Quote:
Originally Posted by dmsfrr
I'm trying to understand, not sound argumentative...
If there isn't that much difference why did Ford and Holley, Autolite (etc.) go to so much design and engineering expense over several years changing and upgrading carbs and distributor advance... if straight manifold vacuum would have worked in the first place?
Is it for reduced emissions and improved fuel economy? or ?
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No that's fine, keep in mind the Loadomatic (Ford) distributor is a separate deal altogether and just confuses things even further. It has no mechanical advance mechanism, it is exclusively operated by vacuum(s) from two different sources from a special proprietary carburetor. This is why there is a performance problem when someone installs a different carburetor on an engine that included the loadomatic. Another issue I believe Ford went to a 1/4" hex drive for the distributor shaft after '56. As far as standard type dual unit distributor using mechanical weights and vacuum advance I'm not sure exactly why different manufacturers used different methods.
"Ported" manifold was utilized/invented because it allows for a steady idle, and long before any pollution controls were ever dreamed of. There is probably more misconception about distributors and ignition timing than anything I can think of, and especially vacuum advance. It's often found disconnected because the tuner doesn't understand it, or "curving". Part of the problem too is so many components and methods (and marketing) geared towards drag racing are mistakenly employed by people who only and ever drive on the street. Huge carburetors and intake plenums, rumpety-rumpety cams and racing distributors are made for high RPM and high speed only, and won't idle well or handle nicely tooling around town.
A mechanics vacuum gauge plumbed into the cabin will illustrate how engine vacuum fluctuates wildly and quickly under different engine load conditions. That's the key to understanding vacuum advance (or retard); the mechanical weights or centrifugal distributor advance is RPM only, vacuum advance is load dependent, they are completely independent of each other. If you were to connect two vacuum gauges, one to manifold, and one to "ported" they would read exactly the same cruising down the highway. Thus the advance at the distributor would be the same.
At idle 30° or more BTDC tends to cause alarm, but there is no load on the engine. And as soon as there is, the vacuum goes away. Most engine tuners recommend a stock engine be connected to "ported" while a modified street engine will prefer being connected to a straight manifold source, mainly for the reasons mentioned.