Triple Disc Torque Converter

How does a torque converter work?

Let’s start with two wall fans facing each other. If we turn one fan on, the wind from this fan will start make the other fan turn, although at a much slower rate than the "drive" fan.

In the case of a torque converter, the drive fan is connected to the engine, and the fan being “driven” is connected to the input shaft of the transmission. In addition, oil is used to transmit the energy between the two fans, as opposed to air in the example scenario.

When a vehicle is stationary, such as at a stop sign, with the transmission in gear and engine at idle, the drive fan that is connected to engine is spinning so slow that it will not transfer enough oil to turn the driven fan. As the vehicle’s brakes are let off and the throttle is held, the engine speed increases, and the drive fan blows more oil at the driven fan. The driven fan then starts to turn and move the vehicle. This important concept is commonly referred to as fluid coupling.

The drive fan will always turn slightly faster than the driven fan, just like the wall fans. If you were to stick a feather into the blades of the driven wall fan, the driven fan will slow down, but not the drive fan. In a real application, this is just like pulling a heavier trailer. The feather in the driven wall fan is essentially like adding a load to the vehicle.


What determines stall speed?

The two major things that determine the stall speed are engine torque and the torque converter.

This is one way to think about it: You’re on a stationary exercise bike that uses a fan for its pedal resistance. The smaller the fan is, the faster you can pedal. You may be able to pedal up to 200 RPM on the bike with a small fan, but only be able to pedal up to 50 RPM if you fit a bigger fan on the bike. The small fan was able to stall you at 200 RPM and the bigger fan stalled you at 50 RPM. Two different fans, two different stall speeds.

Now a professional athlete hops on the bike. With the big fan, they may be able to pedal 100 RPM, as opposed to your 50 RPM.

To compare this to the engine torque and torque converter, you must remember that a torque converter is nothing more than two fans. One fan is connected to the engine and the other fan is connected to the transmission. The fan connected to the engine blows oil at the fan connected to the transmission. Once the engine fan blows enough oil at the transmission fan, the transmission fan will start to rotate. The vehicle will then start to move.

Just like the exercise bike analogy, if you install a torque converter with bigger fan blades into a vehicle with the same engine torque, the stall speed will be lower. If you add more horsepower to the engine, just like the athlete hopping on the exercise bike, the stall speed will increase.

Now let’s talk about air. It's all about the oxygen. You’ll be able to pedal harder on the bike at sea level than on the top of a mountain because you’ll be able to breathe better, since there is more oxygen in the air. Similar to your lungs, an engine is nothing more than an air pump that uses fuel. The fuel an engine uses must react with oxygen in order to push the piston down and turn the crankshaft.

The higher the elevation, the less oxygen the engine has to work with, and the less fuel that can be injected to react with the oxygen. By using less fuel, it means the power input into the converter will be lower, resulting in a lower stall speed. Normally, the higher the elevation, the higher the stall speed that is needed in order to help offset the lack of oxygen that is present at sea level.

The engine turbo also can make a huge difference. Like the converter, the turbo is also a set of fans. The drive fan is located in the exhaust of the engine and it’s the exhaust flow makes this fan spin.

The driven fan of the turbo is connected to the drive fan and sits on the intake side of the engine. The driven forces oxygen into the engine so it can burn more fuel, resulting in more power.

A turbo with bigger fans takes longer to spool up and start to blow more air into the engine. Because of this, it takes longer to get oxygen into the engine cylinders to burn fuel. This is called turbo lag.


What is true, flash, and breakaway stall speed?

To explain stall speed, let’s start with a true full stall speed.

True stall speed is tested by putting the vehicle in drive and holding the throttle wide open while simultaneously holding the brake, so the vehicle remains stationary. When this happens, the torque converter will stall the engine at a certain RPM. When stalled, the engine will not be able to increase RPM until the vehicle is allowed to move. This is a true full stall. Do not test for true stall, as it can damage transmission shafts and overheat the torque converter. We have specialized equipment we use to perform this test.

The next stall speed is generally known as breakaway stall speed. A vehicle in drive is stopped on an incline and the throttle was held enough to hold the vehicle in place, but not enough to move the vehicle uphill. Once the throttle is held enough to start creeping the vehicle uphill, breakaway stall speed has been reached.

For example, if the engine RPM required to hold the truck is 1100 RPM, and an increase of 25 RPM started to move the truck, the breakaway stall speed would be 1125 RPM.

The last stall speed is generally known as flash stall speed. The flash stall speed takes effect under hard acceleration. A vehicle idling in drive and the throttle is then held wide open. The engine starts to accelerate quickly, then temporarily pause at an RPM as it starts to pull the vehicle.

If the engine took 1.5 seconds to get from an idle to 1500 RPM when floored, then took another two seconds to get from 1500 RPM to 2000 RPM, this would mean the flash stall speed would be 1500 RPM.

