5R55S Transmission Rebuild - The Modern Generation

The modernization of classic Mustangs and Fords with a modular motor or EFI is a common occurrence. Take MM&F's on-going Generation Gap project car, where Tech Editor Mark Houlahan is reviving a '68 Mustang with a helping hand from his son Kyle. The first order of business was to convert the '68 from a coupe to a fastback-thanks to Drake Customs and Dynacorn. Moving past the visual upgrades and custom work, the Houlahans plan on stuffing an '06 Mustang GT engine under the hood. For those unfamiliar with the modern generation of engines, the '06 Mustang GT features a 4.6L modular engine that boasts three valves per cylinder, and cranks out 305 hp while knocking down 20-plus mpg.

In keeping with the modernization theme, Houlahan turned to the 5R55S automatic- transmission-the unit that is behind the Three-Valve mod motor in the '05-'10 Mustangs.

"The 5R55S is a physically compact unit having five forward speeds with an Overdrive. Like most five- and six-speed automatic transmissions, it has a very low First gear, followed by a pretty standard gear spread after that," noted Tom Cyr of Performance Automatic. First gear features a 3.22:1 ratio, which is far better than any other automatic transmission before it. The rest of the gearing package includes a 2.29:1 (Second), 1.55:1 (Third), 1.00 (Fourth), and 0.71:1 (Fifth/Overdrive).

The transmission's roots go back to 2003, where it saw life in the Thunderbird. The predecessors to the 5R55S are the 5R55N and 5R55W units that came into service in 2002. The series is part of Ford's new generation of electronically controlled transmissions that began showing up in various passenger cars in 1995.

Getting a junkyard transmission is usually a crapshoot, as its former life is unknown, so we turned to Performance Automatic to freshen up the late-model slush box. The company pulled apart the five-speed automatic transmission and rebuilt it to its Super Street specs.

"For its size, the 5R55S has some very strong planetaries, precision roller and needle bearings, and large Intermediate and Overdrive apply areas," stated Cyr. He continued, "Performance Automatic upgrades all of the clutch materials, both Intermediate and Overdrive bands are reinforced and get a high-performance lining, and a new Overdrive planetary is installed. We also change the input shaft to a 300M unit, as it's standard with our Super Street version."

Performance Automatic (PA) makes it easy to fit this transmission behind any Ford modular or 5.0L engine thanks to its Pro-Fit bellhousing system. PA also added a deep-sump oil pan so it holds more fluid. Cyr explained, "The deep trans pan adds extra fluid and when modifications are made to the torque converter, it creates more heat. Most people don't realize that aftermarket converters hold less fluid. This can be compensated with a deep aluminum pan."

A Ford Racing engine wiring harness was used for this project and it's designed for a manual transmission, which left us with two choices. First was to convert the harness over to an auto setup and re-pin it for the transmission plug-in. It's a process that is time consuming and requires a bit of skill at reading schematics. The other option is to add a transmission controller, which PA offers with its 5R55S packages. Add in the Pro-Fit bell and you can back any 4.6L or 5.0L engine with a 5R55S box. The controller is programmable via laptop and made simple for the novice computer users out there. The transmission can also be upgraded with the PA paddle-shift setup.

Generation Gap continues to blend the old with the new and this Overdrive is ready for a long life of reliable service.

Performance Automatic (PA) makes it easy to fit this transmission behind any Ford modular or 5.0L engine thanks to its Pro-Fit bellhousing system. PA also added a deep sump oil pan so it holds more fluid.