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A few years ago Mikuni published its test results on flow testing of its HS40 and HSR42 carburetors against an unnamed (S&S) company's 1 7/8" and 2 1/4" butterfly carburetors. Since these results are basically favorable to Mikuni, they do point out the various points where flow testing can occur.
Air Flow Results |
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| Carburetor | Carburetor Only | Carburetor with Manifold | Carburetor with Manifold and Head |
| Mikuni HS40 40 mm Venturi |
160 CFM | 150 CFM | 140 CFM |
| Mikuni HSR42 42mm Venturi |
200 CFM | 160 CFM | 150 CFM |
| S&S Super E 7/8" Butterfly 40mm Venturi |
210 CFM | 150 CFM | 135 CFM |
| S&S Super D 2 1/4" Butterfly 45mm Venturi |
230 CFM | 160 CFM | 145 CFM |
It is pretty easy to see that by the time you factor in the carburetor, manifold and intake port that the air flow drops considerably from the "raw" carburetor only numbers. It also shows that carburetor air flow is most effected by the size of the venturi, not the butterfly bore.
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