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Ignition Timing Question

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by cleszkie, Aug 13, 2002.

  1. cleszkie

    cleszkie 1/2 ton status

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    How do I determine the amount of ignition advance to use with a certain aftermarket cam and aftermarket distributor? I aquired a 350 SBC with a Mallory Unilite distributor, Edelbrock Performer intake, Edelbrock 1406 carb, and an aftermarket cam of unknown make. I do have the cam specs though, which were torn off the box the cam came in by the previous owner. I guess my question is this: If I posted the cam specs, could someone out there give me a starting point? I think a stock set up runs about 8 degrees of advance, but I would think that the aftermarket cam would change all that.
     
  2. POFF

    POFF 1/2 ton status

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    The most important thing is the curve and full advance, idle timing is not as important. The curve is really what changes with a different cam. The best way would be to put the engine on a dyno then re-curve the distributor. But, I know I can't spend that kind of cash. The easiest think to do is just to set the full advance. Check the timing at high rpm with an advance timing light. Just rev the engine until the timing stops advancing, that's full advance. I believe the total advance is between 28-32 degrees of centrifugal + about 12-15 degrees of vacuum. I think total advance is around 42 degrees. Anyone?
     
  3. skyblazer

    skyblazer 1/2 ton status

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    I'm experimenting with this also. The best tuning will match the ignition curve to the combustion rate of your engine. I think the best engine performance is achieved when you dial in as much advance as possible without causing pre-ignition or detonation with your engine combination. I'd say that with an aftermarket cam with longer duration, more advance would help because there is more air/fuel to burn at at a specific rpm. Also with a long duration cam, more initial timing will result in a smoother idle and more vacuum.
    My MSD tech manual recommends setting as much initial advance as possible without causing excess starter load and/or engine kick back. Then adjust the total timing. From what I've read most cast iron heads will respond best with 32 - 34* total advance. With my engine, the head manufacturer recommends 36-38* total advance. On my MSD distributor I can select different stop bushings to select the amount of advance (from initial to full) and various spring to vary the rpm range where the advance is increased. I am currently running a 21* bushing. I started by calling MSD tech for some starting points. They suggested setting 15* initial for 36* total advance and using light springs so the dist. achieved full advance by 3000 rpms. That was way too aggresive for a heavy truck. The motor would ping under heavy acceleration and seemed to fight itself until it reached high rpms. I corrected this by taming down the initial advance to 12* (12* + 21* = 33* total) and used the next heavier spring combo which brings in total advance slower (by 3800 rpms). Biiiiggg difference! No more ping and the engine revs better and feels like I unhooked the plow. I think the next step will be to use the smaller 25* stop bushing so I can still use 12* initial and still achieve the recommended 36* -38* total advance (12* + 25* = 37*). Without a dyno, the best indication of too much advance is detonation (engine ping) and too little will cause decreased power. I'd start by going aggressive and check for ping, then tame it down until you get the right combo.
     
  4. cleszkie

    cleszkie 1/2 ton status

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    Thanks Bro. That is good info to get me started in the right direction. Just need to study up to find out how to adjust the advance in the Mallory Unilite (shims and springs).
     

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