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Old 03-21-2010, 02:20 AM
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Here we go again. Many ask why do you need vacuum advance at idle?
I used to ask that same question.
You need it because idle and off idle mixtures are lean, leaner mixtures burn slower, and when the mixture burns slow it needs more spark advance to compensate. Manifold vacuum works off idle just the same as ported vacuum. However, since the idle mixture is lean, it needs that extra spark advance when coming off idle to the main circuits, if not the engine will sometimes sputter. You have to remember, the power valve doesn`t kick in until the vacuum drops under a certain vacuum limit, during regular driving you won`t open the throttle enough to make the vacuum drop low enough for the power valve to enrichen the circuit so the mixture is still lean. Take any chevy vehicle from the mid 70`s to early 80`s. The catalyst says "timing set with vacuum advance disconnected and plugged" if it`s connected to a ported source there is no need to disconnect it, because it doesn`t pull vacuum at idle. When I ran my small block to a ported source, a mild 9:1 compression 350 with headers, small RV cam, intake and carb with 3.23 gears, it wouldn`t even spin a tire from take off, it was a dead player. For a long time I couldn`t figure out why it was so sluggish, until a member of this site told me to connect my vac advance to a manifold source, I did as he asked and the car felt like it picked up 50 horses. It had great bottom end torque and throttle sensitivity.
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