The old 750 and 850 carbs had squared venturi like all Q-Series carbs. Secondary venturi were 1/16-inch larger than primaries on all other old models. 650 through 800s shared a 1-11/16 throttle bore, and venturi diameters were progressively larger. All heck breaks loose with the 850; it was a much larger carb than the 800, with a 1-3/4 bore and venturi that were two steps larger than the linear increases for 800 and smaller carbs. Then the 950 HP came along, with the 750’s venturi and the 850’s throttle bore. It was not a bad carb, just a poorly “sized” one that confused folks. Q-series sizing is linear from the smallest 650 to the largest 1050; a bigger number is a bigger carb. The Q-850 is the same size as the older 950 HP. The old 850 is larger than its linear number suggests, with the same venturi and bore size as the modern Q-1050, which is the ultimate in 4150 Holley performance. The Q-1050 (and Q-1000) will flow more than the old 850 due to the ridiculously excellent blending, recessed air correctors, etc. But the old 850 is still big.