Michael Galimi
April 1, 2009

Superchargers, stroker motors, nitrous, and anything else to make a Three-Valve modular motor scream are all the rage with S197 owners. Seemingly half of the '05-newer Mustangs we encounter are over-the-top these days. It might be a bit exaggerated, but no one can deny the popularity of the S197 and the high-level of mods that its owners perform. The aftermarket has many solutions for engine combinations as well as the rearend, but if you are looking for viable options for the middle, the pickings are scarce. There are the usual five and six-speed manual transmission options from Tremec, with an assortment of clutches from great companies like Centerforce, Fidanza, Spec, RAM, and ACT. Turning to automatics, the usual suspects like a C4 or the immensely popular TH400 conversions are plentiful. But what if you wanted to run an overdrive auto transmission in your street car? As power goes up, the reliability factor of the 5R55S goes down, and unfortunately there isn't much that can be done for big power combos.

The stock S197 auto transmission is praised for its strength in stock form, which is reliable to the mid 500 hp (at the tires) range. We can't say enough about the stock transmission with its five-forward gears, which works great in most applications. In a lot of 10- and 11-second combos, we would even go so far as to say it would hang with a manual transmission. It is stronger than most other factory transmissions (both manual and automatic), given its ability to survive in some pretty heavy-duty combos. Some tricks are used to keep the 5R55S reliable in most applications, like pre-heating the fluid and employing a deep sump pan. When the horsepower and torque climb to big levels, like the ones achieved with stroker motors and supercharger/turbocharger combinations, the reliability factor goes out the window. There isn't much to do inside the transmission to get it to survive the abuse in the long-term.

"The basic design and physical size of the components prevent the 5R55S transmission from being built to hold up behind a powerful engine," proclaims Len Bertrand of LenTech Automatics. "The 5R55S transmission uses bands for the 1-2, 2-3, and 4-5 shifts, which are like drum brakes [in the metaphoric way.--Ed.] The drums don't like a lot of speed, are simple, and prefer to be slow moving. The 4R70W uses clutch packs for the 1-2 and 2-3 shifts, which are like disc brakes, and more high-performance oriented. Along with adding new clutch packs, we use a special valvebody that allows us to split the power through the reverse input clutch, enabling the 4R70W to hold more power. Lentch was the first to pioneer this in its AOD in the mid-'90s." The 5R55S drums cannot be upgraded and fortified in the same manner.

Bertrand further educated us on how the internal workings of the two transmissions are vastly different. He says the 4R70W shifts gears more efficiently and in a better manner because its nonsynchros are similar to a TH400. The 5R55S bands must be synchronized (when shifting from Second to Third) in order to change gears. This is where the computer system is critical, as the transmission functions need to be tuned properly. Bertrand said that many late-model tuners modify the shifting functions so the transmission performs a better shift, but since the bands must overlap during the Second and Third gear changes, it is detrimental to the transmission's lifespan. Eventually, the transmission will give up in higher horsepower applications. The 4R70W, on the other hand, doesn't require the bands to overlap during shifting. The next clutch pack just takes over to engage the next gear. It uses clutch-packs and over-run sprag assemblies, which do not require the timing of the gears, nor does the trans have to wait for one of the bands to release like with the 5R55S. The next gear just engages itself and takes over in the 4R70W.