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Vaccuum Advance?

Discussion in 'The Garage' started by RustBuket, Jun 14, 2003.

  1. RustBuket

    RustBuket 1/2 ton status

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    I'm in the process of replacing the 305 into my truck.( 81 chevy 1/2) I was wondering if it mattered if the vaccuum advance is on a heat sensing switch or not? Right now its set up for only running when the engine is hot.
     
  2. SUBFAN

    SUBFAN 1/2 ton status

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    Vaccuum advance should see constant vaccuum, just vaccuum, all by itself....
     
  3. bigblock44k5

    bigblock44k5 1/2 ton status

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    dont hook up to anything with heat. hook straight to ported vacuum on the carb.
     
  4. RustBuket

    RustBuket 1/2 ton status

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    I haven't hooked up anything yet! lol /forums/images/graemlins/rotfl.gif I just wanted to know if it mattered if I took the vaccuum advance off the heat switch? /forums/images/graemlins/confused.gif
     
  5. bigblock44k5

    bigblock44k5 1/2 ton status

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    wouldnt be good, you wouldnt get advance when it is cold which isnt good.
     
  6. RustBuket

    RustBuket 1/2 ton status

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    [ QUOTE ]
    wouldnt be good, you wouldnt get advance when it is cold which isnt good.

    [/ QUOTE ]

    That is how it is set up now! I want to know if I can set it up so that it gets advance all the time by directly hooking it up to a vaccuum port instead of through the heat switch.
     
  7. bigblock44k5

    bigblock44k5 1/2 ton status

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    [ QUOTE ]
    dont hook up to anything with heat. hook straight to ported vacuum on the carb.

    [/ QUOTE ]

    yes you can, finded ported vacuum on carb and hook it there!
     
  8. RustBuket

    RustBuket 1/2 ton status

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    What is "ported vaccuum" ? /forums/images/graemlins/1zhelp.gif
     
  9. bigblock44k5

    bigblock44k5 1/2 ton status

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    ported vacuum is a vacuum port that has no vacuum whatsoever at an idle. as you give it gas it should gradually increase vacuum.
     
  10. RustBuket

    RustBuket 1/2 ton status

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    Hmmm, ok thanks man!
     
  11. Lonnie

    Lonnie 1/2 ton status

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    I'd recommend manifold vacuum (high at idle & 0 at full throttle). This way you can tune the engine for full power with the (initial/total) distributor advance. The vacuum advance will help part throttle response this way while not overadvancing the timing at full throttle. Ported vacuum was often used in the "emissions era" as high advance at idle increased emissions & ported vacuum is 0 at idle. I have always seen better response/mileage with manifold vacuum.
     
  12. RustBuket

    RustBuket 1/2 ton status

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    manifold vaccuum eh? alright I'll look into that. I've got a free spot where the exhaust manifold butterfly is coming off... (I'm getting headers /forums/images/graemlins/thumb.gif)
     
  13. bigblock44k5

    bigblock44k5 1/2 ton status

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    NO!!! you cannont use manifold vacuum. if you did that you would be totally advanced at an idle. making it hard to start, then when you gave it gas, you would loose all your advance. that would be a real stupid move. if you are really looking for performance you could use no vacuum advance and just use the mechanical advance. you will have to set the timing for total advance. rev the enging to about 2500 rpm and set the timing to 35-40 degrees. this wil make your timing at about 20 at an idle though and would also be hard starting. but under no circumstances use mainfold vacuum.
     
  14. blasterD

    blasterD 1/2 ton status

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    I agree with Lonnie. I have always switched from ported vacuum to manifold vacuum for the ignition advance. I find it gives better throttle response and mileage. The engine seems to run smoother too. At least try it both ways see what you prefer.
     
  15. Lonnie

    Lonnie 1/2 ton status

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    There is virtually no vacuum before the engine starts...therefore no starting problems. The vacuum advance does not work until running with approx 8-10" vacuum. I build race engines & high performance cars for my business.... I have extensive experience of what works & what does not. At least 99% (I didn't say 100% as nothing is fool proof) of the vehicles run better with manifold vacuum.... though I have never found one that didn't unless the base timing was already off.
     
  16. bigblock44k5

    bigblock44k5 1/2 ton status

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    as long as you relize that when you floor it you will loose all your advance and your power as the rpms increase.
     
  17. Lonnie

    Lonnie 1/2 ton status

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    You seem to have a misconception here. You should never use the vacuum advance for full throttle power. If the initial & total timing are both correct you will have the proper full throttle performance. This is the proper way to tune a motor. You then use the vacuum advance to obtain better part throttle performance. A motor needs the most timing under lean cruise conditions not at full throttle. Most Chevy motors like 10-12 deg initial, 32-34 deg total advance in by approx 3000 rpm & close to 10 additional degrees with the vacuum advance at part throttle. If you are tuning by any other method you are doing it wrong. I do not want to get into a debate as I have better things to do..... I do feel that the people asking the questions have the right to a proper answer though. For all those reading, consult some other professional engine builders then formulate your own opinions. You will then see that my advice is correct.
     

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