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Upgrading the Turbo 400 in Your Corvette

Building The Brute: Chevrolet’s Turbo-Hydramatic 400 is the toughest domestic automatic ever done

Jim Smart May 8, 2018
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General Motors’ Hydramatic Division can take credit for having designed and produced the most rugged and dependable domestic automatic transmissions ever done. In fact, the Hydramatics are so darned dependable that Rolls-Royce has been installing them in its high-end luxury rides for the rich for decades. We don’t even know how many other applications around the world are fitted with Hydramatic transmissions.

The two-speed Powerglide transmission was GM’s mainstream automatic for ages. In the mid-1960s, GM developed two “next-generation” three-speed automatics. The Turbo-Hydramatic 400, or Turbo 400, for high-torque/high-horsepower engines was introduced in 1964 in GM brands such as Cadillac, Buick, Oldsmobile, and Pontiac. The Turbo 400 arrived in most Chevrolet vehicles in 1965 and in the Corvette in 1968. The light-duty Turbo-Hydramatic 350 three-speed automatic arrived in 1969.

These two Turbo-Hydramatic transmissions became the industry gold standard for every automatic out there. They performed reliably and flawlessly for decades before tougher fuel economy standards made them obsolete in the ’80s. The 700-R4, 4L60E and 4L80E overdrive automatics replaced these fiercely reliable transmissions. GM knew to adapt the proven gear trains from these two automatics into these high-tech overdrive automatics. The 4L60E employs the Turbo 350’s gear train. The 4L80E, as you might expect, copped the Turbo 400’s internals.

Right off the assembly line, the Turbo 400 is a rugged performer. It can withstand a tremendous amount of torque without breaking a sweat, which made it a logical choice for the all-new C3 Corvette in 1968. Prior to 1968, Corvettes got the two-speed Powerglide automatic transmission.

The Hughes Performance Turbo 400 is all about improving a proven performer. Hughes has plenty of experience with the Turbo 400. It has been building and upgrading them consistently since the company’s founding in 1971. All internal components are carefully inspected, with many of the original hard parts replaced with Hughes Performance components. Hughes replaces the original clutch discs with its special high-coefficient one-sided frictions for long life and greater durability. Kevlar bands have been installed in Hughes transmissions for over 15 years.

Hughes Performance co-developed “High-Static Hold” brake band material in cooperation with BorgWarner for increased holding power, especially in racing applications, for all of its brake band applications.

All hydraulic and valvebody circuits are carefully inspected and modified by Hughes Performance for cooler and quicker operation. Hughes has built countless numbers of Turbo 400 transmissions for a wide variety of vehicles and has earned an enviable reputation from racing to towing and all types of street performance, hot rods and custom applications. You can buy one off the shelf or you can send yours to Hughes for a complete rebuild any way you wish.

We turned our ’68 Corvette’s numbers-matching Turbo 400 transmission over to Hughes Performance’s capable technicians for a complete rebuild. They handed the transmission back to us in its natural finish and factory markings ready for installation. We did encounter one problem with our Turbo 400 case and that was a cracked bellhousing, which was repaired by Hot Rods by Dean before it went to Hughes.

The Turbo 400 transmission, more than 50 years after its inception, remains what is undoubtedly the toughest high-torque automatic ever produced. And, with Hughes’ resources and talents, it will remain in industry legend ready to take on the world. Vette

02 Hughes Performance Turbo 400 1968 Corvette 2/31

1. The rich architecture of the Turbo-Hydramatic 400 transmission demonstrates why GM spent a lot of time developing this automatic to begin with. The Turbo 400’s hard parts can withstand extraordinary amounts of horsepower and torque. In areas of the Turbo 400 where it could use a little help, Hughes installs key components to make them capable of withstanding greater sums of horsepower and torque.

03 Hughes Performance Turbo 400 1968 Corvette 3/31

2. Hughes Performance uses only good cores for its transmission builds. It has become increasingly difficult for Hughes and other builders to find good Turbo 400 cores. If you have a Turbo 400 core or know anyone with good Turbo 350 and 400 cores, you are encouraged to contact Hughes. It will buy good Turbo 350 and 400 cores all day long.

04 Hughes Performance Turbo 400 1968 Corvette 4/31

3. The Turbo 400’s input shaft is an industry winner because it is a tough survivor in stock high-performance applications. Hughes can upgrade your Turbo 400 with a steel billet input shaft for greater torque capacity.

05 Hughes Performance Turbo 400 1968 Corvette 5/31

4. The Hughes Turbo 400 forward drum, rear planet, and intermediate shaft give this transmission greater torque capacity thanks to 4340 billet machined steel shaft (right). On the left is the stock input shaft. Can you see the difference?

06 Hughes Performance Turbo 400 1968 Corvette 6/31

5. Here’s the difference between an upgraded 34-element intermediate sprag and drum (top) and the Hughes extreme-duty 36-element sprag and drum (bottom). Note the 36-element sprag and races are wider. Not only do you get 36-elements, you get more width.

07 Hughes Performance Turbo 400 1968 Corvette 7/31

6. A closer look at the 34- and 36-element sprags side-by-side. The sprag (roller clutch) on the left illustrates greater capacity via an increased number of rollers and greater width.

08 Hughes Performance Turbo 400 1968 Corvette 8/31

7. Here’s a closer look at the 36-element sprag and stock direct drum modified for the 36-element sprag. The combination of the 4340 steel input shaft and the larger 36-element sprag gives you unmatched strength.

