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Lentech 4R70W Overdrive Unit - Gear Drop
Dump That 5R55S Transmission And Back Your Powerful Three-Valve Mod Motor With A Lentech 4R70W Overdrive Unit.
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.