Hello. I would suggest you read over this information. It is pretty straight forward. If the people at Zenith used the Bore - Stroke - RPM etc. info you gave them this is probably how they arrived at their recommendation. I use Zenith universal carburetors on my 3 'teen cars and they are great. I bought my carburetors from Automotive Test Systems. I have talked to ATS and Mike Framer at Zenith. They are good people. I hope this helps you. If you still have questions, I'll try to help. Carburetor & Venturi Sizes Size of carburetor: The carburetor capacity required depends upon the piston displacement (piston area X stroke X number of cylinders) and upon the service for which the engine is to be used. The capacity of the carburetor is gauged by the throat diameter of the large venturi. To find the size carburetor needed for a given engine, find the cubic inches piston displacement from tables on page 1047; table below will then show the carburetor size and venturi size recommended. (For bore and stroke of different size engines see pages 1058, 966, 996; for make and size of carburetor see pages 1055, 966, 996, of Dyke's Auto Encyclopedia.) Example: Given a four-cylinder engine of 33/t" bore, 5" stroke, installed in a pleasure car. The piston displacement, according to table on page 1047, is 221 cubic inches, and from the table below, we find this to require a 12" venturi, in a 1%" size type "0" or "OS" carburetor. If the engine were used on a truck, it would probably be better to use a 33" venturi, in a type "M" or "MB" carburetor. The tables on this page are computed in the case of the Model "O" carburetor for maximum torque and volumetric efficiency at 1,700 r.p.m. and for Models "M" and "MB" carburetors at 1,200 r.p.m. For faster and slower speeds, a corresponding addition or reduction should be made in piston displacement value before selecting the venturi size. Larger venturi sizes may be used on engines which have proper application of exhaust heat on the intake manifold. For fewer than four cylinders on one carburetor, use the same venturi size as required for four cylinders of the same bore and stroke.