Modern-ish transmissions from a Ford C6

Backwards compatibility is a well-known concept in tech circles, but you need not be a software engineer or Playstation 3 loyalist to appreciate it. The Ford C6 automatic transmission is a great example of this in the automotive world because it’s also the foundation of newer automatics in Ford trucks. And thanks to this shared foundation, Ford’s C6 is ready to upgrade to the refinements implemented in its successors.

Before we get into the details, let’s talk about this particular C6’s home: a triple-black 1972 Continental Mark IV that was a punt of an eBay purchase back in the late 2000s. The seller only wanted $2500 for his estate sale find, so money was transferred, road trip commenced, and one test drive had me swooning. The original owner drilled two holes in the hood for bull horns (tragically not included), which became the genesis of an extensive re-fresh: fresh paint, new vinyl roof, high compression D0VE heads, custom-ground RV cam, Hooker headers (intended for a big block ’72 Grand Torino), and wider steel wheels for more rubber to hold it all down to the road. A pretty and fast car eventually needs even more work, so of course the 48-year-old transmission was that weakest link.

Meet Ford’s 4R100, a four-speed overdrive used on gasoline- and diesel-powered trucks until 2004. It’s the most advanced iteration of the three-speed Ford C6, but you need not perform major surgery** to gain some of its engineering superiority. Adding overdrive to a C6 is physically impossible, but the 4R100 can donate its forward planetary gear set for shorter first and second gear ratios. This creates a “wide-ratio” C6. As the owner of vehicles with Ford’s AOD overdrive and its far, far superior 4R70-W (W for wide ratio) replacement, you better believe I wanted what I saw on the Internet. So I called my transmission guru Gary for his feedback. Not only did he concur, Gary’s depth of knowledge was beyond impressive. To wit:

“The 4R100 forward planetary set improves the C6 first gear from 2.46:1 to 2.71:1, and second gear from 1:46:1 to 1.54:1. CT Powertrain Products offers this upgrade as a kit. The input shell to the transmission case can be tight, so modification of the input shell and transmission case may be required.”

The CT kit doesn’t use genuine Motorcraft parts, but the aftermarket bits are high quality and a fraction of the price. Along with the 4R100 forward planetary set, Gary added a 2200 stall converter, roller bearing clutch, shift kit, and the requisite master rebuild kit (always needed when rebuilding an old gearbox).

So what’s the end result of all these hand-picked parts?  The Mark has tall tires/final drive ratio, but that doesn’t stop the new wide-ratio gearbox from launching the big barge with more spirited acceleration in first gear. Launches are way harder, thanks to a torque converter that’s likely 500+ rpm “looser” than the factory converter, though it’s hard to know the exact number with certainty. Both up and down shifts are swift but invisible because of the aftermarket shift kit.

Sure, you don’t need wide-ratio gears, looser converter or even the roller clutch to perform a good C6 rebuild, but none of the 10+ Ford C6-equipped vehicles I’ve driven downshifted this effortlessly at part throttle and wound through first gear this quickly. Who knows, fuel economy might improve with the roller bearing clutch too.

So yes, this C6 now possesses a large chunk of the refinement present in a modern gearbox, which points to one of the upsides present when an automaker doesn’t reinvent the wheel at every turn.  Thank goodness for backwards compatibility!

**While upgrading to one of its overdrive-laden descendants (or a pricey Gear Vendors kit) was considered, the rare and expensive 1989–91 E4OD, or the computer-controlled E4OD/4R100, and finally a 4R70-W conversation will be more than double the cost of a well built C6. All three options require fabricating a new crossmember, a fair bit of trans tunnel modification, hacking up my a brand new exhaust, etc., above and beyond the cost of rebuilding whichever gearbox is chosen.