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Technical 261 Complete Tune Up | Drives Like Crap

Discussion in 'The Hokey Ass Message Board' started by Matt Davis, Aug 14, 2017.

  1. Atwater Mike
    Joined: May 31, 2002
    Posts: 11,618

    Atwater Mike
    Member

    Need slight backpressure from dishonorable non-existent exhaust system. Even Confucius would tell you.
     
    Matt Davis likes this.
  2. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 14,965

    Bandit Billy
    ALLIANCE MEMBER

    I have used engine vacuum in lieu of a timing light to achieve maximum setting. Proper vacuum is the symptom of a well tuned motor. 14 - 16 pounds of vacuum at a minimum and you can get to 20 depending on cam lift and duration. It is a cheap way to diagnose/tune an engine and the least embarrassing way to get the headlight doors open on a corvette.
     
    Matt Davis likes this.
  3. Matt Davis
    Joined: Sep 25, 2016
    Posts: 238

    Matt Davis
    Member

    I got a vacuum gauge and we'll be doing this also this weekend. ;)

    Sent from my Pixel using The H.A.M.B. mobile app
     
  4. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    If you insist... every exhaust system creates more or less back pressure. The more back pressure, the less exhaust gets out and the more stays in the cylinder. This can affect the mixture as it burns. Basically less back pressure = leaner mixture. Burned exhaust that stays in the cylinder dilutes and cools the next charge as it burns. This is the principle of exhaust gas recirculation, or the EGR valve that some engines used to have for pollution control.

    Then again the mixture could be too rich. If unburned gas is escaping with the exhaust and burns on the way out you will get a backfire in the exhaust. Could be rich mixture, could also be lean mixture burning slowly, could be misfire leaving unburned gas in the exhaust.

    A real sharp tuneup man could spend a few hours getting your engine working perfectly as it is. Then all that work would have to be done over again when the new exhaust was on.
     
    Last edited: Aug 16, 2017
    Bandit Billy, RMONTY and Matt Davis like this.

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