Here is a very simplified answer. The mechanical system, no vacuum, counts on weights, springs, and dist rotational speed for advance. VERY generally, the advance curve is achieved by springs and weights, and does not take into account engine load factors. The vacuum system also contains a mechanical advance mechanism, but is governed by a vacuum diaphragm. This diaphragm IS affected by engine load, and variable vacuum caused by butterfly position. For street use, the vacuum system will yield an engine that pulls more evenly, throughout it's normal running range. The mechanical system is better on a performance engine where the ignition advance is over at somewhat lower engine speeds.
The Bosch vacuum system, can be altered into a mechanical distributor, by removing the vacuum system, making an aluminum cover plate for the hole in the dist body, and then changing the weights and springs on a dist machine to match those of the mechanical dist. This is time consuming and not inexpensive. You need a source of parts, access to a good dist machine, and a mechanical unit to replicate. It can be done, but it is NOT as simple as disconnecting and plugging the vacuum line. THere is a lot more to this, but unless you are really interested in busy work, you are much better off locating a mechanical dist, even if you still need to alter it's curve. Hope this helps.
Gordon Raymond, a no bling subscriber
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