At least one cylinder head vendor offers what looks like
essentialy the same head, with several different port volumes.
...
My question is: how were the smaller ones, optimized?
I would guess, something like,,, at a given airflow, design 'A'
is optimzed to have lowest achievable pressure drop.
For design 'B', repeat, except that aiflow is set higher. Etc.
Does anyone know?, please enlighten me.
...
The website of a certain vendor, does not come out and
list what 'upper rpm', he thinks is appropriate for
any certain model.
...
contactpatch

 

GENERALLY, port volumes indicate the peak flow capabilites of heads. Again, GENERALLY, larger peak flow volume indicates a lower flow velocity. Of course, much of that also depends on cam profile, valve sizes, and valve timing, but all else being equal, size = flow.

If you are feeding a lower displacement engine, or a larger engine at lower RPMs, higher flow velocities may be more important than peak flow volume numbers. Conversely, if you are interested in feeding an engine only at high RPM and with a larger displacement, you'll likely want the highest flow you can get.

That is the primary reason for the variety of port volumes offered in the "same" head. SBC engines can have displacements ranging from 262 CID to 453.9 CID, so even though the heads may all interchange (bolt on), there isn't one head that is appropriate for all.

 

(assume fuel injection, not carburator)
I would think that the only (intake side) issue would be, how much mass of air in contained in the cylinder when the intake
valve closes. Why is 'high velocity" necessarily good?
I've never even seem a Mach number associated with their flow numbers.
...
Again, I am trying to rationalize why , these vendors
offer the 'smaller port' sizes. The vendors offer no
recommendations on 'intake manifold runner cross section area',
and usually make only a vague comment about rpm.
...
(Do vendors post some type of 'desktop dyno' file related
to their product, I guess I'll go look)

 

The best analogy in my opinion for port flow versus volume versus area is the waterfalls. Think of your intake port as a waterfalls. Take Niagra Falls, for example. Mate it to a 200cc runner. Now, increase the river size (prior to the falls, upstream) and give it a 220cc label. You increase the overall flow potential. However, when the engine isn't turning as fast, it isn't drawing in as much air as it was. So now keep the falls the same (220cc) but cut the flow. Now how fast is the water moving? Much slower. Because that smaller amount of water (air) has more space to inhibit which will slow it down, hence losing low end velocity. The oposite is also true. Back tot he 200cc falls. Now, decrease it to 180cc in size. You limit the peak flow. However, when you cut the flow through it, velocity doesn't take as big of a hit because there isn't as much space for the air to inhibit. This increases the low end velocity, at the expense of top end peak flow.

To choose the proper runner size, you have to determine the size of the engine and the RPMs you want. The more RPMs you want, go for larger runners, but be prepared to lose bottom end power. The vice versa is also true. If you want low end grunt, get a smaller runner size. You maximize low end velocity, but limit the overall peak flow. The trick obviously being to find the perfect balance of runner and valve size to get the most out of what you are after.

Why don't you ever see velocity numbers? Between runner volume and valve size, the velocity is more or less implied. Like reading between the lines, it is the persons job to "know" the velocity. Much like a cam card. The manufacturer gives you just as much info as needed to make a proper decision.