I hate reading nomograms! So I took the one on the second page of this Holley document on carb selection (for sizing mechanical secondary Holleys):
...and fit a second order polynomial to the inputs (minimum WOT rpm and displacement in cubic inches):
cfm = 0.000886*WOTrpm*disp + 0.000159*disp^2 + 0.0000066*WOTrpm^2 - 0.345*disp - 0.0554*WOTrpm + 439
Checking it against that nomogram, it seems to reproduce the results of drawing a line across from WOTrpm through disp to find the cfm needed, within +/- 6 cfm, when I checked it against the 10 data pairs I used to read off the graph and then check it. It all really hinges on how closely you read that graph to come up with the input data, and how accurate the data they used to make the graph with actually was. I used a graph that I tweaked to exactly match each of the 3 axes of the original graph, zoomed in to very precisely read the data off of lines I drew from several WOTrpm/displacemt pairs over to the cfm axes, as well as one graph for each axes to find the non-linear formula for each of those. Like I said, I geeked out.
Here's a spreadsheet I made up to do the calcuations:
(same file as the download, the link is provided above too)
All of my work is shown on the calcs sheet, if you want to check my math
Just put the WOTrpm (minimum rpm for WOT use) and displacement (in cubic inches) in the yellow boxes and the answer appears in the magenta cell to the right of those.
Watch out, we have no idea how accurate the data in that original graph is! Also, they make no mention of whether the type of boosters are, the throttle plate size, or the venturi size. I believe all of these play in the selection.
Also, note that Holley and BG may have a name for a carb based on a cfm number but the total cfm they flow can largely differ. BG's 650 DP Mighty Demon flows 830 according to them, so if you pull 830 off of that graph, you'd want a 650 Mighty Demon!
By Pete Paraska