Yes, the Rochester RamJet system was mechanical. One article I found mentioned that on a Pontiac Bonneville with HydraMatic, that "a kid on a bike" could beat it off the line. Yet it seemed to work pretty well in Corvettes with manual transmissions.
Kind of like the Chrysler Ram Induction of that time, there were runners inside of the main housing which could be shortened to raise the rpm level of the "tuning" of the intake system. But with all of the Chevy engines it was on (small blocks of less than 350cid), I suspect it was pretty much dialed-in for that size of motor to begin with. I"ve seen mention that on the Sting Ray coupes, they would get 20mpg on the road, with a 4.56 axle ratio, which would be pretty amazing for back then, with that axle ratio, and the level of acceleration performance the vehicle had.
In one respect, I've wondered how things might have progressed IF GM had further replaced their 4bbl engines with the RJ system across the board, IF Bendix/Chrysler had better de-bugged the ElectroJector and used it on more engines, WHAT would Ford have done back then?
As with the later Tuned Port Fuel Injection of the '80s, Bendix pitched their system to GM first, but I suspect that if you back-date the first appearance of the RamJet system in Corvettes to when work on it was first started, it could well have been the Bendix system being promoted to GM that got Rochester Carb working on an "analog" system that would be less expensive to manufacture and such.
Back then, mechanical FI was all over Europe, due to their higher fuel prices and taxes, typically. They had variable-ventur_ carbs, too, plus the "performance" Weber carbs. ALL of which made the best we had in the USA look primitive. BUT our carburetors were pretty reliable and trouble-free by comparison. So, we got "what works that costs less to make".
I need to note that the RamJet fuel injection system on recent/current GM crate motors is fully electronic in nature and bears NO resemblance to the mechanical Ram Jet Fuel Injection used by Chevrolet in the later 1950s, which lasted until about '65 on Corvette 327/375 engines. Two completely different mechanisms!
CBODY67