Mmfp 0904 01 Pl 4r70w  Overdrive Unit Mustang
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.

"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.

By comparison, in racing the 5R55S uses four-forward speeds (plus a fifth overdrive gear) with a gear ratio package of 3.22:1, 2.29:1, 1.55:1, and 1:1 (4th). The overdrive gear comes to this part at a paltry 0.71:1 ratio. The 4R70W carries three-forward gears in racing situations; 2.84:1, 1.55:1, and 1:1. The 4R70W carries a 0.70:1 OD gear. The numerically higher First gear of the 5R55S is great for most 10-second and slower applications, as it helps these heavyweight cars off the starting line. But once in the 9s and quicker, the story changes, as traction becomes an issue with the big horsepower and torque curves. Burcham turned to LenTech Automatics for its conversion kit--a nice little package that includes most of what is needed to replace the 5R55S with a 4R70W transmission. LenTech included most of the larger components, but JPC had to outsource several pieces and parts to make the install a turnkey item--including an '03-'04 Mach 1 transmission wiring harness, a B&M shifter and transmission cooler, and an S197 manual trans computer.

The actual install of the transmission, one-piece driveshaft (with adapter spacer), trans cross member, and shifter were very straightforward and simple. Moving to the wiring, that is where the ballgame changes, but for the worse. It is not overly complicated, but let's be realistic--mating a computer-controlled transmission from one car to another, with two totally different computer systems, can get complicated. There are two options, one easier than the other, yet both are highly effective. The first choice is to go with a full-manual valvebody and handle the shifting with your right arm. This is the easier of the two options, and the one we chose. The other option is to let the transmission do its own shifting--after all it is an automatic--but a separate transmission controller is required. LenTech recommends and sells a controller from Baumann Engineering. The controller is a trick piece that hooks up to your PC and utilizes a Windows-based program to adjust line pressure, torque converter clutch operation, and shift-points for every gear. In either case, the factory Spanish Oak computer system (S197 computer nickname) thinks there is a stick shift trans in the tunnel.

Knight chose a 4R70W Street Terminator Plus, which is rated in excess of 1,000 hp. The electronically controlled trans boasts some really nice features, including LenTech's Stage 2 mid-shaft (26-spline) and beefy 3-4 drum. The drum upgrade increases the torque capacity of the Third and Fourth gear drums. Other internal upgrades include high-capacity clutches for all gears, a beefier overdrive band, and a LenTech valvebody. It also came with a transbrake feature for drag racing. Our unit employs a full-manual valvebody, and we chose a B&M Racing Composite X Pro Stick shifter (PN 81043) as its boss. It is a three-speed shifter with manual reverse lockout as per the NHRA rules. A lever must be moved in order to shift from Neutral to Reverse. It is a safety item so the driver doesn't accidentally shift into Reverse while driving forward. We used a three-speed shifter because our overdrive gear is setup on a button. When in Third gear and ready to get into OD, simply flip the installed switch, and Fourth gear is engaged for your highway driving pleasure.

Now you have to be asking, how did the JPC gang link up the transmission to the factory computer system? In either an auto or manual shifting application, you will need to get a Mach 1 or 4R70W transmission wiring harness (PN XR3Z-7C078-BA). For those going the manual-shifting route, the neutral safety switch plug from your S197 harness goes into the LenTech transmission without any problems. The reverse lights and speed sensor are required to be spliced to the trans sensors since you're combining two different harnesses. LenTech has an instruction manual online that includes a highly detailed wiring schematic. It's so clear and easy that we would rather you reference that diagram directly. There are just a few wires that have to mate up. The lockup and overdrive functions are wired to a button, which we mounted in the cigarette lighter hole and ran like a regular 12-volt power switch.

Kevin MacDonald of DiabloSport handled the tuning, and he informed us that the tune wasn't too different than what one would expect. He said he mostly corrected for the engine's power, 620 rwhp and 578 rwtq. Other than that, the computer simply ignores the transmission, since it thinks there is a manual stick shift box in place. The speed sensors work just fine, as do the reverse lights and overdrive functions. The car starts fine since the neutral safety switch is active.

So what about the driving impression? In a word; fun! That is the easiest way to sum up the experience behind the wheel. For those who have driven a full-manual valvebody automatic ride, it is the same with firm shifts, except you yank the shifter. We love the switch for the overdrive, as you simply run the car up in Third gear and flip the switch as you get to cruising speed. Thanks to the time of year for this article, we were unable to get track performances with the new transmission. Rain and cold weather put a damper on the fun, but Burcham says the car is definitely a nine-second street car.

Converting your S197 to a 4R70W transmission will certainly keep your Mustang on track and out of the transmission repair shop. There comes a time when the 5R55S should be shoved aside for something a little beefier. With winter upon us, now is the perfect time to make the swap to a 4R70W.

Transmission Lingo
Some people might be wondering what is a 4R70W and what is with the goofy name? The transmission name is simple; 4 stands for four-speed, R for rear-wheel drive, 70 is the torque rating (700), and W represents wide-ratio for the gearset. It came in '99-'04 Mustang GT models. Ford also produced the 4R75W, which is the stock '03-'04 Mach 1 transmission and is nearly identical to the 4R70W--with a 750 lb-ft torque rating. The 4R70W trans is a very popular transmission and easy to find, where the 4R75W is a bit more difficult to locate in junkyards and such. Ford also has a 4R100 transmission that comes standard in the Lightning trucks and other models. We have heard of this transmission being used in Mustangs, but the 4R70W is by far the most popular due to the ease of locating a core at a reasonable cost. The 4R70W is also strong enough for just about any combination given the aftermarket's modifications to the internals.