The ported vs manifold vacuum to the distributor deal has graced countless pages on many other websites. The arguements continue to rage back and forth as to which one to use and which one works better?
The improved fuel economy and reduce running temp thing is regurgitated on nearly every thread I've ever read on that subject.
Interesting that after tuning many thousands of different engine combinations over the years, I've never found this to be true in one single instance, and I've seen some pretty hot running, oveheating and detonating engines over the years, and have been asked to help them.
I've done some pretty extensive fuel economy testing over the years as well, clear back into early 1980's. Back in the early 1980's, I was displaced from my family, and had to make a 600 mile run couple of times a month, and I was trying to stretch every penny as far as possible, it made for some very interesting testing.
I had several vehicles over the time that I made that run, and tuned them to DEATH by leaning/richening up the carburetors, and modifying the vacuum units and the mechanical spark advance as well. I'd also swap from a ported to manifold source to the advance.
That's not the only testing I've done, but the results of that early testing taught me two things. Lean mixtures are difficult to burn, and there is no fuel economy advantage whatsoever switching from a well located ported source to a manifold vacuum source running down the highway at 65-70 mph. About all I noticed running MVA, is that the engine rpm's dropped a tad more when the trans was place in gear that if a ported source were used and the throttle plates tipped in a little further.
I've done more closely controlled testing in recent years, and have found that once we increase engine efficiency (optimum compression ratio, optimum quench distance, well chose camshaft, efficient intake/exhaust systems, etc), the engine will not want, like, require, or respond well to a lot of ignition timing anyplace.
Of course we get all sorts of stuff in here to custom tune, and by far and above the very worst are when someone takes a relatively "low" compression engine, tops it with a single plane intake, free flowing headers, and some "whiz-bang" aftermarket camshaft on a tight lobe separation angle. I can't say TURD enough times.
It's just kills me that with all the good information out there these days at your fingertips, that folks still continue to choose piss-poor combinations of parts for their new engine, or upgrading their old engine.
Anyhow, we have, and always recomend to others, when tuning carburetors and distributors, to go after the carburetor first, to make sure it has sufficient fuel across the load speed range for what you are trying to do. Then tune the distributor. 99 times out of a 100, the tuner will find that a lot LESS timing is needed at idle speed once the carburetor is up to par for their particular application.......Cliff
