John – my pleasure, I have a few 8mm pulleys here so was easy to measure the diameters. A 50T is close to 5.0 inch diameter and a 60T is close to 6.0 inches… funny how that worked out. In other words, it calculates to 20 percent overdrive with 50T on top and 60T on crank. For how your 350 ran in quarter with the b&m s/c, I was thinking 4-71 and 20 percent should work well. If you think 20 percent too much to start, bump it up to 5.50 inch diameter pulley on top… would change it to about 9 percent overdriven (not as much fun). Offbeat stuff is “cool” like your intake. I want to make one for a tiny flathead V8-60 (136ci) for a baby roots supercharger I got from France. Alan
p.s. for comparison your b&m s/c with 6” bottom pulley and 3” top pulley is close to 100 percent overdrive…
From Wiki-pedia: positive-displacement superchargers are usually rated by their capacity per revolution. In the case of the Roots blower, the GMC rating pattern is typical. The GMC types are rated according to how many two-stroke cylinders, and the size of those cylinders, it is designed to scavenge. GMC has made 2–71, 3–71, 4–71, and the famed 6–71 blowers. For example, a 6–71 blower is designed to scavenge six cylinders of 71 cubic inches each and would be used on a two-stroke diesel of 426 cubic inches, which is designated a 6–71; the blower takes this same designation. However, because 6–71 is actually the engine's designation, the actual displacement is less than the simple multiplication would suggest. A 6–71 actually pumps 339 cubic inches per revolution (but as it spins faster than the engine, it can easily put out the same displacement as the engine per engine rev).
Here’s some comparison calculations using 6-71 information above with 4-71:
6-71 = 426ci ~ 339ci actual 339/426 = 79.5 percent
4-71 = 284ci ~ 284 x 79.5 percent = 225.7ci actual
b&m = 144ci x 2 = 288ci @ 100 percent overdrive
Answer: 4-71 @ 225.7ci to equal b&m 144 @ 288ci, need to overdrive the 4-71 ~ 20 percent
If a 6 rib works with the b&m s/c, a 10 rib should work for the 4-71 to output the same ci. The belt strength/holding/breaking point would probably be doubled...
