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How much timing is to much

Discussion in 'The Garage' started by Justin Fleming, Sep 19, 2005.

  1. Justin Fleming

    Justin Fleming 1/2 ton status Premium Member

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    I had the k5 305 set to the factory 4deg and it was about 15deg with the advanced hook up to manifold vaccum. I have now bumped it to 25deg with vaccum hooked and she has way more power/response and still does not ping with low grade fuel.
     
  2. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Vacuum goes ported . I bet you if you put it to ported it will ping at part throttle . Mileage will improve though .

    Mine hates advance with low octane .
     
  3. Justin Fleming

    Justin Fleming 1/2 ton status Premium Member

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    I didnt think manifold was ported......Also why do you say I should go ported? and why would it ping? witch port on the q-jet is ported? will ported pull more timing in than manifold?

    thanks
    Justin
     
  4. JNygard

    JNygard Registered Member

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    How much timing is too much? Thats easy advance until you can hear it ping on acceleration. Then back it off a couple degrees.

    Vacuum advance was designed to be hooked to the ported vacuum so there is no vac advance at idle. If you still have your emmision sticker on the radiator support, just trace that back to the carb. Ported vac is a port above the throttle blades so it doesn't create vacuum until the blades start opening creating the venturi vacuum effect.

    I use to run about 8-10 degrees initial timing without vac advance.
     
  5. RustBuket

    RustBuket 1/2 ton status

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    Agreed, its supposed to be ported so it advances as the RPMs go up where you need it. Its not pinging under acceleration because where you have it hooked up, it will actually pull less vaccuum under load.
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    MOST use ported vacuum..

    Some years and models ran manifold vacuum to the vacuum advance..My 86 305 motor in my 79 C10 seems to run better and smoother with manifold vacuum rather than ported..If I use ported vacuum it pings..I think its because the motor originally had the "ESC" Hei distributor and electric Q-jet carb,and I ditched both and used a 1976 305 HEI with vacuum advance and an Edelbrock 600 cfm performer carb--which wouldn't fit without removing the EGR valve,-(the EGR helps prevent pinging by lowering the comvustion temparature--but kills a bit of power too)--and using an adapter to the stock intake..those "ESC" distributors ran a lot of advance,using the module's "advance curve",and the computer..

    I bet the actual amount of advance the canister delivers isn't that much different at cruising speed with ported VS manifold vacuum,and with manifold vacuum it RETARDS the spark when you floor it,(kills the advance from the canister anyway)-and the mechanical advance weights take over..my truck idles much smoother with it fully advanced at idle,it still gets SOME advance at cruise,and it wont ping wide open like before without the additional vacuum advance with ported vacuumm--I swear its better on gas too!..

    If I knew how I would have just left the "ESC" distributor intact--I think the van I got the motor from had a computer--I didn't want to mess with all that nightmare...I drove the van before I stripped the motor out--It was a bit peppier than my C10,but not much--I think it was less weight that made the difference more than anything..I can still smoke the tires with it in my truck fairly easy--my truck is light in the rear--it will swap ends if you punch it around a corner ,especially if its wet out.. :crazy:

    All this said,all I can say is try both ported AND manifold vacuum,and see which way it seems best for your motor..the ported "port" on a Q-jet is on the front of the carb,higher than the throttle butterflies--it will have no vacuum at idle,and it will just above idle..a vacuum gauge makes finding it easy.. :crazy:
     
  7. Justin Fleming

    Justin Fleming 1/2 ton status Premium Member

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    Anyone else have any input
     
  8. slimlynn1

    slimlynn1 1/2 ton status

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    Once you step on the gas ported and manifold vacuum is the same. If you want to smooth out the idle on an engine with a large cam you want to use manifold vacuum so there is advance at idle. At wide open throttle either vacuum will go to zero.
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Yep. Vac advance is for cruise. Doesn't make an ounce of difference for acceleration. (well, it might be a hair, but... :))

    High advance at idle smooths an engine out, (manifold vac to the vac advance) but I guarantee that causes some sort of emissions problem which is why most everything "light duty" went ported at some point.
     
  10. rcamacho

    rcamacho 1/2 ton status

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    Advance

    Do a search, there are plenty of articles on proper advance profiles for Chevy V-8's. Generally you need 32-36 degrees of mechanical advance and 10-15 degrees of ported vacuum advance.
     
  11. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    Till it pings then back off 2 degrees. ;)
     

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