TH350 vs TH400: Differences, Identification, and Which Is Stronger

Two three-speed automatics, two names one number apart, and a swap meet full of people who still mix them up. The Turbo Hydramatic 350 and Turbo Hydramatic 400 are the backbone of the classic GM rear-drive world, they live under everything from grocery-getter Novas to blown big-block gassers, and choosing between them for a build comes down to a handful of real, measurable differences. This is the builder's version: what actually separates the two, how to spot each one in a junkyard without a service manual, and which one you want under a car that makes power.

Working on other GM slushboxes? See our TH400 wear-point guide and the 4L80E heavy-duty guide for the overdrive descendant of the 400.

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The Short History: Different Jobs From Day One

The TH400 came first. Per Novak's transmission reference, it debuted in 1964 in Buick and Cadillac cars, spread to Chevrolet and Oldsmobile for 1965, and was GM's premium heavy-duty automatic. It was so widely trusted that it ended up behind engines in Jaguars, Rolls-Royces and Ferraris, not just GM iron. In 1990 GM renamed it the 3L80 under the new naming convention (three speeds, longitudinal, 8,000-lb GVW class), and heavy-duty truck variants like the TH375 and TH475 shared the same core.

The TH350 arrived later, for the 1969 model year, as the replacement for the aging two-speed Powerglide. It was designed as a lighter, more compact, more efficient three-speed for the bread-and-butter cars, and it stayed in production into the mid-1980s before the overdrive 700R4 took over. GM also spun off downsized derivatives, the TH200, TH250 and TH375-B, for smaller-displacement cars chasing fuel economy. So the two units were never really competitors; the 400 was the strong one and the 350 was the efficient one. Everything else follows from that.

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Size and Weight: You Can Feel the Difference

These are not the same size, and it matters the second you try to fit one in a car built for the other.

• TH350: 21-3/4 inches long, roughly 120 lbs dry, cast aluminum case with an integral bellhousing.
• TH400: 24-3/8 inches long, heavier dry and heavier still filled because it holds more fluid, cast aluminum case, and Novak calls it the largest of the common GM automatics.

That is about 2-5/8 inches of extra length on the 400. In a swap, that translates to a shorter driveshaft and, in most chassis, a crossmember that has to move. The 350's shorter case and lower weight are exactly why it became the darling of street rods and lighter cars where every pound over the rear axle and every inch of tunnel clearance counts.

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How to Tell Them Apart in the Yard

You almost never have a build sheet when you are staring at a used core. Here is the fast field ID, in the order I actually check.

1. The Pan

The TH350 pan is close to square with one corner, the passenger-side rear, cut off at an angle. It reads as a simple five-sided shape. The TH400 pan is bigger and busier, an irregular outline with several angled sides. Both happen to use around 13 bolts, so do not count bolts and call it a day. Read the shape: simple square-with-a-clipped-corner is a 350, complicated multi-angle is a 400.

2. The Kickdown

This is the tell that never lies. The TH350 uses a mechanical kickdown, a cable or detent linkage that operates the downshift. The TH400 uses an electrical kickdown switch with a connector on the case. If you see an electrical kickdown plug and no cable provision, you are looking at a 400. If there is a cable bracket, it is a 350. B&M's installation instructions for GM non-electronic transmissions say the same thing from the installer's side: on a TH400 you make the electrical connection from the kickdown switch, and on a TH350 you connect the kickdown cable and adjust it to factory specs. The same sheet lists typical converter depth, bellhousing face to converter mounting pads, as 1-1/8 inch for a TH350 and 1-3/16 inch for a TH400, so do not assume a converter seated correctly just because it looks like the last one you installed.

3. The Modulator and Overall Bulk

Both carry a vacuum modulator at the right rear of the case. The giveaway there is proportion: the 400's case, bellhousing and tailshaft area are visibly beefier. Set a 350 and a 400 side by side and the size difference alone answers the question. The TH350-C lockup version, introduced late in the 1979 model year, adds an electrical plug on the left side for the converter clutch, so do not confuse that lockup connector with a 400's kickdown switch.

