When it comes to automatic transmissions, few names carry as much weight in the performance world as the TH350 and TH400. Built by General Motors during the golden era of muscle cars, these three-speed automatics became the backbone of hot rods, drag cars, and heavy-duty workhorses alike. Both have a proven reputation for strength, reliability, and adaptability, but deciding between them isn’t always straightforward.

At first glance, the TH350 is the lighter, more compact option, while the TH400 is the heavy hitter designed to take massive torque loads. But the story runs deeper than just weight and horsepower ratings. Each transmission was engineered for a specific role in GM’s lineup, and today both remain popular choices among enthusiasts, racers, and restorers. To make the right call, you need to understand how they differ in design, performance, and application.

GM’s Hydramatic automatic transmission debuted in Oldsmobiles in 1939. This design would evolve to become the Turbo-Hydramatic in the ’60s.

A Brief History

General Motors made history in 1939 when Oldsmobile introduced the Hydramatic, the world’s first mass-produced automatic transmission. Its four-speed, dual-range design delivered smoother shifts and a far easier driving experience than the manual gearboxes of the era. The Hydramatic quickly proved its durability as well. During World War II, it was adapted for military vehicles, accelerating the development of stronger and more sophisticated automatic transmissions.

By the 1960s, GM had advanced the design with the Turbo-Hydramatic series. The heavy-duty TH400 debuted in 1964, built to handle the torque of big-block engines and the demands of towing. Its reputation for strength and reliability made it a staple in performance and commercial applications alike. A few years later, in 1969, the lighter TH350 arrived, striking a balance between durability and efficiency, making it a favorite for smaller cars and moderate-performance builds.

The TH350 (top) and TH400 (bottom) look similar, but are vastly different in size. (Image source: TCI)

The TH400 gained a reputation for being one tough transmission very quickly, but not just among hot rodders and drag racers. There were several outside of GM that sought the need for a simple and robust automatic transmission throughout the 60s, 70s, 80s, and into the early 90s. The TH400 could be found in some Jeeps, the Avanti II, Checker Marathon and Checker taxi cabs in New York City, Rolls-Royce and Bentley luxury cars, V12-powered Jaguars, the V12-powered Ferrari 400 and 412, and even the Nissan Prince Royal — a 7,000-pound ultra-luxury Japanese limousine made in the late 60s for the royal imperial household and emperor of Japan.

Although both transmissions were eventually retired in favor of more fuel-efficient designs, their legacy endures. The TH400 and TH350 became the backbone of American high-performance builds throughout the ’60s, ’70s, and ’80s, and they remain highly sought-after today. Enthusiasts and builders still rebuild and adapt them for modern street machines and race cars, proving that GM’s early automatic technology continues to earn respect decades later.

This cutaway of a TH350 from ATI Performance shows the inner workings of the popular three-speed automatic.

Design Differences: TH350 vs. TH400

Although both are three-speed automatics made from aluminum housings, their construction tells you everything about their intended purpose. The TH400’s case is made from a thicker casting, filled with larger internal components, and a 32-spline output shaft, making it rugged by design. Its stock torque rating of 450 lb-ft is conservative, and modified builds can easily support over 1,000 horsepower. Its clutch packs and band offer more surface area to hold higher loads for heavy-duty operation, making it a favorite in drag racing and towing applications. But the tradeoff for this capability is heavier weight and a higher rotational mass, which saps a little more power than its TH350 little brother.

The TH350 is slightly smaller and around 20 pounds lighter than the TH400. It was designed for cars in the 350-horsepower-or-less range, making it perfect for small-block V8s and mid-size GM cars of the late ’60s and ’70s. The case is made from a smaller and slightly thinner casting, filled with smaller internal components, and a 27-spline output shaft. With aftermarket parts and tuning, the TH350 can be built to rival the TH400, but its lighter construction and smaller size remain an advantage in performance builds where weight reduction, less rotational mass, and limited space matter.

TH350 Gear Ratios:

• • 1st Gear: 2.52:1

  • 2nd Gear: 1.52:1

  • 3rd Gear: 1.00:1

TH400 Gear Ratios:

• • 1st Gear: 2.48:1

  • 2nd Gear: 1.48:1

  • 3rd Gear: 1.00:1

On the left is a transmission yoke for the 27-spline TH350 and on the right is the much larger 32-spline TH400 yoke. (Image source: Holley)

Modifications and Aftermarket Support

Both the TH350 and TH400 benefit from decades of aftermarket support, with builders offering everything from shift kits to heavy-duty clutch packs and torque converters. Because both transmissions are so common, rebuilds are relatively affordable, and finding a competent transmission builder is rarely an issue.

With the right upgrades, the TH350 can be built to handle 700-plus horsepower while retaining its lighter weight. Popular mods include shift kits and a higher stall torque converter to up the power capacity to around 450 horsepower. Aftermarket valve bodies offer superior fluid control to handle higher line pressure for higher load capacity in racing environments. Combine the aftermarket valve body with higher-capacity clutch packs, upgraded drums, and new shafts made from stronger alloys, and you can have a TH350 that can tackle the drag strip with a 700-plus horsepower capacity.

Already a powerhouse, the TH400 can be pushed into four-digit horsepower territory with simple aftermarket upgrades. Most professional transmission builders retain the stock case well past 1,000 horsepower. Drag racers often use manual valve bodies with a transbrake to fine-tune launch RPMs and improve consistency at the strip. The aftermarket also offers cases and bellhousings to handle 2,500 to 3,000 horsepower and more.

Longevity and Reliability

The TH400 has earned its place as one of the most reliable automatic transmissions ever made. That’s why it became the go-to for big-block muscle cars, tow rigs, and race cars alike. The TH350 is no slouch, either. In street-driven cars, it offers plenty of durability, and with proper maintenance, it can hold up for decades. For daily driving, its efficiency and lighter weight often make it the smarter choice. In stock or lightly modified form, both of these transmissions can last over 100,000 miles with ease.

Daily Driving vs. Racing vs. Heavy-Duty Use

Choosing between the TH350 and TH400 largely comes down to how you intend to use your vehicle. For daily driving duty and medium performance use under 500 horsepower, the TH350 is going to be the better choice. A TH400 can be daily driven like it was in millions of cars and trucks from the factory, but if you don’t need the extra capacity, the TH350’s lighter weight and lower internal drag are preferred.

Once your power output exceeds around 500 horsepower, or you plan to drive your project hard at the local drag strip or off-road trails, the TH400 wins hands down. Its strength, durability, and proven track record make it the right choice for handling massive torque.

Which One Should You Choose?

At the end of the day, the TH350 and TH400 are very capable transmissions with massive aftermarket support. Pick the TH350 if you’re building a lightweight street car, a bracket racer, or a performance machine where shaving weight is more important than brute strength. With the right aftermarket parts, it can still handle serious horsepower while staying compact and efficient.

The more clutches that can be fit inside the case and drums, the more power it can hold. Some lighter-duty OEM applications came with fewer clutches than normal, like you might find in a TH350 behind a six-cylinder engine.

Choose the TH400 if your car is running a high-torque big-block, you’re planning on towing or hauling heavy loads, or you want bulletproof durability for drag racing at extreme power levels. Its heavier build pays off in sheer reliability.

Decades after their debut, both the TH350 and TH400 remain legends in the performance world. Whether you’re restoring a muscle car, swapping drivetrains, or chasing quarter-mile times, GM’s Turbo-Hydramatic transmissions continue to deliver the perfect blend of performance, strength, and history.