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  1. #1
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    Can Cobb AP monitor engine vacuum?

    Can the Cobb AP read actual vacuum in the intake manifold. Any sensor in there, or do I need to hook up a vacuum gauge like how the P3 is connected? I'm trying to chase down why my STFT values are suddenly maxing out at 34 and somewhere I read a vacuum leak could cause this. Click here to enlarge
    @Cobb ?
    2008 E90 335xi AT 58K MILES - FAILING XFER CASE MOUNT
    MHD CUSTOM E50, N20 TMAP, FUEL-IT TBI & STG 2 LPFP, cPE CHARGE PIPE, HELIX IC, FORGE DV, BMS DCI, BMS T-STAT DELETE, BMS OCC, RB PCV, KW-V1 COILOVERS, APEX ARC-8, 235/265 MICHELIN PSS, ALPINA B3 TCU, SCOOPSClick here to enlarge

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    Click here to enlarge Originally Posted by Jeffman Click here to enlarge
    Can the Cobb AP read actual vacuum in the intake manifold. Any sensor in there, or do I need to hook up a vacuum gauge like how the P3 is connected? I'm trying to chase down why my STFT values are suddenly maxing out at 34 and somewhere I read a vacuum leak could cause this. Click here to enlarge
    @Cobb ?
    The AP reads vaccum off the TMap in the charge pipe. If you want vacuum from the manifold just buy a cheapo vacuum gauge from any auto parts store and run a small hose from the nipple to it. We have an analog boost gauge in the car so we see it all the time. You should be seeing somewhere between 10-21 in hg depending on where the car is ilding, how warmed up it is, etc.

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    I was testing my diverter valves yesterday and I noticed a spike in "Boost Mean Abs" when letting off the throttle, but no spike in "MAP" at the same point in the log. I think "MAP" reads the pressure sensor in the manifold itself, and "Boost Mean Abs" reads the pressure sensor in the throttle body. MAP reports pressure in millibars.
    Eppur si muove.

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    Click here to enlarge Originally Posted by ajm8127 Click here to enlarge
    I was testing my diverter valves yesterday and I noticed a spike in "Boost Mean Abs" when letting off the throttle, but no spike in "MAP" at the same point in the log. I think "MAP" reads the pressure sensor in the manifold itself, and "Boost Mean Abs" reads the pressure sensor in the throttle body. MAP reports pressure in millibars.
    It doesn't. You can unplug the map sensor in the manifold and the cobb will continue reading the same. I know all this because I had a very weird vacuum condition recently. Analog boost / vacuum gauge directly off the manifold reading normal 10-20 in hg. Cobb ap reading 0 vacuum or maybe .1-.3 inches. Notice the point there, less than 1 inhg. Well I figured out the issue and it was after I swapped the map senor on the manifold thinking it was bad etc. Has to do with these Billet GTX compressor wheels. But that will be posted in another post when I get the chance, I was 100% it was getting the signal from the manifold map too until I unplugged it and no change, I hooked a gauge to the charge pipe and it was showing the same as the AP, while my manifold had the normal 20 inhg of vacuum, this told me the ap does in fact read from the charge pipe.

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    Click here to enlarge Originally Posted by VargasTurboTech Click here to enlarge
    It doesn't. You can unplug the map sensor in the manifold and the cobb will continue reading the same. I know all this because I had a very weird vacuum condition recently. Analog boost / vacuum gauge directly off the manifold reading normal 10-20 in hg. Cobb ap reading 0 vacuum or maybe .1-.3 inches. Notice the point there, less than 1 inhg. Well I figured out the issue and it was after I swapped the map senor on the manifold thinking it was bad etc. Has to do with these Billet GTX compressor wheels. But that will be posted in another post when I get the chance, I was 100% it was getting the signal from the manifold map too until I unplugged it and no change, I hooked a gauge to the charge pipe and it was showing the same as the AP, while my manifold had the normal 20 inhg of vacuum, this told me the ap does in fact read from the charge pipe.
    Can you recall which parameter were you looking at on the AP?
    Eppur si muove.

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    Click here to enlarge Originally Posted by ajm8127 Click here to enlarge
    Can you recall which parameter were you looking at on the AP?
    Go to live data, go up 2 clicks, second from the bottom, its just titled boost.

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    Interesting. In the logs I have, there is a clear correlation between MAP and Boost Mean Abs only when the throttle is open. When the throttle closes, Boost Mean Abs goes basically to barometric pressure, while MAP goes deep into vacuum (22 inHg).

    Also in my logs, the parameter "Boost" is simply the parameter MAP, converted to PSIA minus the barometric pressure. Boost and MAP track each other perfectly.

    I wonder if this is just a difference between firmware versions. I also am using the AP V2.

    Checkout the attached spreadsheet.
    datalog15.zip

    Here's a Cliff's Notes graph:
    Click here to enlarge
    Eppur si muove.

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    The channels I've used are req boost, mean absolute, and boost... the first 2 are CP sensor, boost is in the manifold -baro as ajm said. Haven't used MAP.

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    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    The channels I've used are req boost, mean absolute, and boost... the first 2 are CP sensor, boost is in the manifold -baro as ajm said. Haven't used MAP.
    Boost isn't the manifold. Go ahead and unplug while looking at that channel. It will still keep reading, I have an analog gauge hooked up to the manifold, right below where that sensor is, it reads perfectly around 20 inhg. Boost is reading zero, it used to read -9-10 psi which converts to right around 20 inhg. I thought the same as you guys, thought I had a bad map, swapped it for a brand new one. No change, put a barb in my charge pipe, read the same boost. Tell you what. I will put the barb back in and put my vacuum pump no it, and watch the ap. This should tell us 100%, even thought I think I already know what my issue is.

