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Hot Rods Cam vs Vacuum

Discussion in 'The Hokey Ass Message Board' started by 69fury, Mar 3, 2017.

  1. 69fury
    Joined: Feb 24, 2009
    Posts: 1,419


    The recent thread "cam timing" has me thinking a bit about my OT daily driver. 350 pre-vortec head Chevy-pretty basic stuff. I've put a light on the timing tab and it's right on spec- but maybe the previous owner gave the dizzy a twist to account for timing chain slop. This situation would be correct ignition timing with a retarded cam. My question is- what effect does retarded cam timing have on engine vacuum and mileage. I have a good working understanding of cam dynamics, but I cant get my head around the vacuum/
  2. First of all, is it a stock unmolested motor? How many miles does it have on it? Does it seem to run OK or does it get bad gas mileage? How much does the car weigh?

    To answer your question, an overly worn cam chain will retard the cam, which reduces cylinder pressure and thus lowers torque at any given rpm. It will give an overall soggy performance. You will end up pushing the pedal down more to get the same performance and that reduces vacuum and that reduces gas mileage.

    If indeed the cam chain is that worn, the rest of the motor needs attention too. Rings and valve job might bring it up to snuff. Of course, it you're that far into it, do bearings and cam and lifters at the same time. Then you can pick a nice mild cam and get more than you had to begin with. Just my 2 bucks... :)
    Last edited: Mar 3, 2017
    69fury, AHotRod and gimpyshotrods like this.
  3. 69fury
    Joined: Feb 24, 2009
    Posts: 1,419


    Stock mill, I think 225K on the ticker. Heavy vehicle, I'm currently stuck with over 5,000 daily driver (GMC Yukon). and I only live 5 miles from work so in the winter, it's just getting warmed up when i shut it down. 350 TBI, not an oil burner, seems in decent repair-cant tell if it's a soggy mill , or just underpowered for the weight. I was hoping to see if cam timing (worn chain) would bring up mileage/pep. Thanks for your insight. -rick.
  4. I am running the cam slightly retarded in my 355. I didn't do it for mileage I did it for revs. Retarding your cam will or can move your power band up slightly.

    When I motor was fresh about 150k ago it was pulling 16" at idle. But camshaft timing is only part of the equation. Good compression, head flow, intake carburetor, bore stroke all of that is where you get your numbers. I can tell you this, the motor has consistently gotten over 20 on the highway, light vehicle or heavy. But there is way more than what I mentioned in that equation as well. Lower end setup, quench, heads etc. That and I just got lucky on the build, I was not shooting for mileage when I built the motor it is just a by product of the build.

    I am not saying this to be mean but one cannot choose one component. A motor and or a car is a synergetic thing. The sum of the whole is greater than the sum total of the parts.
    Montana1 likes this.

  5. falcongeorge
    Joined: Aug 26, 2010
    Posts: 18,341

    from BC

    Reducing cylinder pressure at idle will reduce idle vacuum. Increasing cylinder pressure at idle will increase idle vacuum.
    Last edited: Mar 6, 2017
  6. Los_Control
    Joined: Oct 7, 2016
    Posts: 766

    from TX

    Chains wear and stretch, not to expensive to buy. Hard to say how much effort is required to change it on "your" vehicle. But it would not be uncommon to replace a timing chain and gears.
    With Fords, you can move the lower crank pulley back and forth and watch the rotor in your distributor cap, see how much play is in your chain.
    With a chevy this requires long gorilla arms or a helper.

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