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[quote="Eddie_W"]Hi, I'm somewhat puzzled. The ditributors on my engines are mechanical advance and therefor are tunable with springs and stops. The vacuum is only an addition for road economy applying extra advance to fire lean mixtures on overun and light throttle.
On modified engines the vacuum can be disconnected to prevent them from actuating at full throttle which can be a possibility.
Modified engines will normally require less total advance because of better combustion characteristics ( faster flame front from better cylinder filling and more homoginised mixture from better squish) Total advance (static plus distributor) is the crucial setting.
Examples from the past:
   The old Lotus Twincam in racing form would produce max HP at 38 degrees total and last about 4-6 laps. At 36 degrees it would lose about 3HP and last all day.
The ford BDA rally engines would run between 28 to 32 degrees total. Less than the Twincam because of better squish.

Some supercharged and Turbocharged motors  run dizzies with top end retard  althogh these days it's all usually handled be an ECU.
Why anyone would recommend points over electronic triggering is beyond me. Electronic maximises the dwell angle which is the period that the primary windings in the coil are being saturated. An important factor in the amount of spark delivered. Points can only be a compromise in this area because of the mechanical limitations of lobe shape to avoid points bounce (just like valve bounce) and the reason that performance cars pre electronic ran special heavy duty points with a tougher spring. 
The original Mini Cooper S is an example.
Most dizzies with mech advance(like the 333s) have a stop to limit the advance and this can be modified with ingenuity. The rate of advance or curve is modified by the control springs used.
Hpoe this helps
regards Eddie[/quote]

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