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    Click here to enlarge Originally Posted by VargasTurboTech Click here to enlarge
    it used to read -9-10 psi
    What do you mean? What changed? Did you update the AP?
    Eppur si muove.

  11. #11
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    I think AJM is close to the answer. I took another look at the Cobb. There is a MAP output (manifold air pressure ?) which reads a low of about 400 (units? Maybe millibars? Maybe mm Hg ?) idling to about 1000 with low revving at about 2000 rpm easy driving, to more than about 2000 WOT. Sudden off accelerator I can hear the diverter valves open and the MAP value goes down to a low of about 220. If in fact the units here are millibars or mm Hg absolute pressure, (14.7 psi = 1013 millibars = 760 mm Hg) then it makes sense that MAP is in fact the absolute pressure in the air manifold.

    And by the way, there is a sensor on the manifold near cylinder 2 or 3 which to me looks like the TMAP on the charge pipe. Could it in fact be the MAP sensor?
    2008 E90 335xi AT 58K MILES - FAILING XFER CASE MOUNT
    MHD CUSTOM E50, N20 TMAP, FUEL-IT TBI & STG 2 LPFP, cPE CHARGE PIPE, HELIX IC, FORGE DV, BMS DCI, BMS T-STAT DELETE, BMS OCC, RB PCV, KW-V1 COILOVERS, APEX ARC-8, 235/265 MICHELIN PSS, ALPINA B3 TCU, SCOOPSClick here to enlarge

  12. #12
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    I'm pretty sure it's millibars.

    The sensor is part #8 :
    http://www.realoem.com/bmw/showparts...73&hg=11&fg=40

    It is the same sensor as in the charge pipe as far as I know.
    Eppur si muove.

  13. #13
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    I will tell you, I still have my weird vacuum readings, boost which used to read -10psi at full vacuum still only reads .1-.5. I can unplug the sensor from the manifold and it still reads perfectly fine, goes up and down with vacuum fluctuations but not nearly as high as it should be, but my analog gauge plugged into the manifold is reading perfect. I am at a loss now, replaced the sensor, didn't fix it. Tried another AP, didnt fix it. reflashed the car to stock, didn't fix it. Its annoying because it throws the cars boost reading off, it looks like its high in the logs too.

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    When monitoring boost on my V3 AP it reads vacuum. When cruising at 0 throttle I am around -11. When at idle I see -8.
    Click here to enlarge
    PTF Tuned Cobb Stage 2+ | H&R / Koni Sport | BBS LM

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    Yeah, that was my point. That is what it is supposed to read. Its what Its always read. Then it decided to do what it wants.

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    Click here to enlarge Originally Posted by VargasTurboTech Click here to enlarge
    Yeah, that was my point. That is what it is supposed to read. Its what Its always read. Then it decided to do what it wants.
    Did it start acting up with the new shop car, or after Wedge helped you reflash the DME? Maybe @Josh@Cobb could offer some insight.
    Eppur si muove.

  17. #17
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    Click here to enlarge Originally Posted by ajm8127 Click here to enlarge
    Did it start acting up with the new shop car, or after Wedge helped you reflash the DME? Maybe @Josh@Cobb could offer some insight.
    No, it worked fine after the reflash. I thought it might have been the flash too, but it worked fine after, I cannot even pinpoint exactly what I had done to the car when it first started. Thats how many stupid problems this car has had. Chalk it up to one more, but this wont keep me from tuning

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    Actually let me clarify, the manifold MAP only has like 1.5bar range I think. So the "boost" channel is a combo of the 2 sensors.

  19. #19
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    I made a video of what our ap is doing. I will post it up. It very well maybe the beta firmware, not sure.

  20. #20
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    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    Actually let me clarify, the manifold MAP only has like 1.5bar range I think. So the "boost" channel is a combo of the 2 sensors.
    Are you sure about that? I've got data showing the monitor "MAP" reading all the way up to 31 psia which is 2.1 bar. I thought the N54 sensors were 2.5 bar.
    Eppur si muove.

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    The MAP in the charge pipe is 2.5 bar I beleive, but the transducer in the manifold is much narrower, something like 1 or 1.5 bar.

  22. #22
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    I see. It would surely be useful if someone from Cobb could tell us what these monitor mean instead of use trying to infer their meaning based on logs.
    Eppur si muove.

  23. #23
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    So you are saying its a combination reading, from the cp and the manifold? That would explain why it continues reading even with the manifold unplugged. You will see what mine is doing when I get a chance to post the video.

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    Here is the video. As you can see the cobb shows no vacuum. And as I tip into boost it actually shows positive boost as the gauge reaches 0, once we actually get into boost the cobb starts coming back down and corrects itself to line up with the gauge, so in boost its accurate, in vacuum its about 3 psi off and will not show vacuum.


  25. #25
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    Click here to enlarge Originally Posted by VargasTurboTech Click here to enlarge
    Here is the video. As you can see the cobb shows no vacuum. And as I tip into boost it actually shows positive boost as the gauge reaches 0, once we actually get into boost the cobb starts coming back down and corrects itself to line up with the gauge, so in boost its accurate, in vacuum its about 3 psi off and will not show vacuum.

    http://youtu.be/PfurVrNuJPQ
    That is strange, man. When displaying "Boost" my AP will show all the way down to -11 or so psi, then all the way up to 16 or so which is about where I'm targeting now.

    What happens when you select "MAP" instead of "Boost?
    Eppur si muove.

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