The 6bbls I’ve run on the dyno always used the pretty soft springs, and the secondaries would pop open surprisingly early when opening the throttle while bringing the motor up against the load.
Never bothered to take a reading off the secondary vacuum lines though.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
