Not sure how much of it was from internal friction and rotating weight, but when I had a C6 in my Fairmont, it was a fairly good piece, had all the low drag Torrington bearing thrust washers, but still had a stock steel drum. When I swapped out the C6, to a Jerico 4 speed, with no other changes to the car, the 1/4 mile numbers improved by over 5 tenths, and 6 MPH. And that was with a 8" torque convertor, and a Art Carr transbrake/reverse pattern valve body, launching at WOT at 5200 RPM. Many of the Stock and Super Stock Ford racers are using big $$$ custom built C4s behind their FEs and 429/460 engines, but some are running equally expensive C6s, that have had all the heavy internals, replaced with much lighter, MoPar 904 guts. Ritchey Pauley told me that his Pro Trans C6 was about as good as his C4 in back to back testing. As for Powerglides, several years ago, NHRA started to allow cars that were only available with 2 speed automatics, use a 3 speed. So, now , most former Powerglide Stock and S/S Chevys are running either a trick lightweight Turbo 350, or Turbo 200, instead of a trick Powerglide. The racers that I know that has switched from a 2 speed to a 3 speed, say that the lower first gear ratio, gets the car off the line much harder, and the closer ratio drops, help keep the engine in the power band.