The good ol’slush box. Late model transmissions, particularly automatics, have advanced way beyond the years of the two-speed Powerglide, in fact there are now 8-speed automatic transmissions that provide a deep first gear with close-ratios and dual overdrives, all of which help the makers achieve the ever-increasing MPG requirements. While these modern equivalents are impressive, many of these transmissions are based on decades-old designs. Take the 727 Torqueflite, considered by many to be one of the most dependable automatics ever built. While it has not been built for 30 years, the overdrive used in most new MOPAR offerings is based on the same design, and even share many internal components.

We recently went through an early ‘70s 727 transmission. Building a transmission is not for the faint of heart. Transmissions require specialty tools and are unlike anything else in a car. You should not attempt to rebuild an automatic transmission using a 4-page magazine article; you need a real tech book that is dedicated to that particular transmission. There are 4 major designs on 727, most of the internals are the same, but there were changes along the way that require specific parts and setting for that year range of transmission. We set out to show you a few of the tips and tricks for building a high-horsepower 727 transmission. Some of these you don’t even have to get inside the case to perform, you can upgrade your trans from under the car.

There are several areas that are critical to survival for a street\strip transmission- convertor, clutches, steels, bands, servos and valve body. There are other components, but this is where the power gets transferred from the engine to the driveline.

Convertor- The torque convertor is spec’d by the engine and not the transmission. The convertor needs to match the camshaft and gearing of the car. For our 727 (which will sit behind a 600-hp 383 Mopar), we picked up a Street Fighter convertor from TCI. This should provide a 3000-stall, which will put the engine right in the middle of its power band.

Clutches- There are several levels of clutches, determined by their color. Blondes are light-duty clutches. This is what you would find in a stock transmission and stock rebuild kits. Don’t use these, they do not have the holding power you need. Moving up the chain are high-energy clutches, Red Eagles, and Raybestos Blue Line. These clutches provide increasing levels of holding power and are made of Kevlar\metallic blends. Most of the time, Red Eagle clutches are good for just about anything, but monster power needs the Blue Line clutch, which has the most hold of them all. Our rebuild kit came with blondes, but we swapped those out with some Alto hi-energy clutches.

Steels- between each clutch (there are several clutches in each pack), rests a steel plate. This is essentially the flywheel to each clutch. With use, these see a lot of heat and eventually they warp and glaze over. There are two main types of steels- stock and Kolene. A Kolene steel is an otherwise stock steel plate that has been treated with a salt bath nitriding process. This increases the nitrogen content in the metal, giving it better wear and fatigue resistance to the tune of 200-500% over standard steel. If you have the option, pick up a set of these, they are better.

Bands- Just like the clutches, the bands are made from different materials. Kevlar\metal blends offer the best holding power of all. For our 727, we went with an overseas Kevlar band because they are just as good as the Raybestos brand, but less costly. For Ford and GM trannys, stick with the Raybestos, the overseas versions just don’t hold up as well for some reason.

Servos- The forward and reverse servos in the 727 are particular weak points. This is what applies the forward and rear bands inside the transmission. If these break, you lose the ability to drive, which happens a lot on these transmissions. It is the single biggest weak spot on this transmission. We upgraded our servos with a set of billet servos from our local Transtar transmission component supplier. The forward servo increases holding power by 11% and comes with a heavy duty strut band. The reverse servo simplifies the application and eliminates the breakage issue.

Valve body- The stock valve body is more than capable of handling its job in a performance transmission, but it needs some tweaks. The brains of the transmission, this is the part that controls the shift points. Part of our TCI master rebuild kit is a shift kit, which allows us the modify the shift patterns, firming up the shifts and tightening the shift timing. Unlike GM and Ford shift kits, this one requires you modify the valve body itself, so be careful.

We spent an entire day rebuilding the 727 in a transmission shop. We took our 727 to a professional transmission builder; this is the best way to go. You need to research your shops though, as most tranny shops only build stock units, you need to get references for local builders of high-performance transmissions, a stock tranny just won’t hold up to 500+ horsepower.