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Gear Ratios: Closer Than People Think

Both are three-speeds with a 1.00 direct third, and the low gears are nearly identical:

• TH350: 2.52 first, 1.52 second, 1.00 third.
• TH400: 2.48 first, 1.48 second, 1.00 third.

The 350's first gear is a hair deeper, which technically helps initial launch off the line by a sliver. In practice the difference is nothing you will feel from the seat; it is well within the range you would swap with a rear-gear change. Neither unit has overdrive, which is the real ratio story here. If you want highway cruising RPM, that is the argument for a 700R4 or 4L80E, not a reason to pick between these two 3-speeds. Choose the TH350 or TH400 for strength, packaging and simplicity, and handle final drive with the rear gear.

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What Makes the TH400 Stronger

The strength gap is not marketing, it is architecture. Novak rates the TH400 conservatively at about 450 lb-ft of input torque, and the reason it holds up is the way the gear train is supported and how low/reverse is applied.

The TH400 uses a bolted cast-iron center support that keys into the case and splits the unit into a stout front and rear section. It applies low and reverse through a band wrapping a drum, which is a robust, well-proven way to hold reaction torque. The TH350, by contrast, does without that rigidly mounted center support in the same form, leans on a large-diameter sun shell, and uses a low/reverse clutch pack with a smaller-diameter low/reverse roller clutch. None of that is fragile for what the 350 was designed to do, but it is a lighter-duty layout.

You can see how seriously the 400's gear train is taken by looking at how it is built. Gears Magazine (ATRA) documents that converter charge pressure of around 60 psi plus lube pressure produces roughly 199 pounds of rearward thrust on the forward clutch housing, which is why builders pay close attention to the No. 22 thrust bearing and the case thrust washers. The direct clutch housing is designed with 0.020 to 0.025 inch of float, and the reaction carrier normally floats 0.050 to 0.080 inch between its thrust surfaces. High-end builders machine that down to as little as 0.002 to 0.006 inch to pin the carrier and kill runout on hard-launched cars. That is the kind of gear train that survives being fed 1,000-plus horsepower with a billet input shaft, a manual valve body and a good sprag, which is exactly why the TH400 has been the default automatic in Pro Street and Pro Mod cars for decades.

The lineage tells the same story. The 4L80E, GM's electronically controlled overdrive workhorse, is the direct descendant of the TH400 and shares much of its gear-train architecture. Sonnax's TH400 parts catalog lists sprag and planetary components that fit both the TH400 and the earlier 4L80E, so hard parts interchange across the family. When a transmission's internals get carried forward into a heavy-duty overdrive unit two decades later, that is a good sign the original was overbuilt.

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Where the TH350 Wins

Stronger is not the same as better for every car. The 350 wins on the things the 400 gives up.

Weight and packaging. Roughly 120 lbs and 21-3/4 inches means less mass over the axle and an easier fit in tight tunnels and short-wheelbase cars. In a light street machine that will never see four-digit horsepower, that is real.

Parasitic loss. A lighter unit turning less fluid and fewer heavy rotating parts costs a little less power to drive. The difference is modest, but on a mild street build it is a fair point in the 350's column.

Cost. Cores, master overhaul kits and hard parts for a 350 generally run cheaper than a 400, and it is a slightly simpler unit to freshen. A basic rebuild is an affordable way to get a dependable three-speed behind a small-block.

The 350 is not a weakling. Its known soft spot is the intermediate one-way clutch: Transmission Digest notes that classic and upgraded street units benefit from a heavy-duty intermediate outer race with better alloy steel and heat treatment for break-resistance under higher torque. Address that and keep the fluid clean, and a 350 behind a healthy small-block will live a long life.

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Fluid and Basic Service

Neither of these units is fussy about fluid. Both were factory-filled with GM Dexron-family ATF, and per ATRA's Gears Magazine, GM has indicated that transmissions originally using Dexron II/III can now be serviced with Dexron VI, which makes Dexron VI the correct modern service fill for both. Worth knowing what changed underneath that statement: the same article puts Dexron II/III at 6.8 to 8.0 cSt at 100 C and Dexron VI at 5.8 to 6.4 cSt, so the modern fluid is measurably thinner at operating temperature. On a healthy 350 or 400 that is not a problem, but on a tired unit with worn bushings and loose clearances it will show up as slightly lower pressure and a faster leak-down. There is no exotic low-viscosity spec to chase the way there is on a modern eight- or ten-speed.

