I stole this from the Corvette forum (http://www.corvetteforum.com/forums/c1-and-c2-corvettes/647366-vacuum-explained-manifold-venturi-and-ported-new-article-by-lars.html)
The most sensible explanation I've read:The timing advance curve requirements for an engine will vary a bit from one engine to another depending on cam, compression ratio and other efficiency factors. But in general terms, most V8s will produce peak power at WOT with 36-38 degrees of ignition timing. Peak fuel economy and drivability at cruise is achieved with about 52-54 degrees of advance. Best idle quality has a much wider range depending on cam & engine, but tends to be in the 12-24 degree range. Lowest emissions usually occur with timing in the 4-8 degree range.
When tuning, it is important to realize that the upper limits on timing are determining factors for how to set things up: You want the total WOT timing (the maximum timing the engine will see with vac advance disconnected and with the centrifugal advance fully deployed) to be not over 38 degrees. 36 is the best setting for most applications. Once this has been set, it automatically determines what your initial advance ends up being unless you physically alter the length of the advance curve. In most cases, once total advance has been set to 36, the initial advance will end up being about 12 degrees-or-so. And, since most vacuum advance control units pull in about 16 degrees of vacuum advance at cruise speed (where the full centrifugal advance will also be deployed), the 36-degree setting will produce 52 degrees of total combined advance at cruise with the vac advance fully deployed.
But what if your engine/cam combination idles best at 26 degrees advance? Radical cams often require quite a bit of advance at idle. If you simply bump the initial timing up from 16 to 26, your total WOT advance will go from 36 to 46. The total combined timing at cruise will go from 52 to 62. This is not acceptable, and can result in severe engine damage from detonation at WOT, and the car will chug and jerk at cruise from the over-advanced condition. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2 in. Hg below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.
If, however, your engine idles best in the 12-16 degree range due to a mild cam, plug the vacuum advance control unit to a ported vacuum source to eliminate the vacuum signal at idle. You will still obtain the 36-degree WOT total, and you’ll still have 52 at cruise. Also, if you need to pass an emissions test, use the ported source to reduce your hydrocarbons.
By playing with the total length of your centrifugal advance curve, selecting between ported or manifold vacuum, and carefully selecting a matched vacuum advance control unit that meets your specification requirements, you can achieve an optimum idle, excellent off-idle throttle response, and the best fuel economy possible.
