I set the filters to weed out the bad hits.
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    Here's one aspect about 'histograms' I've never heard of talked about on forums: If your fuel trims lets say are -30 -30 -30 -30 30 30 30 30, what is the avg value, the one that everyone tells you to use? 0. Perfect trims right? Then why does the car drive like poo? Cuz you relied on a tool without understanding it fully.

    I've had a statistics teacher that would take points away from you if you did not draw out a distribution before doing the math. It was harsh, but it forced you to look at data before you start mangling them.

    This brings us to even a bigger point: tuning suites are TOOLS, not SOLUTIONS. They make your job easier, they do not do the job for you.

  13. #13
    Senior Tuner mbray01's Avatar
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    Thats all I am trying to say, that you shouldn't be changing the tool, if you don't truly understand how the tool works, or why the tool does things a certain way.
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    Quote Originally Posted by redhardsupra View Post
    Here's one aspect about 'histograms' I've never heard of talked about on forums: If your fuel trims lets say are -30 -30 -30 -30 30 30 30 30, what is the avg value, the one that everyone tells you to use? 0. Perfect trims right? Then why does the car drive like poo? Cuz you relied on a tool without understanding it fully.
    Average trims is all these people know. They've been told that variations in the trims are 'noise' and that these variations are to be expected... it's not considered an issue and that's why it's never talked about on forums.