Good question. Though the guts in the OD section are much larger and have more components than the output shaft on a 904 or 727, much like a forged vs cast crank in your engine... More weight takes more power to get rolling.... Would be my guess.
I just got the brand new sonnax trans parts in the mail today, and there's lots of stuff that says 727/518 only, won't work for an a 500, parts like HD servos .... And they don't show equivalent parts that do work.

There must be extra mass to rotate in the OD section and more bearings involved? That OD is probably like adding almost whole nuther trans. If you are not drag racing, it shouldn't be noticeable. I did pick up about 15% MPG with the lockup A500.

Lou

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Home of Slant6-powered fun machines since 1988

oh yeah there definitely is.... 2 more clutch packs in the tail, that the 3 speed didn't have.... not "light" either.

I know the A500 and A518 share an identical overdrive unit and it is interchangable between the 500 and 518. My understanding is that there are a few extra hydraulic passages to run the OD in the tailshaft housing, but otherwise the main body of aan A500 is basicaallly identical to a 904.

Not quite. An od unit won't bolt to a 904/727 as they "puffed out" the front of the tail shaft section.

right now I have an A904 with just a manual valve body swap and an Edge convertor ( not rebuilt - unknown mileage ) behind my turbo 225. So far it has been issue free, fingers crossed it stays that way until I make up my mind on what to do. Would love for the Gill Welding guys to make a run of slant to la / magnum bell housing adapters ( like the A100/A108 had ) in aluminum. think an A500 would be great in my 66.

I just got an original one of those adapters. Had to buy a whole motor and trans for it. I sold both of those.
Now to decide which trans to use in my truck