"The valvebodies are also really different. The 5R55S is what I call a second-generation type of electronic-controlled transmission, and the 4R70W is a first-generation. The biggest difference is in the valvebody area, the 4R70W has a more traditional type valvebody, while the 5R55S has what I call a solenoid-body," inserts Bertrand. To sum up his description of the valvebodies, the 4R70W valvebody is a traditional unit, but solenoids control the fluid movement inside the valvebody. The computer can control how firm or soft the shifts are and at specific rpm levels. In the 5R55S valvebody, there are a series of solenoids that are wired into the computer system. Each solenoid is connected to a gear, and the computer activates that solenoid at the appropriate time to make the gear shift. The solenoid opens and feeds fluid directly to the clutch pack and servos. The maze looking valvebody is not present in the 5R55S, like it is in the 4R70W. The S197 trans also has a series of speed sensors to provide information to the computer so it can regulate the shifting. Tuners modify those controls to have the trans shift harder and faster, but according to Bertrand, the transmission will eventually break. It goes back to the internal workings with the overlapping and synchronization of the bands during shifting. The input shaft is very small, as are the stator support and other integrated components. The parts aren't large enough to allow the substantial upgrades necessary to make it a practical option, especially when the 4R70W is available and capable of doing the job.
LenTech rebuilds 5R55S transmissions, and it helps the transmission live longer and healthier. There, however, comes a time when it has to be replaced. Bertrand went on to suggest that for longevity, he recommends removing the 5R55S in anything over 450 hp at the tires. He said in 500-600 rwhp applications, the 5R55S becomes too risky, and failure is only a matter of time. He also said that supercharged and turbocharged applications are more prone to failure at that level than a naturally aspirated engine. He said torque and vehicle weight are what kill the transmissions. We know how easy it is to make power with a Three-Valve engine these days, and the S197's girth is no big secret either.
Justin Burcham offered his two cents on the option for a replacement overdrive transmission, "We have done many TH400 transmission conversions, which is a three-speed GM automatic transmission. It'll handle any power you can throw at it, but that trans is not what all of our customers desire. It is for the hardcore strip cars. We have built a lot of cars with big power, but the owner wants to keep the overdrive for street use, which is when we started looking into 4R70W transmission conversions. Properly built 4R70Ws can hold great horsepower, offer three speeds for racing, and still retain overdrive for highway driving."
One look at the Internet message boards and you can see that 4R70W conversions are becoming more and more popular in the S197 segment, warranting us to take a closer look at what it takes to install an older-style OD trans in an '05-newer Mustang complete with the Spanish Oak computer system. Our curiosity to get a closer look at this popular conversion led us to JPC Racing, where Burcham and his staff were tackling a project for Leander Knight Jr. It involves a serious, ProCharger-blown, stroked 4.6L engine--a perfect candidate for this transmission. "Not only will the 4R70W transmission hold more power than a 5R55S trans, the gearing inside the transmission is better for getting a high-horsepower car down the track," comments Bertrand. In private testing with a turbocharged car, the folks at LenTech found the transmission to be three-tenths better than the 5R55S. The one less gear allowed the engine to be loaded, and kept the turbo working hard. That was in what some would call a mild turbocharged car, and the 4R70W will only work better as the power goes higher.