Vacuum Advance - Manifold or Ported

[SuperHart]

To answer Excaliber's question, I checked my vacuum advance module with a vacuum gauge and hand-held pump. The vacuum advance began to move at 10 in. of Hg. Therefore, I would have at least early vacuum advance with a manifold vacuum as low as 10". Even with dual #8007 Holleys on my 351W I still pull 18" of vacuum at idle. I have no idea how much vacuum I have at various cruising speeds, though. Maybe one day I will be energetic enough to hook up the vacuum gauge in the car so I can find out. To answer another question, I have dialed in my mechanical advance of 26* to be all in by 3200 RPM.

I run a stock Ford distribnutor with a Pertronix conversion and the Crane adjustable timing kit. When the stock distributor is disassembled down to the weights you will find that the weight plate has two slots in it, one usually marked with the number 13 and the other with the number 18. Depending on which slot you place the advance limiter tab in you can set up the distributor to provide either 26* or 36* of mechanical advance (13 or 18 multiplied by 2). The RPM at which the mechanical advance comes in is determined by which combination of springs provided in the Crane kit you choose to use.

The only time you might have both mechanical AND vacuum advance dialed in at the same time is if you are cruising at more than 3000 RPM. Even so, most engines can handle a lot of timing at that speed without difficulty because the load on the engine is low in that situation. Increasing engine load (going uphill, for example) would require giving more throttle thereby opening the throttle plates on the carb and lowering the manifold vacuum, rapidly causing cancellation of the vacuum advance.