January 2025

The Turbo 350 (TH350) transmission is a 3-speed automatic transmission developed by General Motors in the late 1960s and widely used through the 1980s in various GM vehicles. It is known for its durability, compact size, and versatility in performance and street applications – and predictably, it can be found in many American classics, particularly large trucks from the 70s and 80s. 

For its power-to-weight ratio and relative simplicity, the TH350 has been a popular transmission to mod and rebuild for high-performance applications and show cars, including classics like the 90s Camaro. Its only downside is the lack of the overdrive gear introduced in later GM transmissions – but if fuel economy is pretty low on your list of considerations, the TH350 remains a popular and fairly accessible choice of transmission for older cars. 

However, there’s a lot to improve on the stock TH350, such as the stall converter. When looking for a stall converter/torque converter for a TH350, choosing the right one is critical for performance and drivability. Below are the key considerations when purchasing a stall converter for your Turbo 350 and engine combo:

Stall Speed

The stall speed is the RPM range the converter allows the engine to reach before it fully engages and begins driving the wheels. 

Why does this matter? Simple: if you want faster, stronger launches, you want the transmission to engage the engine via the “clutch” (or stall converter, in this case) close to the relative peak of its torque curve, which depends on the engine and the car you’ve got. 

For performance engines, the stall speed should be at most 700 RPM below the engine’s peak torque RPM. Here’s what you’ll usually want to look for: 

  – Street applications: Look for a converter with stall speeds of 1,800–2,500 RPM.  

  – Mild performance engines: Look for a converter with stall speeds of 2,500–3,500 RPM.  

  – High-performance/racing: Look for a converter with stall speeds of 3,500 RPM and above. 

  – Heavy towing: Ask around for specific stall speeds and stall converters designed for heavy towing and heavy-duty use, to prevent stalling/bogging down under high loads, or at very low speeds. 

Stall converters aren’t created equally, and your car’s general usage type or application matters for what kind of stall converter you should be purchasing. If you’re rebuilding an older car or fixing it up for general street driving, you should prioritize drivability and efficiency. Moderate stall speeds work well for street applications.  

A higher stall converter maximizes launch by allowing the engine to reach its powerband before engagement – perfect for quick launches, drag racing, and explosive power transfer from the engine. Meanwhile, a low to medium stall speed and a high torque multiplication factor ensure performance without overheating when towing. 

To keep it short: lower-stall converters are ideal for daily driving. High stall speeds are better for racing or high-performance applications but may reduce street drivability. If you’re installing a TH350 on a towing vehicle, you might want a stall converter designed for towing.

Vehicle and Engine Setup

The torque converter you’re choosing must be compatible with your overall setup, including your engine modifications, rear-end gear ratio, total vehicle weight, and tire size. 

Consider compression ratio, camshaft profile, cylinder head flow, and intake/exhaust tuning. Aggressive cam profiles often require higher stall speeds to maintain drivability and proper engine operation at idle and low RPM, which changes what you’re looking for in a stall converter. 

Make sure that the rear axle gear ratio works with the stall speed. Low rear-end gearing (numerically higher ratio, e.g., 4.10:1) pairs better with higher stall speeds, while taller gears (numerically lower ratio, e.g., 3.08:1) match with lower stall speeds.

Weight matters a lot. Heavier vehicles (e.g., SUVs or full-size cars) require a lower stall speed than lighter vehicles, even with similarly performing engines. To the same point, larger diameter tires effectively increase the final drive ratio, impacting stall converter selection. 

Transmission Compatibility

The TH350 transmission has specific design requirements for torque converters. First, make sure that the torque converter matches the TH350’s input shaft spline (30-spline) and flexplate bolt pattern. Also, most TH350 transmissions use a non-lock-up converter. Be sure not to buy a lock-up torque converter unless your transmission has been modified to accept it.

Build Quality and Features

It goes without saying, but if you’re already in the market for an aftermarket stall converter, then go for quality: look for high-strength construction, such as furnace-brazed components, heavy-duty bearings, and high-quality welds. 

Some other upgrades to consider if you’ve got room in the budget include furnace-braced fins, and anti-ballooning plates. These help minimize the risk of breakage or converter distortion, which can further damage and even ruin your transmission. 

