I'm glad you asked that question.
Originally Posted by KB Cool
If you were to compare all of the available cap tube sizing information, (which isn't much) you'd find right off the bat that none of it agrees.
Likely the most popular chart out there is Supco's. I used it for longer than I care to remember and it worked OK for the most part. Their chart was created back in the 1960's, primarily by trial and error...and extrapolation. I have high regards for Henry Ahrens for his work back then and I'm sure he'd be thrilled to see what we now have to work with.
This new Tecumseh program reflects the results a lot of new studies about how cap tubes actually work and how they can be modeled using computers. There have been a few cap tube programs floating about in the last ten years or so, but this is the first one to use the latest info. I've had very good results in the field using it.