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.
We're making progress on our...
We're making progress on our 2010 Dodge Challenger bracket car and can't wait to get it to the track. This month we'll build a 904 TorqueFlite transmission for our car and show you what parts it takes to handle late-model Hemi power.
01 1973 and later 904 transmissions...
01 1973 and later 904 transmissions are the best for performance builds, but they're also more difficult to find since many were the undesirable lockup styles. To make things easier, we got a race prepped case from A&A Transmission, which offers several advantages over a stock case. The case is completely cleaned and glass beaded, and offers improved lubrication and venting as well as a machined provision allowing for line-pressure adjustment from outside the transmission. Additionally, the bell-housing portion is cut out for easy access of torque converter bolts. For the money, this case gives some great advantages when starting your transmission build.
02 A nice feature of the...
02 A nice feature of the 904 when compared to the 727 is that the outer race of the over-running clutch is riveted in the case, instead of pressed in like the 727. This eliminates the need for a "bolt-in" conversion race which is a necessary upgrade when building a race 727.
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.
|A&A race prepped 904 case||$275.00|
|A&A billet four-piston kickdown servo||$110.00|
|Mopar Performance adapter Flex Plate||$119.95|
|SFI approved shield||$315.00|
|TCI 904 Racing master overhaul kit||$152.81|
|TCI 904 Pro trans-brake valve body||$831.25|
|TCI 904 deep aluminum pan||$158.39|
|TCI Outlaw shifter||$350.00|
|A&A 1350 904 rear yoke||$225.00|
03 Another trick part we...
03 Another trick part we got from A&A is this billet aluminum dual-ring kick down servo. This servo won't rock in the bore or jam like the factory unit, comes with a heavy duty spring and sealing rings, and offers a more positive application of the kick down band for firmer shifts.
04 We'll be using clutches,...
04 We'll be using clutches, steels, bands, bushings, gaskets, and seals from a TCI master overhaul kit in our 904 transmission. We also ordered a new kick down lever to replace the factory unit. Over the years, we've found TCI transmission internals to be top quality and well suited for racing applications.
05a The TCI trans-brake valve...
05a The TCI trans-brake valve body requires modifying the high-gear piston by drilling a hole in a specified spot and...
05b ...also enlarging a hole...
05b...also enlarging a hole in the transmission pump housing.
06 The solenoid for the trans-brake...
06 The solenoid for the trans-brake controlling the low/reverse band is electrically actuated, so the case must be drilled, and tapped with a pipe tap to accommodate the electric connection. TCI provides all the necessary pieces along with the valve body.
07 The early model 904 core...
07 The early model 904 core transmission we used for some of our internal parts had factory four-pinion planetary gear-sets. For a performance or racing application, factory four-pinion planetary gears are certainly recommended unless you have the funds to upgrade to aftermarket planetary gears. Stay away from factory three-pinion planetary gears as they are prone to fail in high-powered applications.