Installation and Maintenance

Stall converters are relatively easy to install, provided you’ve got one designed for your respective transmission – again, pay attention here to the bolts and input spline. 

As for maintenance, while the TH350 is generally air-cooled, you can use aftermarket transmission coolers to extend the longevity and RPM range of your TH350 and get a little more out of your transmission in terms of both performance and lifetime. This is especially relevant if you’re building a car with a higher stall speed in mind, which can produce more overheating issues if not properly planned for. 

Conclusion

Choosing the right stall converter for your Turbo 350 is a matter of matching your options (given the TH350 itself, your budget, and the availability of parts in your area) with your needs. When selecting a stall converter for this transmission, factors like stall speed, vehicle weight, engine setup, and drivability needs are critical to maximizing performance and longevity. Working with transmission rebuilders from the get-go ensures that your car is ready to perform optimally. 

SUMMARY: Upgrade to a rebuilt valve body for firmer shifts, improved durability, enhanced control, and better performance for towing or racing applications. Whether fixing problems or optimizing performance, a better valve body revitalizes your transmission system.

Struggling with a faulty 4L60E? You could be looking at a bad valve body. Issues like delayed shifting, slipping gears, and overheating can make driving frustrating and even lead to costly repairs if left unchecked. When the valve body fails, the results can include rough transitions, erratic gear changes, and even transmission damage over time. Worse yet, ignoring these signs often causes bigger, more expensive problems down the line – and costs you more than a few replacement parts. 

On the other hand, even mint-condition stock valve bodies might not necessarily be the best fit for a 4L60E attached to a project car. If you want to put more horsepower on a historic vehicle, then you’ll want to give its historic transmission an equal facelift. 

What’s a Valve Body Do?

The valve body is a critical component of an automatic transmission system in vehicles. It serves as the central control unit that directs hydraulic fluid to various valves, which engage the transmission’s clutches, bands, and gears. By doing so, it controls the shifting of gears in response to driving conditions, throttle position, and other factors.

As an electronic transmission, the valve body of the 4L60E receives signals from sensors and actuators that monitor the vehicle’s speed, load, and other parameters. It uses these signals to determine when to send pressurized hydraulic fluid to specific areas, enabling smooth gear changes. 

Common Issues with Stock 4L60E Valve Bodies

The 4L60E has certain common issues related to its valve body, including: 

Shift Delays or Harsh Shifting

This is often caused by problems with the shift solenoids or issues with fluid pressure in the valve body. The solenoids might be clogged or malfunctioning, preventing the smooth engagement of the gears. The vehicle may experience delayed, rough, or hard shifts between gears, especially between the 1st, 2nd, or 3rd gears. Fluid pressure isn’t always caused by a bad valve body – a shredded torque converter, for example, can damage your transmission and bring pressure down considerably. 

Slipping Gears

If the valve body fails to direct enough hydraulic pressure to the transmission components (like the clutch packs and bands), this can cause the transmission to “slip” out of gear or have trouble staying in gear. 

Fluid Leaks

Over time, seals and gaskets in the valve body may wear out or get damaged, leading to fluid leaks. Puddles of transmission fluid beneath the vehicle or low fluid levels in the transmission could result. This can cause poor lubrication and improper operation of the transmission.

Valve Body Blockage or Contamination

Over time, metal shavings or other contaminants from the transmission can build up in the valve body – such as through the aforementioned torque controller. This can cause the valves to stick or fail to move properly. 

Electronic Control Issues

In the 4L60E, the transmission control module (TCM) and valve body work in tandem to manage shifts. If the TCM malfunctions or the wiring to the shift solenoids is damaged, the valve body may not receive proper signals to shift properly. You could have transmission shifts that don’t happen at the correct RPM, or the transmission may not shift into higher gears.

Sticking Valves

Sticking valves in the valve body can result from poor fluid maintenance, causing varnish, debris, or other contaminants to build up inside the valve body. This typically manifests in the form of delayed or missed shifts, harsh shifting, or the transmission staying stuck in a lower gear (limp mode).

Torque Converter Clutch (TCC) Problems

The TCC solenoid in the valve body can also malfunction, causing the converter clutch to engage or disengage at improper times. You may feel shuddering, slipping, or a noticeable loss of power at certain speeds, especially while cruising at highway speeds.

