904 Torqueflite Transmission - New Old School

Of all the components that make up a Mopar performance vehicle, the automatic transmission is likely the least understood both by the general public, enthusiasts, and even some mechanics. It's little wonder though, as so many books and magazine articles are devoted to engine modifications and ways to enhance your car's power and performance, yet very few are dedicated to the inner workings of transmissions. Let's face it, money spent on components like aftermarket camshafts, cylinder heads, and carburetion will certainly enhance a vehicle's performance to a greater extent than the same money spent on transmission upgrades. But at some point that powerful big-block, small block, or Hemi in your Mopar will overpower a stock TorqueFlite transmission, requiring the transmission to be upgraded to handle additional power and torque.

While the 727 and 904 automatic transmissions may seem like black magic to some, the reality is that they operate on basic hydraulic and mechanical principles and can be modified to handle extreme levels of horsepower if the right components are chosen. So when it came time to decide which transmission to build for our 2010 Dodge Challenger bracket car, we carefully evaluated considerations such as the car's weight, tire size and gear ratio, as well as engine power and torque before choosing a transmission. And while some may argue that a 727 would be the logical choice for this application, we picked a 904 for its lighter total weight and lower internal friction.

The aluminum 904 was first introduced by the Chrysler Corporation in 1960, and is a design that evolved from the Chrysler PowerFlite, which was a two speed automatic, and then the cast-iron three-speed TorqueFlite which was durable but heavy. Originally featuring a rear hydraulic pump, pre-1966 TorqueFlites could actually be push-started since the pump was driven from the output shaft rather than by the torque converter as in modern versions. Both the 727 and 904 versions of the TorqueFlite are similar in design, although none of the components are interchangeable between the two with the exception of the valve body, though calibration is different.

Finding a 727 or 904 in a scrap yard is relatively easy. For our relatively high-powered application, however, a 1973 and up 904 unit is preferable since it contains beefier shafts and a larger rear bearing. Finding one of these 904 core transmissions can be more difficult given the fact that many of them were equipped with a lockup torque converter, which means the input shaft is incompatible with aftermarket performance converters like the 8-inch ATI unit we'll be using. With this in mind, we called A & A Transmissions and had them send us a race-prepped 904 case and input shaft, saving us the trouble of searching the scrap yards for a suitable transmission case. A & A also supplied the SFI approved transmission and bell-housing shield to meet NHRA and IHRA requirements.

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