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question for cam minded people

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by spoolnaround, Jan 5, 2005.

  1. spoolnaround

    spoolnaround 1/2 ton status

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    How much vacuum does it take to make power brakes work? Where would you draw the line for duration for enough vacuum in a cam? does lobe center affect vacuum?
     
  2. stevewix

    stevewix Registered Member

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    finally someone asks a question i know something about!


    we'll start with lobe sep:

    wider lobe separation will make MUCH more vacuum, also they idle and run smoother and make a fair amount of low-end.

    personally:
    i've run 750+ hp big-blocks on power brakes, the biggest tip i can offer is use a vacuum canister, the larger the better. as for a specific number, i want to say 10 in/hg and below sucks with power brakes (i could be waaay off on the units, but i'm pretty sure 10 was the lowest i went)

    ALSO- amount of vacuum depends on adjustability of the carb, size of the motor, lobe separation/duration of cam and spark timing.

    please give me the carb model, cubic inch, cam size etc. maybe i can offer some pointers.

    big all-out drag motors (i.e. 550+ CI) make TONS of vacuum.

    do you ever tune idle mixture with a vacuum gauge hooked into the manifold? that's a great way to do it and you know how much vac you have.
     
  3. bigblock44k5

    bigblock44k5 1/2 ton status

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    Correct with lobe sep. Just to put it in simpler terms, the greater the amount of time that both the intake and exhaust valves are open at the same time, the less vaccuum you will have. Called valve overlap. When the intake valve opens and the piston moves down to suck in air, your exhaust valve might still be open(depends on cam specs)for a little bit, therefor weakening the signal at idle, but giving better volumetric efficiency at higher rpms.
     
  4. jarheadk5

    jarheadk5 1/2 ton status

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    I'll second the 10 in/hg of vacuum minimum.
     

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