Benefits of Upgrading Your 4L60E Valve Body

Aside from replacing a damaged valve body, there are quite a few performance improvements to look forward to when switching to a better non-stock valve body for your 4L60E. 

If you’re thinking about improving your transmission, then a valve body upgrade can net you firmer shifts, enhanced durability, better control, and more. 

Shifting Performance

A custom valve body can provide firmer and more aggressive shifts, which are beneficial for performance vehicles or those used for towing or racing. Firmer shifts reduce the slip time between gears, which can improve vehicle acceleration and transmission responsiveness.

Some custom valve bodies allow you to modify or fine-tune the shift points to match your driving preferences or the specific needs of your vehicle. For example, you could program the transmission to shift at higher RPMs for performance or at lower RPMs for smoother driving.

Enhanced Durability and Reliability

Custom valve bodies often include upgraded parts, such as stronger valves, springs, or solenoids, which can improve the longevity of the transmission. These modifications make the valve body less prone to wear, especially in heavy-duty applications like towing or high-torque engines.

A custom valve body can be designed to offer more precise fluid management, which improves hydraulic pressure consistency. This can help prevent issues like slippage, overheating, or inefficient shifting, ensuring smoother operation over time.

Increased Transmission Control

If your vehicle uses a more advanced system with electronic controls, a custom valve body can provide better integration with the Transmission Control Module (TCM), allowing for more consistent and responsive shifts. This can be beneficial for performance cars or vehicles with high horsepower where precise transmission operation is crucial.

Some custom valve bodies allow users to adjust the hydraulic pressure in the system. This can be useful if you’re pushing the transmission to handle higher power outputs, such as with performance modifications or forced induction (turbocharging, supercharging).

Faster Shifting

A custom valve body can be designed to decrease the time it takes to complete a gear shift. In high-performance settings, quicker shifts translate to better acceleration and quicker lap times, particularly in racing or spirited driving.

Improved Towing Capability

Custom valve bodies can also optimize shifting behavior for vehicles that tow heavy loads. This can involve firmer shifts and better control over how the transmission handles the stresses associated with towing, reducing the chance of overheating or excessive wear.

Better Transmission Cooling

Certain custom valve body designs feature optimized pathways for the fluid, reducing resistance and helping to maintain proper fluid flow. This can lead to better cooling, reducing the likelihood of overheating, especially in performance or towing situations.

Conclusion

Upgrading to a custom valve body can transform your vehicle with firmer shifts, enhanced durability, and optimized control for high-performance or towing scenarios. Whether fixing stock issues or boosting performance, a valve body upgrade ensures smoother, more reliable transmission operation – and better translates the power of a stronger engine to your axles.

SUMMARY: The 6L80 transmission, a reliable 6-speed GM system, can develop performance issues due to its stock torque converter’s wear. Upgrading to a high-performance converter enhances heat management, torque multiplication, and durability, addressing common failures. Learn why replacing your 6L80’s torque converter is crucial for longevity.

One of the most prevalent problems of early 6L80s is the torque converter’s tendency to wear out, leading to slipping, vibration, and transmission overheating as metal shavings from poor torque converter durability often lead to a compromised transmission pump, causing a drop in pump volume and pressure, meaning the transmission begins to struggle to get in (and stay in) gear.

Replacing the torque converter on a relatively unused stock 6L80 can improve durability and extend the transmission’s lifetime while being a practically mandatory upgrade for 6L80s intended for high-performance or heavy-duty use. A high-performance torque converter with a billet cover enhances power efficiency, improves torque multiplication, and ensures durability in demanding conditions.

Why Bother Replacing Your Torque Converter with a Billet Cover?

The torque converter’s job is to enable translation between the engine’s flexplate and the transmission via a fluid coupling, as opposed to the mechanical clutch of a manual transmission. In the 6L80, the internals of the stock torque converter contain a single-disc lockup clutch plate which, over years of use, flexes and eventually shreds itself apart against the piston, flinging metal chunks throughout the transmission’s fluid pump.

Many trucks built with the 6L80 transmission have been reaching serious points of transmission failure over the last few years, highlighting the consistent problems with the 6L80’s torque converter. If you’ve been experiencing some transmission performance problems in your truck – especially as it reaches above and beyond 100,000 miles – then the torque converter on your 6L80 may be the main culprit, particularly if you use your vehicle for towing and hauling.

