904 race tranny Recipe

What you want to look for is an A999 transmission. These were the 'heavy duty', non-lockup versions of the 904 that generally came behind pass cars with 360s in the mid-'70s. '(74-'78) The difference between a regular 904 and the 999 is the 5 clutch drum. Garden variety 904s generally came with 3 or 4 clutches. Find a low gear planetary set from a later 904 (early '80s) and you're golden. If you have more cubes though, it may not necessary to have so much torque multiplication in the drive line.

Unfortunately though, finding a 'real' A999 is very difficult because they are rare as hen's teeth. Plus, even if you do find one and the case has the right number, there's no way to verify what's inside until you open it up since they are the same externally as a regular 904. You can build them from scratch but you need to be aware of specific parts and how they work together. That is a lot of work in itself.

I have two 999s. One is a spare and one in the car right now. I also have a low gear planetary set on my shelf. They are great transmissions and can be built to the moon with all sorts of high-dollar parts.

They respond to modifications like a a 727. The main advantage of the 904 is less rotating weight in the gear train which helps to prevent explosions. It's rare to hear of a 904 blowing up but a 727 is like a ticking time bomb with the steel front drum.

Here are some threads from Moparts about 904/999 race transmissions, should tell you all you need to know.

Everything you never wanted to know about 904s.

Racers/builders.

How to identify a 999 from a 904.