Installing a 904 (or small-block 727) behind a late-model Hemi engine is actually a very simple process requiring only a couple of specialty pieces. With the exception of a single bolt, the bell-housing pattern is the same, but the rear crankshaft flange of the late-model Hemi protrudes further aft than that of a small-block, requiring either a spacer or special flex-plate. The late-model Hemi crankshaft also utilizes eight bolts to hold the flywheel or flex-plate, but the spacing and bolt-circle is different than early Hemis. Fortunately, Mopar Performance has the proper flex-plate for this application, which has both the proper attaching bolt holes and offset, eliminating the need for a transmission spacer. These flex-plates are SFI approved and available from a variety of sources including Mopar Performance dealers such as Mancini Racing Equipment.
With our pieces coming together, the next step was to build our transmission. Assembling a 904 properly requires not only knowledge and experience, but some special tools as well. To handle the task we enlisted local Mopar shop and transmission specialist Tod Struck of Inline Performance Specialists. Tod has put together plenty of 727s for us over the years, and his attention to detail ensures the transmission will operate to the limits of the parts inside it. For those parts, we made a call to TCI.
We're not about to let you know how much power the 426-inch late-model Hemi we built for this car made, at least not yet, so you'll have to look for the details on the engine in a future issue of Mopar Muscle. What we will tell you, however, is that the engine we built with the help of Indy Cylinder Head made enough power that it should propel our 2010 Dodge Challenger to sub-ten second elapsed times in the quarter mile. Elapsed times under 10-seconds will make this car too quick to run in the Pro, or foot-brake bracket classes, so in order to be prepared to run in Super-Pro, a trans-brake is necessary. The trans-brake uses an electric solenoid to actuate the reverse band while the shifter is in first gear, and once the button controlling the reverse solenoid is released the car almost instantly leaves the line. In the Super-Pro (electronics) class, the trans-brake is used in conjunction with a delay box, and we'll explain that in a later article.
08a Automatic transmissions are built in sub-assemblies which are then installed into the
08b Race transmissions require specific tolerances which can be found in the TCI overhaul
09 Installing our TCI trans-brake valve body and deep aluminum pan finishes up our 904 bu
With this in mind, and knowing a good valve body is the key to quick reaction times and sharp shifts, we ordered a Pro-brake trans-brake valve body from TCI. This valve body requires some internal modifications to the transmission to work properly, but Tod at Inline Performance Specialists handled the mods with no problems. For the clutches, bands, and other associated parts needed to complete our transmission we ordered a 904 Master Racing overhaul kit from TCI as well. This kit includes all the normal high-quality pieces needed to rebuild our 904, including the clutches, steels, bands, seals, gaskets, and even bushings. Since we're taxing this 904 with a somewhat heavy car and high-powered engine, we also chose a TCI deep aluminum transmission pan to help dissipate the heat generated, especially in the later rounds of racing. For shifting, a TCI Outlaw shifter with a three-speed reverse pattern will be installed in the Challenger.
Using the above transmission components, along with factory four-pinion planetary gear-sets, shafts, and drums, Inline Performance Specialists got busy performing the necessary modifications and assembling our transmission. When it comes to a racing transmission, we would consider this 904 to contain the minimum modifications necessary to handle a moderately powered, somewhat heavy drag car. Of course there are plenty of parts such as billet shafts, aftermarket planetary gear-sets, and even rollerized components that we could have used to make this transmission even more bullet-proof, but using such components drives the cost up exponentially and shouldn't be required for our application. One of the things we enjoy doing here at Mopar Muscle is testing parts, and these parts will certainly be tested when we take this car to the track. We'll let you know how this 904 holds up to late-model Hemi power in future issues of Mopar Muscle.