by Robert S. Hoover
A vacuum advance distributor senses changes in the engine's LOAD -- and does so almost instantaneously. Mechanical advance distributors, which sense only changes in rpm, are meant for special purpose applications, such as as drag racing or poorly designed multi-carb set-ups.... or moving a loaded Type II with a 25hp engine.
If you try to use a purely mechanical advance distributor in your daily driver you will have to jack-up the output of the accelerator pump, since providing a jolt of raw gas is the only way you can bump up the rpms, which is the only way you can get the advance to kick in. But before the revs can build up you waste a lot of gas. This plays hell with your fuel economy and shortens your engine's useful life at the same time.
People who run mechanical advance distributors usually have no idea how well their Volkswagen can perform.
When you go back to a vacuum advance distributor you usually see a remarkable improvement in both performance and economy. The performance-change being the typical perception of an engine that is now suddenly more responsive than before. The improvment in economy comes about because you're no longer throwing away most of the accelerator pump's output each time you shift gears or pull away from a stop.
The fact John Muir.... and all the tits & ass VW magazines.... say the mechanical advance distributor is the best thing since canned beer is no reason to assume this is true. Muir advocates any number of things that are incorrect, including many that are detrimental to your vehicle.
I've written about the vacuum vs mechanical distributor situation many times but most people still don't get it. Volkswagen -- and all other auto makers -- use vacuum-advance (or vacuum/centrifugal) because for normal driving such distributors do a better job than a purely mechanical-advance device.