Is there somewhere online, some type of chart that
displays: RPM vs the optimum header (primary) tube length,
hopefully with several tube diameters?
My suspicion is that so called 'long tube headers'
are too short.

 

http://www.headerdesign.com/index.asp

Register for the site above and put in your parameters to find the optimal header length.

 

Larger primary sizes shift the torque peak higher in the RPM range.

A shorter pipe for a given diameter kind of "rocks" the whole graph around the peak, lower RPM having less torque and higher RPM gaining some.

The opposite happens with a longer primary pipe for a given diameter.


EDIT: I got it backwards.

here's another link about header pipe sizes and lengths:

http://www.popularhotrodding.com/tech/0310phr_burns/

 

Here is a estimator equation for torque peak RPM for a given runner cross section (primary pipe area) and cylinder volume:

Torque Peak RPM = (88,200 x avg runner cross section)/volume of 1 cylinder

 

I happened across some intenet site, with a header length
calculator, I put in the numbers for 5000 RPM,
and was reported an optimum lenght of 60 inches.
Does that seem right?
I fully realize that lenght, is not what header vendors
want to sell you.