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Thread: Boost Reading when Engine Braking

  1. #1
    Join Date
    Aug 2008
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    Boost Reading when Engine Braking

    Hi,

    I've not been out in my Roadster for a while so remembering what it was like was a joy the other day when I took it out. However, I did notice the boost pressure would go quite low when changing down the gears using engine braking - around -.75 sometimes a little less. I can't remember noticing this before so wonder if I just didn't notice it or whether this is a new or about to be problem. All else seems fine, turbo spins up well and goes like a little rocket.

    It's supposedly not been chipped or modded and is a stock non-Brabus one so what boost pressure should I expect during normal (if a little enthusiastic) driving?

    Thanks

    Joe
    2003 Smart Roadster Coupe
    1997 TVR Cerbera 4.2 AJP
    1987 Toyota Hilux Pickup
    1980 VW T25 Camper
    1974 Lotus Elan Plus2 130/5
    1972 TVR Vixen 2500
    1972 Maserati Indy 4.7
    1969 Rover P6 3500
    1968 Jensen Interceptor
    1963 Daimler Dart

  2. #2
    Join Date
    Mar 2012
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    0.75 what?

    Choices being PSIA, PSIG, Bar etc etc.

    I suspect it will be low, you have no exhaust to turn the turbo and the throttle body has its valve closed.

  3. #3
    Join Date
    Nov 2007
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    I assume 0.75 bar vacuum. That equals 22 inches of mercury which is very good.
    Drives a Smart Cdi - 65 to 85 MPG

  4. #4
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    Quote Originally Posted by tolsen View Post
    I assume 0.75 bar vacuum. That equals 22 inches of mercury which is very good.
    Hmm, 0.75 bar absolute would be around 9 PSIA, surely 0.75 vacuum would be denoted by a minus sign.

    It's too late / early for brain work.

  5. #5
    Join Date
    Nov 2007
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    I think a negative vacuum is positive pressure.
    Drives a Smart Cdi - 65 to 85 MPG

  6. #6
    Join Date
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    Quote Originally Posted by tolsen View Post
    I think a negative vacuum is positive pressure.
    I think a negative vacuum is impossible.

  7. #7
    Join Date
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    Quote Originally Posted by Mjolinor View Post
    Hmm, 0.75 bar absolute would be around 9 PSIA, surely 0.75 vacuum would be denoted by a minus sign.

    It's too late / early for brain work.
    The OP did use a minus sign. Either way it's perfectly normal and about the same value shown on my wife's Roadster on the overrun.

  8. #8
    Join Date
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    The OP should also be asking this question on www.theroadster.net where people actually have a clue about Roadsters!
    ==============================
    2004 Roadster-Coupe SB2 Brabus
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    ==============================

  9. #9
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    Quote Originally Posted by mikerj View Post
    The OP did use a minus sign. Either way it's perfectly normal and about the same value shown on my wife's Roadster on the overrun.
    So what pressure is it then?

  10. #10
    Join Date
    Jan 2015
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    Devon
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    -0.75 bar, relative to atmospheric pressure. The gauge scale ranges from -1 to +2 bar.

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