The capacity numbers are where the two units really differ. Per GM's 1976 light truck service supplement (ST-330-76), after draining the sump you add approximately 2-1/2 U.S. quarts to a TH350 but about 7.5 pints (3-3/4 quarts) to a TH400 to bring the level back to the mark. TCI's published capacity chart lists the transmission side alone, stock pan and no converter, at 4 quarts for the TH350 and 6 quarts for the TH400. The converter adds more on a dry fill: TCI puts a new 12-inch converter at about 4 quarts and an 11-inch at about 3.5, so a completely dry stock unit lands around 8 quarts for a 350 and 10 for a 400 before the cooler and lines. Always set the final level on the dipstick at operating temperature rather than trusting a fixed number; GM notes the level rises more than 3/4 inch between 60 F and 180 F, and that overfilling causes foaming and fluid loss through the vent. Fresh Dexron VI and a clean filter on a regular interval is most of what a stock 350 or 400 asks for. Our TH350 fluid type and capacity guide breaks the 350 numbers down part by part.

For pressure diagnosis on either unit, both have accessible line-pressure test ports and respond well to a gauge check before you condemn anything internal. We cover that process in the TH400 pressure testing guide and the TH350 pressure testing guide.

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Which One Should You Run?

Here is the decision the way I give it on the phone. If the car makes real power, tows, or you have any intention of chasing horsepower later, run the TH400 and never think about it again. The strength is there, the aftermarket is bottomless, and the same parts pool feeds the 4L80E world. If the car is a lighter street build with a small-block that will stay reasonable, the TH350 gives you less weight, a little less parasitic loss, lower cost, and more than enough durability with the intermediate race addressed. And if you want highway overdrive out of either decision, that is when you step up to a 700R4 or 4L80E instead. Pick for the job, build it right, set your endplay, and either one will outlast the rest of the car.

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FAQ

How do I tell a TH350 from a TH400 without pulling the pan?

Look at three things. The TH350 pan is roughly square with the passenger-side rear corner cut off at an angle; the TH400 pan is a larger, irregular shape with more angles. The TH350 has a mechanical kickdown cable or detent linkage on the case, while the TH400 has an electrical kickdown switch and connector. And the TH400 is physically bigger, about 24-3/8 inches long against the TH350's 21-3/4 inches.

Which is stronger, the TH350 or the TH400?

The TH400 is stronger. Novak rates it conservatively at about 450 lb-ft of input torque, and its bolted cast-iron center support and low/reverse band give it more capacity than the TH350's clutch-pack low/reverse and lighter internals. A built TH400 with a billet input and manual valve body is a proven 1,000-plus horsepower race unit. The TH350 is durable for street and mild performance use but was designed for lighter torque loads.

Will a TH350 bolt up where a TH400 was, or the other way around?

Both share the common Chevrolet bellhousing bolt pattern in Chevy applications, so the engine side often interchanges, but the two are different lengths and use different kickdown systems, so the crossmember location, driveshaft length, and kickdown hookup all change. The TH400 is about 2-5/8 inches longer than the TH350, which means a shorter driveshaft and usually a crossmember move when you swap one for the other.

What fluid do the TH350 and TH400 use?

Both were factory-filled with GM Dexron-family automatic transmission fluid. ATRA's Gears Magazine reports that GM has indicated transmissions originally using Dexron II/III can now be serviced with Dexron VI, so Dexron VI is the correct modern service fill for both units. Note that Dexron VI runs 5.8 to 6.4 cSt at 100 C against 6.8 to 8.0 cSt for the older fluid, so it is thinner at temperature. There is no separate low-viscosity spec to chase like there is on newer transmissions.

Is the TH400 the same as the 4L80E?

Not the same, but closely related. The 4L80E is the electronically controlled overdrive descendant of the TH400 and shares much of the same gear-train architecture. Many hard parts, including sprag and planetary components, interchange between the TH400 and the earlier 4L80E, which is why builders treat them as cousins.