The Motion gear drive kit was developed because back then, there was a definite attitude in enthusiast magazines that vacuum secondaries were for station wagons, not for performance. We know more nowadays of course. Vacuum secondary carbs produce just as much power as a mechanical secondary carb, they're just harder to dial in because they don't have a secondary metering block, just the plate. Drilling holes was time consuming, and swapping in a metering block was easy. Besides, you drill some holes in that vac secondary plate too large, you're sort of screwed. That's why most shops opted to replace the plate with a block, and then go to the gear drive. More of a shot off the line, in race trim, with slicks, but there's a lot to be said for vac secondaries on a street car. Once the butterflies are opened up, there should be no difference. The problem is tuning that secondary....
Just for the record, the larger Holley 3-barrel is a 1030 cfm unit, not a 1050. The only difference between the Holley 950 3-barrel (3916) and the 1030 cfm (4604) is that the 1030 uses dump tubes for the secondaries, no boosters, similar to the way the Q-jet works. The primary circuits are exactly the same as an 850.
I've been using a 1030 on the street for several years now, and have swapped back and forth with a 950HP. The 950HP is more responsive, understandable as it has smaller venturi. Wide open throttle, seat of the pants, can't tell a difference.
I know that's not scientific, just wanted to throw away a couple pennies today, so that was my .02 cents.
