Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Ported Vacuum or Manifold Vacuum?
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Old 09-04-2008, 10:46 AM
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To give a slight explanation of it, I have delt with some GM vehicles that ran better with it on a ported source, but most ran best with manifold. Idle and off idle cruise mixtures are lean, and lean mixtures burn slower than rich mixtures, this is where the extra advance comes into play. When I was running my cutlass, I had it on a ported vacuum running a mild 350 chevy. The thing was a slug on low end, I kept wondering why it had no bottom end power, it had a RV cam and 9.5:1 compression, headers, a dual plane intake and a small 625 demon jr carb. it had really poor throttle response and drank gas. Then one day on here I complained about it and was told to set the base timing with the vac advance disconnected and plugged to 12 degrees, then reconnect the vac advance to a manifold source, I did so and it felt like it picked up 50 horses, it was really sensitive throttle response wise and a pleasure to drive. Fuel economy went up as well. Since then I`ve stuck to manifold vacuum unless it runs better on a ported.
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