I have been wondering forever how much more rotating mass a 12 bolt has over a 7.5 10 bolt. What size ring gear does a 12 bolt have? I am trying to figure out how much more engine power a 12 bolt takes to turn.

 

don't quote me on this but i think 8 7/8"

 

Correct.

 

There's more to it than rotating mass.... all the bearings are bigger, there's more preload, etc.

"They" say (and as we all know, when "they" say something, it's just got to be true) that a 12-bolt usually shows up as about a 10-12 RWHP hit on a chassis dyno. A F*rd 9" is in the same range. But I've never done a back-to-back comparison myself, so take that for what it's worth.

 

RB, what exactly are you saying about the dyno thing with the 12 bolt "hit"? It shows up as a 10-12 loss? I know I've read something about the 12 bolt causing less frictional drivetrain loss because of it's pinion offset. It's small, something like 1.5 (?) inches compared to something like 2.25" (?) for a Ford 9". So if you could elborate more on what your saying I'm sure we could learn something. Thanks.

 

I think he is just saying that you loose that much power from turning the larger heavier parts so it never gets to the ground.

 

Right.... "they" say that on average, a car with a 12-bolt will measure about 10-12 HP lower than an otherwise identical car with a 10-bolt. The real-world difference between a 12-bolt and a 9" are obscured by indivivual variation from one specimen to another. There's alot more losses going on in there than just the gear mesh, and most of those other losses are alot bigger than that one.