Mike, great analysis as always!!  I’m 5.5 years late to the party here.  Great discussion too!

Going back to your very first post, you noted that the Venturi is oval (40mm x 27.4mm), and you asked for verification of your math…

If the area of an oval (or eclipse) is 984.6, the square root is 31.4mm.  How did you get 35.6?  

The shape of the Venturi isn’t exactly oval as the sides are straight and not curved or rounded.  But I agree with the oval description.

I did a little different math to calculate the area and got 875.4.  I pretended there was a 27.4 x 27.4 square in the center (area 750.8).  Then I pretended the remaining top and bottom half moons would create a circle with a radius of 6.3mm (area would be 124.6). Sum of the 2 parts is 875.4.  Square root is 29.6.  I think this is actually a bit overestimated because what I’m referring to as “1/2 moons” would be more like “1/3 moons”.  Those 1/3 moons would make more of a football shape than a circle.  So my circle area calculation of the remainder is actually kinda generous.

Either way, I’m getting something between 29-31mm equivalent.  Where is my math mistake or do I have it right?  How did you get a 35.4mm round equivalent out of an area of 984.6?

I’m going through all of this because I’m sensing the PWK38 on my bike is too big for a stock motor or lightly modified (just a dyna and UNI filter).  While it might scream and love it at 5500+rpm, I’m thinking the velocity of air flow through the carb is too slow at lower RPMs with the big 38mm carb on a stock or almost stock setup.

I’m considering experimenting with a 36mm or maybe even a 34mm carb, but I need to gather some real data on performance with the PWK38.  Like timed acceleration from 40 to 60mph in 3rd gear.  And/Or maybe 50-70 in 4th.

I determined the angle of the segment by sketching the venturi on graph paper and measuring it with a protractor.

The remainder of my crude calculations.