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 make the other fan turn, although much slower than the "drive" fan. In the case of a torque converter, the drive fan is bolted 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 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 will not 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 and the driven fan starts to turn and moves 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, or straw, 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.
Torque Converter Lock-up
Once the truck is up to speed there is a mechanism, called a lock up 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 (impeller) is turning 2500 RPM and the driven fan (turbine) is turning 1800 RPM. The efficiency of this converter, at this speed, is 72% (1800 divided by 2500). 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 would mean that the impeller would be turning 2500 RPM and the turbine would be turning 2200 RPM. When the converter clutch locks the turbine to the front cover we would only see a rpm 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.
Torque Converter Stall Speeds
To explain "stall speed", let’s start with a true full stall. If the transmission were in drive, 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. We have specialized equipment which is used to perform this test. Do not test for true stall, it can damage shafts and overheat the torque converter.
The next stall speed is generally called "break away" stall speed. If a truck is stopped on a hill and held in position using light throttle as opposed to brakes we are almost at the "break away" stall speed. If the engine RPM required to "hold" the truck was 1100 RPM and an increase to 1125 RPM started to move the truck then the "break away" stall speed is 1125 RPM.
The last stall speed is generally referred to as the "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 1500 RPM in 1.5 seconds when floored, and then took another 2 or 3 seconds to get from 1500 to 1700 RPM, this would mean the "flash stall" speed was at 1500 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, in most cases, will increase fuel mileage. Once up to speed, the computer will command the lock up clutch "on", and the driven fan will lock to the front cover of the converter. At this point the drive fan, driven fan and engine are turning the same speed which means all engine power will be delivered to the transmission and back to the wheels.
Upgrading a Stock Converter
In a stock torque converter, the clutch has 1 clutch plate with about 37 square inches of clutch lining. At Goerend Transmission we like to use 3 clutch surfaces that total about 105 square inches of lining, we call this a triple disc converter. This triple disc will hold dramatically more torque than a single disc.