Results 1 to 5 of 5

Thread: Vacuum at idle..... kpa vs inch/hg

  1. #1
    Advanced Tuner
    Join Date
    Jun 2011
    Location
    Montreal, Canada
    Posts
    758

    Vacuum at idle..... kpa vs inch/hg

    I feel a bit dumb asking this but I am just getting frustrated.

    At idle HPT registers 60kpa. I have a gauge that is showing about -12 inch/hg at idle. Does this match and how is the conversion properly calculated. No online converter I have tried gives me these results.

    I looked at some logs of my previous setup, and they also showed around 60 KPA at idle, but I seem to remember the gauge showing around 16. I am just trying to figure out if the gauge is right, or something is skewed now.

    Thanks for being patient. My best 3 years of school were grade 8 !!!
    2007 Corvette C6 Vert. A6
    LME LS402, Pat G custom cam, ATI 10% OD Damper
    Circle D triple disc 2600, 3.42 Diff
    YSI, 3.0 pulley, ID 1000's
    Alky Control Meth,
    ARH 1 7/8 headers,
    1009 RWHP @ 7000, 817 RWT @ 6000

  2. #2
    Senior Tuner DSteck's Avatar
    Join Date
    Mar 2008
    Location
    St. Louis, MO
    Posts
    4,856
    100 kPa = 29.5 in/hg
    60 kPa = 17.7 in/hg

    Those are in absolute values. Your MAP reading in kPa is absolute pressure, but your gauge reads, well, gauge pressure. For gauge pressure, atmospheric is considered zero, so when you're in vacuum, it's a negative number.

    29.5 - 17.7 = 11.8 which rounds to 12

    DSX Tuning - Authorized HP Tuners Dealer
    http://www.dsxtuning.com
    http://www.facebook.com/dsx.tuning
    Just say no to bull s***.
    IF YOU WANT HELP, POST A FILE!

  3. #3
    Advanced Tuner
    Join Date
    Jun 2011
    Location
    Montreal, Canada
    Posts
    758
    Quote Originally Posted by DSteck View Post
    100 kPa = 29.5 in/hg
    60 kPa = 17.7 in/hg

    Those are in absolute values. Your MAP reading in kPa is absolute pressure, but your gauge reads, well, gauge pressure. For gauge pressure, atmospheric is considered zero, so when you're in vacuum, it's a negative number.

    29.5 - 17.7 = 11.8 which rounds to 12
    Thanks Dave, just the guy I thought might chime in. That is the calculation I got too. Maybe I am mistaken in remembering seeing 16 before, because this means the gauge is about right. Is -12 vacuum normal with a mild cam. For some reason I thought it would be more.
    2007 Corvette C6 Vert. A6
    LME LS402, Pat G custom cam, ATI 10% OD Damper
    Circle D triple disc 2600, 3.42 Diff
    YSI, 3.0 pulley, ID 1000's
    Alky Control Meth,
    ARH 1 7/8 headers,
    1009 RWHP @ 7000, 817 RWT @ 6000

  4. #4
    Senior Tuner DSteck's Avatar
    Join Date
    Mar 2008
    Location
    St. Louis, MO
    Posts
    4,856
    It depends on the cam, how much airflow you're using to idle, barometric pressure, and whether an elephant in Zimbabwe farts. Point being... it's in the general vicinity. Raise your rpm or use more spark and less airflow, and you'll see the vacuum increase.

    DSX Tuning - Authorized HP Tuners Dealer
    http://www.dsxtuning.com
    http://www.facebook.com/dsx.tuning
    Just say no to bull s***.
    IF YOU WANT HELP, POST A FILE!

  5. #5
    Advanced Tuner
    Join Date
    Jun 2011
    Location
    Montreal, Canada
    Posts
    758
    Quote Originally Posted by DSteck View Post
    It depends on the cam, how much airflow you're using to idle, barometric pressure, and whether an elephant in Zimbabwe farts. Point being... it's in the general vicinity. Raise your rpm or use more spark and less airflow, and you'll see the vacuum increase.
    Thanks..... I thought I heard an elephant fart... so everything is good. Runs perfectly so who cares about vacuum. LOL
    2007 Corvette C6 Vert. A6
    LME LS402, Pat G custom cam, ATI 10% OD Damper
    Circle D triple disc 2600, 3.42 Diff
    YSI, 3.0 pulley, ID 1000's
    Alky Control Meth,
    ARH 1 7/8 headers,
    1009 RWHP @ 7000, 817 RWT @ 6000