At that point, however, replacing your torque converter is a bandaid solution for a problem requiring invasive surgery, or even a total replacement. If your transmission fluid pan is chockful of metal fragments, chances are that the pump and housing have been damaged to the point of requiring re-machining.

An upgraded torque converter is ideal for various applications, ensuring it meets the diverse needs of different vehicle types, including towing, hauling, and high-performance driving.

However, if your 6L80 is doing just fine, then replacing your torque converter earlier rather than later can save you a small fortune in repairs or replacements, and greatly improve the longevity of your build.

In short, replacing the stock torque converter with an upgraded version addresses many of the 6L80’s inherent issues, such as heat management, torque multiplication, and longevity. To sweeten the deal, non-stock torque converter replacementsfor your 6L80 can greatly improve performance and transmission longevity by offering different stall ranges tailored to your vehicle’s setup.

Steps to Replace a Torque Converter in a 6L80 Transmission

Preparation

Start by gathering what you need, including a socket set, torque wrench, and transmission jack. You’ll need your replacement torque converter, a change of transmission fluid (plus your catch tray/pan), and if necessary (depending on what your converter kit calls for), you may need replacement seals, gaskets, and some fittings for your cooler lines.

Make sure your vehicle is set up on stable level ground and set up your jack for adequate working clearance. Disconnect the battery, drain your transmission fluid, and double-check for metal debris.

Removal Process

Use a marker to label the position of the driveshaft relative to the rear axle flange for proper alignment during reinstallation. Then, use the socket set to loosen and remove the bolts securing the driveshaft to the rear axle. Support the driveshaft to prevent it from falling when detached.

Slide the driveshaft out from the transmission tail shaft carefully, being mindful of any remaining fluid that might leak. Next, start by unplugging the wiring harness attached to the transmission. Label connections if needed to simplify reassembly. Detach transmission cooler lines by using line wrenches or quick-disconnect tools. Again, be prepared for a small amount of fluid to drain.

Place a transmission jack beneath the transmission and secure it with straps to prevent movement during removal. Loosen the bell housing bolts in a star pattern to prevent stress on the transmission case. Keep the bolts organized for later use, and make sure you don’t lose them.

After removing and lowering the transmission onto your work surface, examine the transmission’s input shaft for wear or damage before removing the old torque converter. Carefully pull it straight off the input shaft. Avoid tilting or jerking motions to prevent damage.

Installation Process

Fill the torque converter with clean, new transmission fluid (approximately 1-2 quarts). This primes the converter and ensures immediate lubrication during operation. Verify that the torque converter splines and mounting surfaces are free of debris or imperfections that could impede installation. Carefully slide the new torque converter onto the input shaft. Rotate the converter gently to engage the splines and pump drive. Push the converter fully onto the input shaft until you feel it “click” into place at three stages: splines, stator, and pump drive. Check clearance to ensure proper depth.

Using the transmission jack, raise the transmission and align it with the engine block. Start threading the bellhousing bolts by hand to avoid cross-threading. Reattach the wiring harness, cooler lines, and any brackets. Ensure all bolts and fittings are torqued properly. Slide the driveshaft into the tail shaft and reattach it to the rear axle flange. Tighten the bolts securely.

Home stretch now! Add transmission fluid through the dipstick tube or fill port as specified in your car’s service manual, or according to 6L80 specifications. For reference, the 6L80 holds about 13.2 quarts and uses Dexron VI.

Start the vehicle and allow it to idle while shifting through all gears to circulate fluid. Get your temperature up to about 100 before going through your gears. Don’t get it too high! If you’ve brought your transmission temps up to over 120, turn the engine back off and wait a while before trying again. Check for leaks at the cooler lines, transmission pan, and torque converter housing before giving your car a road test.

Finally, drive the vehicle to verify smooth shifting and proper engagement. Monitor fluid levels again after the test drive, as some air may have worked its way out of the system. Monitor temperature again, to see if it’s within normal levels. Monitor your transmission for new and exciting (i.e., frightening) noises. If everything seems in order, then you’ve done well!

Conclusion

Upgrading the torque converter on a 6L80 transmission is a vital step for addressing inherent reliability issues and extending its life, particularly in heavy-use or high-performance vehicles. With proper preparation and care, the process ensures smoother shifts, better power delivery, and enhanced transmission longevity.