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What should the vacuum be on the hose that goes to the brake booster?

Discussion in 'Technical Q&A' started by 308GTS, Jan 12, 2004.

  1. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Does anyone have the vacuum reading that the plenum/intake should draw on the booster? I want to make sure that I am getting the correct amount of vacuum for my assist on my brakes. I suspect that a missing cylinder could cause more effort in braking and lose some assist.
     
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  3. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
  4. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,357
    Texas
    Full Name:
    Steve Magnusson
    What model/year/version? (Where are the Profile police when you need them;))

    Have to say that I don't quite understand your question (i.e., how a "missing" cylinder would contribute to an inefficient intake vacuum process). Are you saying that you've measured the vacuum level in the line to the booster (between the checkvalve and the booster), and the checkvalve is working OKish but the overall vacuum level is lowish?

    I think worst case (for something in a single cylinder to influence the brake vacuum process) would be on a carbed 308 where the brake vacuum is drawn off a single cylinder, but if that cylinder's intake process was so leaky that it didn't produce a high vacuum under engine rundown conditions (that the checkvalve should then hold in the booster for braking use) you'd need a serious rebuild/repair anyway.

    Please give more information/problem description/etc....
     
  5. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Steve, what I am asking is what should the reading be on a vacuum gauge if I were to start the engine and have a guage connected to the hose that connects to the plenum.
    Ok so the only loss of vacuum could be from a leak, line leak, or booster otherwise look elsewhere.
    I was mainly wanting a # reading to compare. Thanks.
     
  6. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    Steve Magnusson
    (Since you say "plenum" I'll assume it's an injected year/model F) If you measure directly at the fitting on the intake plenum you should measure something like ~16 inches Hg vacuum at idle and it would vary as you the gently blip the throttle (maybe dropping to 10 inches Hg and rising to 20 inches HG vacuum on the rundown). In the line between the checkvalve and the booster you should measure a vacuum level corresponding to the highest level seen at the plenum fitting -- so if you blip the throttle a few times it should "pump down" towards the 20 inches Hg level and hold fairly constant after shutoff.
     
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  8. 328GTB

    328GTB Formula Junior
    Silver Subscribed

    Jun 4, 2002
    288
    Texas, USA
    Full Name:
    Carmine
    Steve, I don't want to hijack this thread, but would a replacement hose that's rated at 28 in/Hg be sufficient? What should a temperature rating be? Aeroquip makes a similar black cloth braided hose ~12x20mm with the quoted vacuum rating and a temp rating of 212 degF.

    Thanks
    Carmine
     
  9. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
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    Steve Magnusson
    I've never instrumented up the booster side to see the maximum vacuum value obtained during actual vehicle running (where due to engine braking you can have higher vacuum levels than by just statically blipping the throttle) so can't say for sure. If the manufacturer refers to it as being used in brake booster applications I'm sure it's OK.

    The temp value is probably OK -- but engine bays do get up in the 200 deg F ballpark. If you had an application where the hose was more exposed to the radiant heat transfer from the headers I'd be concerned.

    One point I'd make is that the labor is the dominate cost/hassle to replace the long hose so I'd err on the side of an over-designed hose. It won't match the original, but IME high-quality brake booster hoses usually have an extra inner liner and much bigger OD/ID ratios (like 7/16" ID and 1.38" OD) than the "stock" F brake booster hose. If you want to keep the stock-look, you might consider adding another connection somewhere above the rear wheel well so the (easily-replaceable) underbonnet/underfender hose sections are stock while the long buried hose section is better quality/life -- just a thought...
     
  10. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    2,220
    TN
    Yes it is an injected 308 (QV). How much of a loss should be expected if I test vacuum at the booster 1 inch Hg or less? Thanks for the help. I want to see if I am loosing any vacuum anywhere. Where would one purchase a high quality brake vacuum hose in this length?
     
  11. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
    Full Name:
    Philip
    On my carb'd 308 (which has good leakdown and compression stats), I seem to recall 15 ins or so of vacuum when we measured it -- consistent with Steve's report.

    I usually get the Wurth hose from Continental. Randy stocks it in bulk and will cut a piece to the length required.
     
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  13. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,357
    Texas
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    Steve Magnusson
    The vacuum measured in the line at the booster end (between the checkvalve and the booster) should match the vacuum measured in the line at the engine end (between the checkvalve and the booster) -- no "losses" to account for.
     

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