Frequently Asked Questions:
How does a torque converter work?
Let’s start with two wall fans facing each other. If we turn one fan on, it now becomes the drive fan. The wind from the drive fan will make the other fan turn, although. The fan that is not turned on is known as the driven fan. In the case of a torque converter, the drive fan is bolted to the engine and the driven fan being 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 stationary, such as at a stop sign, with the transmission in gear and the engine at idle, the drive fan is spinning so slow that it can't transfer enough oil to the driven fan to make it turn. As the engine speed is increased, the drive fan blows more oil at the driven fan, which allows the driven fan to be turned move the vehicle. This important concept is commonly referred to as fluid coupling.
The drive fan will always turn a little faster than the driven fan, just like the wall fans. If you were to stick a feather into the driven fan blades, it would slow the driven fan down, but not the drive fan. In a real application this is just like pulling a heavier trailer, the straw in the driven fan is essentially adding a load.
How does lockup work?
Once the truck is up to speed, there is a mechanism called a lockup clutch, that will lock the fans together. In actuality, the driven fan is locked to the front cover of the torque converter, 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 the fluid coupling. When the drive and driven fan are not locked together, heat is generated in the converter. The greater the load and RPM difference, the greater the heat generated. 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. Suppose we have a converter where the drive fan, known as the impeller, is turning 2,500 RPM and the driven fan, known as the turbine, is turning 1,800 RPM. The efficiency of this converter, at this speed, is 72% (1,800 ÷ 2,500 = 0.72). The efficiency is constantly varying, depending on the RPM of the converter, the power input to the converter and the output load, or towed weight. When the converter clutch locks the fans together, the engine rpm will drop 700 RPM.
If we use a converter that is more efficient, such as a low-stall converter, we will be able to achieve a higher efficiency rate. For example, an 88% efficiency rate could mean that the impeller would be turning 2,500 RPM and the turbine would be turning 2,200 RPM. When the converter clutch locks the turbine to the front cover, we would only see a drop of 300 RPM, as opposed to 700 RPM. A lower RPM drop is substantially easier on the converter’s clutch lining and will reduce glazing. In addition, because the fluid coupling of the converter is more efficient, more power, less heat and better fuel economy are delivered before the converter locks up.
How does stall speed work?
To explain stall speed, let’s start with a true full stall. If the transmission were in drive, and the brakes were held down so the vehicle will not move, and the throttle was held wide open, the torque converter will stall the engine at a certain RPM. When stalled, the engine will not be able to spin any faster unless the vehicle is allowed to move. This is a true full stall. Do not test for true stall, it can damage shafts and overheat the torque converter. We have specialized equipment which we use to perform this test.
The next stall speed is generally called break away stall speed. If a vehicle is stopped on a hill and held in position using light throttle as opposed to brakes, then we are almost at the break away stall speed. If the engine RPM required to hold the vehicle was 1,100 RPM and an increase to 1,125 RPM started to move the vehicle, then the break away stall speed would be 1,125 RPM.
The last stall speed is generally referred to as flash stall speed. The flash stall speed takes effect under hard acceleration. From a standing start, if you were to floor the throttle, the engine would start to accelerate quickly and then pause at an RPM as it starts to pull the truck. If the engine went from idle to 1,500 RPM in 1.5 seconds when floored, and then took another three seconds to get from 1,500 to 1,700 RPM, this would mean the flash stall speed was 1,500 RPM. When we lower the stall, we want to lower the break away speed as well as the flash stall speed. This will make the engine work at a lower RPM for a given road speed and will increase fuel mileage in most cases. Once up to speed, the computer will command the lock up clutch on, and the turbine will lock to the front cover of the converter. At this point, the impeller, turbine and engine are turning the same speed which means all engine power will be delivered to the transmission and back to the wheels.