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Measure The Diameter of the Exhaust Throat - What's Your Problem?

Durable C4
Chris Rabben; Las Vegas, NV: My question is regarding the C4 transmission in my '69 Mustang. It's time for a rebuild and I'd like to step out of my comfort zone and do the work myself, but I'm having a really hard time finding any books or information on doing a performance rebuild. It has to survive behind a 700hp nitrous small-block, so I need to put something pretty stout together. Are any full rebuild kits out there to handle this power level? I'm going to run a transbrake but just about all the brakes I find are for '70-and-newer C4s. Are these compatible with my '69 trans? I know the input shaft should be changed to the '70 26-spline, but what needs to be changed to swap that over?

Jeff Smith: This sounded like an intriguing question, Chris, so we immersed ourselves in the world of the racing C4. While 700 hp seems like a lot of steam for the little trans that was originally used only in front of small-block cars, the automatic transmission world has built some serious durability into these three-speed autos. The consensus opinion is that unless you have significant experience building performance automatics, the horsepower you propose almost dictates that you leave the buildup to one of the many C4-knowledgeable shops. That's why we're not going to go into a ton of detail on all the specific parts necessary to convert a C4. A shop relatively close to Las Vegas is Mike's Transmission in Lancaster, California. Mike's has a reputation for durable C4s and offers four different levels of C4 automatic starting with the Street/Strip version that goes for $950. The Competition version is $1,595, and the Ultimate C4 lists for $2,495.

For those of you on the East Coast, Dynamic Racing Transmissions (DRT) has earned an equally well-deserved reputation for building durable C4s. DRT also offers various levels of C4 including a trans DRT called the Mighty Mite that sounds like what you're going to need. Both companies recommend the hardened 26-spline input shafts, a solid forward drum, five- and six-clutch packs for the forward and direct drums, a large-diameter servo, and wide bushings. DRT also recommends what it calls a rollerized upgrade that exchanges roller bearings for the factory-style thrust washers and bushings. Roller bearings are capable of greater thrust loads and they also reduce friction, which not only reduces lost power but also lowers heat buildup in the transmission. DRT also has an external transbrake for these transmissions. Talk to Harold Miller at DRT and he can give you more specific advice for your application, including torque converter recommendations. Both of these companies can answer your specific questions concerning input shaft changes, transbrake applications, and other differences between the early and later C4 automatics.

More Info
Dynamic Racing Transmissions

North Branch, CT

Mike's Transmission
Lancaster, CA

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