When we lower the stall of a converter, it can also lower the breakaway stall speed as well as the flash stall speed. Lowering the stall makes the engine work at a lower RPM for any given road speed and in most cases, will increase fuel mileage.


Lock Up & Speed Ratio

Once a vehicle gets up to speed there is a mechanism called a lock up clutch that locks the driven fan of the torque converter to its front cover, which is bolted to the engine. When this occurs, the drive fan and driven fan turn at the same RPM, with no loss of power in fluid coupling.

When the drive and driven fan are not locked together, heat is generated inside the converter. The greater the RPM difference between the drive and driven fan, the higher the temperature will reach. This heat is essentially lost power which results in a lower transmission life, performance, and fuel economy.

The loss of energy in this process can be calculated. A converter’s drive fan (impeller) may be turning at 2500 RPM and the driven fan (turbine) may be turning at 1800 RPM. By dividing the turbine RPM by the impeller RPM, we can find the efficiency of the converter, which is 72% at the given speeds of the impeller and turbine. This efficiency rate is also known as a converter’s speed ratio. When the vehicle gets up to speed and the lock up clutch engages, the engine and impeller RPM match the RPM of the turbine.

Low stall converters will be able to achieve a higher speed ratio before lock up. When low stall converters lock up, their RPM drop is substantially less, therefore easier on the converter’s clutch lining and will reduce clutch glazing. Because the fluid coupling of a low stall converter is more efficient, they are able to transfer more power, create less heat, and deliver better fuel economy than high stall converters.


Should I use a threadlocker on my torque converter bolts?

While you don't need to use a threadlocker, such as Loctite, on your torque converter bolts, we would recommend using Loctite Blue if you choose to use a threadlocker.


Why is my torque converter overheated?

Similar to how an engine produces heat that must be removed by the radiator and water pump, the torque converter generates heat within the transmission and relies on proper fluid flow for heat removal. Excessive line pressure with no line to cooler bypass may cause the torque converter and cooler to not receive adequate fluid flow. This leads to torque converter overheating, lock up clutch damage, which can then drag the engine down when the vehicle is stationary at idle. Replacing an overheated torque converter in this situation won't fix the underlying issue. The cooling circuit problem must be identified and resolved.


Diagnosing Heat-Related Torque Converter Lock-Up Issues

Lock-up complaints are often blamed on the torque converter, but real-world diagnostics don’t always point that cleanly in one direction.

The issue presents as a temperature-dependent lock-up problem. When cold, the converter clutch applies and holds reasonably well. As operating temperature increases around 115°F, lock-up performance begins to degrade. Under light load, the clutch can still be felt engaging, but it lacks holding strength. Under throttle, it slips entirely, with engine RPM rising independently of vehicle speed.

At face value, this looks like a failing converter clutch. However, testing tells a more complicated story. The converter was checked on a lock-up test fixture using applied air pressure and measured torque resistance. Even at relatively low test pressure, it demonstrated strong holding capacity, well within what would be expected for the application. This suggests the converter is capable of holding torque, at least under static conditions.

That creates a gap between bench results and real-world behavior. When a converter passes static testing but the transmission slips under load, nit points to a system-level issue rather than an immediate component failure. Heat changes fluid viscosity, internal clearances, and sealing effectiveness. Any weakness in the apply circuit, whether inside the converter or elsewhere in the transmission, may only appear under these conditions.

In cases like this, it becomes important not to jump straight to condemning the converter. The correct next step is verification: either testing under load (dyno) or physically inspecting the converter internals. This kind of problem underscores a broader principle in transmission diagnostics: lock-up slip under heat doesn’t automatically mean the converter is bad. It often indicates that something in the transmission system such as pressure supply, sealing, or internal leakage.

The takeaway is simple. If a failure only occurs hot and under load, you’re no longer just diagnosing a part, you’re diagnosing the entire transmission system.


How Torque Converters Shape Power Delivery in Performance Builds

In performance-oriented drivetrain setups, the behavior of a torque converter plays a major role in how power is delivered from the engine to the transmission. While discussions often focus on stall speed alone, the reality is more nuanced. Converter design influences how the engine builds RPM under load, how efficiently power transfers as speed increases, and how responsive the vehicle feels in real-world driving.
A useful way to understand this is by looking at how a converter behaves across different operating conditions rather than at a single fixed point. Engine torque, vehicle load, and speed all interact to determine how quickly the engine can rev and how effectively that energy is transferred through the drivetrain.

At low vehicle speeds, such as when accelerating from a stop, the converter allows a certain amount of slip between the engine and the transmission. This slip is what enables the engine to build RPM before the vehicle fully responds. A converter that allows more slip in this range will let the engine rev higher under load. This is often described as being “looser,” but more broadly, it reflects how aggressively the drivetrain allows the engine to get into its powerband early.

