Big block and small block are the two general sizes of V8's available. Generally the small blocks go from 260 cubic inches to 427 ci. Big blocks started at 350 ci and go up to 502, 572, what ever the bore and stroke allow. The 600 and 850, refers to the amount of cubic feet of air per minute that the carb allows in to the engine. 600 cfm would be 600 cubic feet per minute. I wouldn't put that on anything bigger than about 427. 800 cfm, well you get the idea. I wouldn't put an 850 cfm on anything smaller than a 427, unless it had nos, or boost. Hope that helps.