How to Swap a 700R-4 Overdrive Transmission into a C10

We’ve all been there—cruising down the highway in an old truck, just wishing you had “another gear.” The hum of 3,000 rpm on the interstate can be concerning to those of us who constantly stare at the temperature gauge or perhaps the gas gauge, and the persistent strain can take its toll on an old-school engine. In our case, we’re working on a 1971 Chevrolet C10, which has a fresh 355ci small-block Chevy with aluminum heads and a sizable camshaft. While the engine had received some serious attention, the TH350 transmission was completely stock, aside from a stall converter to work with the big cam.

The truck ran pretty well, but had a few issues that were hard to pinpoint. Our concern came when it was time to really test out the new engine on a nice, straight back road. At wide-open throttle in low gear, the engine seemed to bog, and it never came to life, even with close to 400 hp on tap. It wouldn’t spin the tires, no matter how hard we tried. The engine was firing on all eight cylinders, but the truck felt like it was tied to a tree stump when we’d take off. As we were scratching our heads in disbelief that it wouldn’t do a burnout or set you back in the seat, we went out for one more testdrive, and the transmission finally let go. What we quickly realized was that the transmission was causing the bogging issues before it gave up.

The truth of the matter is that we wanted to upgrade to an overdrive transmission either way, and this breakage gave us the perfect opportunity to make it happen. The natural choice for our application was a GM 700-R4, a simple four-speed automatic based off of the long-running Turbo Hydramatic design. The .70:1 overdrive ratio would calm down our peppy small-block during highway cruising, and the lower 3.06:1 First gear ratio (compared to the TH350’s 2.52:1 First gear) would liven up the acceleration. Of course, we could’ve also opted for the 200-4R overdrive transmission, but they’re getting increasingly harder to find and not exactly known for their strength.

We were faced with a couple of options. Grab a core from the junkyard and have a local shop go through it with factory-style replacement parts, or buy an aftermarket unit. After some studying and price checking, we called up the folks at Phoenix Transmission Products in Weatherford, Texas. They offer transmissions for lots of applications, including Chrysler and Ford, and had plenty of options for GM transmissions, including several versions of the 700-R4.

After a conversation with Greg at Phoenix Transmission, we decided on the “SS” model, the street/strip version, rated for 450 hp and 500 lb-ft of torque. Phoenix Transmission modifies the transmission with high-load capacity sprags, high-energy friction and band, and increase pump volume and pressure. To keep us from piecing together the components, Phoenix Transmission offers a complete package for our C10, including the new transmission, a high-performance lock-up torque converter, a COMPUSHIFT Mini module, a G-Force crossmember, and a Lokar stainless steel TV cable, complete with the appropriate correction bracket for our Demon carburetor. That means the only thing we had to add to the kit was a new driveshaft and a few quarts of Dexron III transmission fluid, and our C10 was back on the road.

The final result of our transmission swap is a classic truck that hits the highway with ease, thanks to overdrive and a lock-up torque converter. It also lights the tires with ease now that the new transmission isn’t holding back the rowdy small-block. We were pleased with the simplicity of the swap, and the performance of the Phoenix Transmission-prepared 700-R4 is undeniable. Now it’s time to stretch the legs on our C10 with some longer trips and some more back road adventures.
































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