As speed increases, however, efficient power transfer becomes more important than initial responsiveness. Ideally, a converter transitions from allowing slip to tightening up, reducing the difference between engine speed and transmission speed. This improves efficiency, reduces heat, and ensures that more of the engine’s power is actually driving the vehicle forward rather than being lost in the fluid coupling.

What makes certain converter designs effective is their ability to balance these two behaviors. A well-matched setup will allow the engine to build RPM quickly at low speeds, helping with acceleration and, in turbocharged applications, improving spool while still coupling efficiently as speed rises.

Engine torque also plays a critical role in this behavior. Higher torque levels tend to drive the converter harder, changing how quickly it moves through its operating range. This is why the same converter can behave differently depending on the power level of the setup. Rather than thinking of a converter as having a single fixed stall speed, it is more accurate to view it as a dynamic component that responds to the load placed on it.

In practical terms, this means that selecting the right converter is less about chasing a specific number and more about matching its behavior to the goals of the build. Setups that prioritize quick response and strong low-speed acceleration benefit from designs that allow the engine to rev more freely under load. Applications focused on sustained power delivery and efficiency may favor tighter coupling characteristics.

Ultimately, the goal is to create a system where the engine and transmission work together seamlessly. The converter acts as the bridge between them, shaping how power is introduced, managed, and sustained across the entire driving range.


Torque Converter Clutch Drag Caused by a Stuck Pressure Regulator Valve

A repeat transmission issue led to a deeper investigation into converter clutch release pressure and valve body operation. The primary complaint was a torque converter clutch remaining applied at idle, causing the engine to stall at stop signs. The condition was most noticeable at low RPM, where converter release pressure is highly sensitive to hydraulic leakage and pressure loss.

The converter passed lockup testing and showed no signs of mechanical failure. However, inspection of the valve body revealed metallic debris throughout the fluid along with contamination on the outside of the valve body. The pressure regulator valve was also found stuck in an open position.

Tracing the hydraulic circuits showed the stuck valve created a direct exhaust path for both line pressure and converter charge oil. With converter release oil exhausting internally, the converter clutch could not fully disengage at idle.

Although the converter contains a front cover bushing, that bushing is not known as a common failure point. This shifted suspicion toward internal transmission bushings, thrust washer damage, or contamination circulating through the cooler system.

The pressure regulator valve controls line pressure by balancing pump pressure against spring tension and hydraulic feedback oil. Under normal operation, the valve constantly moves in small increments to regulate excess pressure while maintaining converter charge oil and converter clutch release pressure.

Hydraulic feedback oil is routed through passages in the separator plate and channel casting to the backside of the valve, allowing the valve to continuously balance pressure demand and exhaust flow. The valve should never remain fixed in a fully open position.

In this case, debris contamination prevented the valve from returning to its closed position. With the valve stuck open, both line pressure and converter release pressure were continuously exhausted.

At higher RPM, increased pump volume partially compensated for the leakage. At idle, however, converter release pressure dropped low enough that the clutch could not fully separate. The converter clutch remained applied or partially applied, causing the engine to stall when the vehicle came to a stop.

Despite the drivability complaint, the converter still functioned normally on the lockup tester. 

Although converter clutch drag may have created additional heat and wear inside the converter, that alone would not explain the metallic debris discovered on the valve body. This reinforced the need to locate the original contamination source.

The amount of debris found on the valve body indicated material was being generated somewhere within the transmission system. Possible sources included internal transmission bushings, thrust washers, pump components, or contamination trapped within the cooler system.

The transmission required complete inspection for damaged bushings and thrust washers, including the pump assembly, stator support, planetary assemblies, and rotating hard parts.

The converter was cut open for inspection, but the converter bushing was verified as acceptable. 
The final repair recommendation after inspection of the converter and valve body, was inspecting all transmission bushings and thrust washers, and carefully checking the pump assembly for wear or damage.

If no internal transmission damage could be identified, the cooler system became the next likely source of contamination. Coolers can trap metallic debris and recirculate contamination into rebuilt units, especially after previous failures. In situations where contamination cannot be completely eliminated, replacing the cooler is often safer than attempting to flush it.

This case highlights an important diagnostic principle in transmission repair. A vehicle stalling at stop signs due to a locked converter clutch does not automatically indicate a failed converter. Hydraulic regulation problems, valve sticking, contamination, pressure loss, low fluid level, or a displaced filter can all create the same symptom.

It also demonstrates how a single stuck pressure regulator valve can mimic major component failure by exhausting converter release oil and preventing the clutch from disengaging properly.

Most importantly, any time metallic debris is found inside a transmission, the source must be identified. Replacing components without locating the origin of contamination often results in repeat failures.

This case highlights the importance of diagnosing converter clutch release hydraulics before condemning major transmission components.