09 Hughes Performance Turbo 400 1968 Corvette 9/31

8. Before you is the difference between the OEM forward drum (left) and the modified forward drum (right) engineered for the larger Hughes billet input shaft. Note the increased number of splines in the modified forward drum.

10 Hughes Performance Turbo 400 1968 Corvette 10/31

9. This is the Hughes steel billet input shaft for the Turbo 400, which has a spline count for the torque converter on one end and an increased number of splines for the modified forward drum. You’re not going to break this shaft.

11 Hughes Performance Turbo 400 1968 Corvette 11/31

10. Here’s the Hughes steel billet input shaft stuffed into the Hughes-modified forward drum.

12 Hughes Performance Turbo 400 1968 Corvette 12/31

11. There really is a difference in clutch hubs. Stock cast-iron clutch hubs can be fragile, even in a Turbo 400. On the right is a stock GM cast-iron clutch hub. On the left is the Hughes forged steel clutch hub with a Torrington bearing for reduced friction. By contrast, the cast piece on the right has a thrust washer.

13 Hughes Performance Turbo 400 1968 Corvette 13/31

12. Hughes replaces OEM clutches with special high-coefficient one-sided frictions for longer life and greater reliability.

14 Hughes Performance Turbo 400 1968 Corvette 14/31

13. Torrington bearings are great friction reducers and a vast improvement over thrust washers. Here, Hughes uses a Torrington bearing in place of a thrust washer on the clutch hub.

15 Hughes Performance Turbo 400 1968 Corvette 15/31

14. Here’s another look at the intermediate shaft options for the Turbo 400. The stock intermediate shaft on the left is suitable for street/strip applications. On the right is the Hughes steel billet shaft when horsepower begins to ramp up to 600-1,000.

16 Hughes Performance Turbo 400 1968 Corvette 16/31

15. If you’re planning rough and tumble off-road, high-performance driving you need these Raybestos intermediate and direct clutches and plates.

17 Hughes Performance Turbo 400 1968 Corvette 17/31

16. A good rule to remember on clutch frictions is gray Raybestos clutches for off-road and red clutches for street and strip.

18 Hughes Performance Turbo 400 1968 Corvette 18/31

17. We’re building a street Turbo 400, which calls for red clutches and plates.

19 Hughes Performance Turbo 400 1968 Corvette 19/31

18. A word on friction bands goes like this. Hughes Kevlar front bands (front) offer tremendous holding power. The larger, thicker band is the Turbo 400’s reverse band.

20 Hughes Performance Turbo 400 1968 Corvette 20/31

19. Although a lot of transmission builders grab planetary cores off the shelf, Hughes doesn’t do that. Hughes rebuilds its planetary cores as necessary using the parts shown here. We’re talking new pins, washers, bearings and gears. In short, new planetaries.

21 Hughes Performance Turbo 400 1968 Corvette 21/31

20. The accumulator piston and springs are replaced. The accumulator stores control pressure for shift function. If you have a broken accumulator spring, you may get a harsh shift.

22 Hughes Performance Turbo 400 1968 Corvette 22/31

21. Servo pistons are what apply pressure to each of the friction bands. This is the reverse band servo piston, which gets new seals.

23 Hughes Performance Turbo 400 1968 Corvette 23/31

22. This is the Hughes HP2215 Turbo 400 trans brake kit for drag racing.

24 Hughes Performance Turbo 400 1968 Corvette 24/31

23. Hughes Performance goes through every single valvebody; machining and inspecting all surfaces before the valvebody is reassembled and installed. This is the Turbo 400 cast-iron valvebody.

25 Hughes Performance Turbo 400 1968 Corvette 25/31

24. Old-school automatics like the Turbo 350 and 400 get adjustable vacuum modulators for precise shift control. Vacuum modulators function based on load and throttle position.

26 Hughes Performance Turbo 400 1968 Corvette 26/31

25. The Turbo 400 governor, which operates based on vehicle speed (output shaft speed), works hand-in-hand with the vacuum modulator and kickdown to control shift points.

27 Hughes Performance Turbo 400 1968 Corvette 27/31

26. Here’s a Hughes Turbo 400 freshly completed and ready for the transmission dyno.

28 Hughes Performance Turbo 400 1968 Corvette 28/31

27. All Hughes Performance automatic transmissions are dyno-tested behind an engine and performance tested. This means the torque converter and front pump are primed and good to go. Pressures are tested. If the performance doesn’t measure up, the transmission is returned to the line and disassembled for inspection.

29 Hughes Performance Turbo 400 1968 Corvette 29/31

28. Our Turbo 400 had a cracked bellhousing, which Hot Rods by Dean managed to save by welding and return into service.

30 Hughes Performance Turbo 400 1968 Corvette 30/31

29. This is our Turbo-Hydramatic 400 fresh from Hughes Performance. Normally, Hughes Performance will paint the casting in a gloss black unless you specify a natural finish. Inside are all-new Hughes Performance hard and soft parts along with a new vacuum modulator and torque converter.

31 Hughes Performance Turbo 400 1968 Corvette 31/31

30. Hot Rods by Dean has mated the Turbo 400 to our 350ci small-block, and this union is ready for installation.

Photography by the Author

Sources

Hot Rods By Dean
Phoenix, AZ 85027
623-581-1932
www.hotrodsbydean.com
Hughes Performance
Phoenix, AZ 85009
602-257-9591
www.hughesperformance.com

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