Updated: Oct 11, 2022
prior to 1955, none of the dynaflows had a "passing gear." it was like just driving around in high gear all the time. so when '55 came around, and they gave the dynaflow the variable pitch feature, it was a night and day improvement in performance. in fact, the 1955 century was the fastest car you could buy that year. the dynaflow did not actually receive a passing gear, in that when you put your foot to the floor, you didn't downshift, but the converter changed stall via a 2 speed converter. this simulated and felt like dropping into low, at WOT when you put your foot to the floor. this feature and design carried on from 55 to 63. people either love or hate the dynaflow. buick was always about luxury novelties, just like the gas pedal start feature for instance. that being said, the real reason the nailhead even exists is because of the dynaflow. the premise of the nailhead and intended design was to simply move a heavy car, quickly from stop light to stop light with a transmission that didn't shift. low end power was number one priority. the nailheads restrictive design was not an accident. i personally never cared for the dynaflow. if i was going to buy a dynaflow car, it would be the first thing i'd get rid of. people can argue, yeah the late dynaflow was actually quicker from a stop than the 400 trans that came out for 64, if you physically shift into low when taking off from a dead stop. that being said, no one can really argue that the 400 and 300 nailhead trans are far more practical and get better mpg around town and are just plain more fun to drive. die hard stock buick lovers will tell you the dynaflow is what makes the buick, its the charm of the whole car. i totally respect that point of view 100%.
when the 400 trans came out for the nailhed for '64, it was a non-vp, single year version. one year only case and valve body. only one low physical shiftable gear. then when the 65 version came out, it was variable pitch with the 2 speed converter which carried on through 66 (for nailheads). the variable pitch feature on these were used entirely differently, which is what can be confusing when explaining it to a lot of people. the VP had two operations. there was a micro switch on the firewall gas pedal linkage with deliberate slop in the actuator rod bushing. when your foot was off the gas pedal, the micro switch would engage, changing the stall of the converter so that it took less pedal effort to hold the car back at traffic lights. this is a great feature. its like having an instant swapable set of rear end gears, so to speak. the second feature is the VP activated part throttle via receiving power from the kickdown switch. that feature i never bother with simulating when we do carb swaps with our plug and play edelbrock kickdown switch. you can also feed power to the variable pitch on custom applications with a toggle switch so you can leave it flipped on all the time if you feel like it. (this causes no ill effect to the transmission.) or you can get power from the brake light switch when brake is engaged. be aware than many transmission shops will try to talk you into removing the variable pitch feature, because they don't want to deal with it, or simply don't understand it. furthermore the 300 smaller variable pitch converter is swapable into the variable pitch 400, for a higher stall. i prefer the 65 and 66 400s for the two physical low gears and the variable pitch... but if you're building a custom car or a hot rod, and a '64 trans (jeep also used a 64 non-vp variation, as well as cadillac for one year and even rolls royce with an adaptor) is what you happen to come across as a "runner," then i would go for it. its still a great transmission. run a tall rear end gear and you don't even need an overdrive and you'll still burn rubber for a block. take note that ONLY nailhead bellhousing transmissions fit nailheads. (chevy and BOP patterns do not.) 53 to 56 (up to 59 and chevy huge trucks) used the 322/264 larger bellhousing. 57 to 66 is the same bellhousing pattern. much more info on trans swaps in other articles on here. 55-56 322/264 cars do not need a special linkage adaptor for carb swaps, as the slider is build into a bell crank behind the cylinder head. the 57 to 63 dynaflows do however, as the slider was built into the carb lever. you cannot just take the variable pitch actuator rod and mount it directly to the carb, or you will only get half throttle and drive around with the VP engaged 24/7.