The very early repair manuals discussed "ported" vacuum source - that is, above the throttle plates, as a method to ensure a smooth steady idle. This predates any sort of smog considerations. Sometimes you'll hear people claim this port business was an EPA requirement foisted on everyone. That isn't really the case.

It is true that after the federal clean air mandates were imposed on the automakers especially by the early 1970s, everyone needed to use a ported connection to have any hope of getting their engines to pass. One concession they made, was a solenoid that would revert to a manifold vacuum connection of the distributor in the event of engine overheating. Very thoughtful of them, since lots of engines were now running on the ragged edge.

A number of strategies were employed, including a reduction in compression ratios, cylinder combustion chamber redesign, EGR, retarding of ignition curves, and camshaft modifications to include "late" valve timing, and very lean fuel mixtures, as well as excessively rich fuel mixtures depending.

What all this tended to do was create a very poorly running, inefficient engine that wasted prodigious amounts of fuel (during both a currency devaluation and an Oil Embargo ..)

You can try to experiment a bit and see what you think. The general rule that seems to hold is factory stock engines are best setup with a "ported" or timed vacuum source. Performance engines, particularly those with radical camshaft (and consequently very low manifold vacuum) may indeed benefit from a fulltime manifold vacuum connection to ensure a streetable engine. Remember, a true drag engine doesn't include vacuum advance - because "part throttle" just isn't part of their vocabulary. Thus, the vacuum advance mechanism itself is just something else to get in the way - and possibly fail - at an inopportune time.

Keep in mind "Total Timing" is also a drag racing term. Again, because vacuum advance doesn't exist in their world.

The 36° or 38° "total timing" we talk about on Y-Blocks isn't counting vacuum advance. This is strictly referring to the initial timing, + the internal distributor mechanical weights and springs. A lot of people get confused on this subject. The vacuum advance is always tuned separately, after the mechanical curve is satisfactory.

When cruising on flat ground 50° BTDC or more is typical with OHV engines. The engine will run cooler and smoother with appropriate ignition timing advance at idle and cruise and everywhere in between.

Another common error is errors when setting the ignition timing. There won't be any vacuum advance at idle in a factory stock engine, provided the RPM is set to factory specification.

Most people like to bump it up a little. For example if I recall Y blocks are factory 450-550 RPM. About 600 to 650 seems to run smoother for most - and if you check with a timing light, it's easy to see why. The vacuum advance, even connected to a "ported" connection, will start to pull in a fair amount of timing just past factory RPM specs. My experience with walking people through this stuff is they would be far better served with learning what is an appropriate timing curve, than worrying about what port to connect to, because without understanding, it doesn't matter what port is used, it won't run right anyway.

You're right, modern engines of course are computer controlled. But that isn't the question. The gasoline, after all, has no way of knowing any of this.

The characteristic of NOX emissions in particular, which spike (relative to HC and CO) when the engine tuning and air fuel ratios are optomized for power and economy is well known, this is what the early smog regulations were designed to address. They basically crippled the motors with compression reductions, late valve timing - and lots of ignition timing jiggery-pokery.

Modern EFI especially means engines can run very lean air fuel mixtures, far leaner than any carbureted engine, I get that, but I'm still curious how they were able to reduce NOX emissions to acceptable levels, the kind and amount of pollutants is a basic characteristic of fuel combustion, regardless of how it gets there.

They don't need to retard the ignition timing anymore, they have reasonable compression. They can run maybe 18-1 AFR on the highway. This should make the NOX worse, not better. See where I'm going with that? Again, I figure it must be in large part due to better catalysts.

It would be interesting to see what the engineers thought back then trying to meet all the federal clean air rules. The basically traded power and economy to reduce NOX, and then burned all the wasted fuel in the tailpipe in the catalyst. Not the most elegant solution.

Quotes From The Referring URL In My First Post -

1. Lean Fuel Mixture - Lean fuel mixtures cause high NOx. A lean fuel mixture exists when less fuel then required is delivered to the combustion chambers or when more air then necessary is added to the fuel. In either case the lack of gasoline needed to cool the combustion chambers down is not present. Combustion temperatures increase causing high nitric oxide emissions. A lean fuel condition may be due to a vacuum leak/s and/or defective fuel control components, such as the Air Flow Meter, Engine Coolant Temperature Sensor, and O2 sensors.

5. Engine Overheating - Inadequate engine cooling can will high NOx. If your vehicle's cooling system is not working efficiently, (i.e. bad radiator, thermostat, hoses) high NOx will be created. Remember high NOx nitric oxide is created when an engine's combustion chamber temperatures reach over 2500F. You will want to make sure your vehicle's cooling system is working properly, and your vehicle's temperature gauge is always indicating normal.

Too fat of a fuel curve will create excess HC and most likely overheat the CAT(S) and burn them.  

You say HOLLEY? Is it the SNIPER?

You really do not have to worry about being EMISSIONS COMPLIANT with just a CFI FUEL SYSTEM do you?