I've been very happy with the two Q-F double pumpers I've used. They were vastly superior to the single Holley I've had. But to be fair, that was an entry level vacuum secondary carb that I never could get to set up and run well. Q-F's are a very similar design as Holley, in fact some parts are interchangeable. But they were sold as upgrades to the Holley style product. Including such features as built-in sight glasses, machined aluminum parts vs. castings, interchangeable jets, bleeds, etc. in places were it was necessary to drill to adjust a Holley. There are now premium Holley's that are probably on par with the Q-F's. Plus the situation is a little more muddled I guess since Q-F is now part of the Holley Corporation. But I would still highly recommend Q-F. I specifically used the SS-650 and SS-750-AN which has the annular boosters.