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Understanding torque converters and selecting the best
Most modern drag race cars use an automatic transmission. Selecting the right torque converter for your automatic transmission can make all the difference in how your drag race or street/strip vehicle performs.
A performance torque converter can help you to:
The torque converter is an integral part of the total vehicle combination. While many vehicle combinations and applications are very similar and the best torque converter may be an off-the shelf buy, it is normally a wise step to consult an expert for your specifications, and choose the best torque converter for your particular application. It is extremely important to communicate with your converter builder in order to get the right torque converter for your race car.
For many quarter-mile racing enthusiasts, and especially backyard mechanics, there seems to be a mystery about an automatic transmission's torque converter. Many people do not understand how it functions, let alone how one can work to your advantage when trying to improve a car's performance and efficiency. The torque converter is really not all that complicated, once you understand how it functions. This adaptable coupling between your engine and your drive-train can be modified or reproduced by transmission experts to increase your transmission's use of the supplied engine horsepower. But, before you go out get an expensive new torque converter, we believe it's important to understand what you are spending your money on.
So what does a torque converter do?
How a Torque Converter works
There are four components inside the very strong housing of the torque converter:
The housing of the torque converter is bolted to the flywheel of the engine, so it turns at whatever speed the engine is running at. The fins that make up the pump of the torque converter are attached to the housing, so they also turn at the same speed as the engine.
How the parts of the torque converter connect to the transmission and engine
The pump inside a torque converter is a type of centrifugal pump. As it spins, fluid is flung to the outside, much as the spin cycle of a washing machine flings water and clothes to the outside of the wash tub. As fluid is flung to the outside, a vacuum is created that draws more fluid in at the center.
At idle or lower engine speeds, the force of the oil discharged by the impeller is not great enough to turn the turbine with any degree of efficiency. This allows the vehicle to remain motionless with the transmission engaged in gear. The fluid then enters the blades of the turbine, which is connected to the transmission. The turbine causes the transmission to spin, which moves your car. The blades of the turbine are curved. This means that the fluid, which enters the turbine from the outside, has to change direction before it exits the center of the turbine. It is this directional change that causes the turbine to spin.
As the turbine causes the fluid to change direction, the fluid causes the turbine to spin. The fluid exits the turbine at the center, moving in a different direction than when it entered. The fluid exits the turbine moving in the direction opposite to what the pump (and engine) is turning. If the fluid were allowed to hit the pump, it would slow the engine down, wasting power. This is why a torque converter has a stator.
The stator resides in the very center of the torque converter. Its job is to redirect the fluid returning from the turbine before it hits the pump again. This dramatically increases the efficiency of the torque converter. The energy of the redirected oil acts on the impeller in the same direction as engine rotation, thereby multiplying the torque output of the engine.
The stator has a very aggressive blade design that almost completely reverses the direction of the fluid. A one-way clutch (inside the stator) connects the stator to a fixed shaft in the transmission (the direction that the clutch allows the stator to spin is noted in the figure above). Because of this arrangement, the stator cannot spin with the fluid -- it can spin only in the opposite direction, forcing the fluid to change direction as it hits the stator blades.
As both, engine speed and transmission input shaft (turbine) speed increase there is a point, at which both the pump and the turbine are spinning at almost the same speed (the pump always spins slightly faster). At this point, the fluid returns from the turbine, entering the pump already moving in the same direction as the pump, so the stator is not needed. Even though the turbine changes the direction of the fluid and flings it out the back, the fluid still ends up moving in the direction that the turbine is spinning because the turbine is spinning faster in one direction than the fluid is being pumped in the other direction.
At these speeds, the fluid actually strikes the back of the stator blades, causing the stator to freewheel on its sprag or one-way clutch so it doesn't hinder the fluid moving through it. When the stator freewheels, the torque converter ceases to multiply torque and becomes simply a fluid coupling between engine and transmission.
Stall Speed is commonly referred to as the RPM speed at which the converter will hold back or limit the engine when the automotive transmission is prohibited. By not allowing the engine further RPM gain, the increase in engine RPM “stalls” or in other words, ceases to increase. The important thing to remember is that “stall” speed is a direct relationship between the engine’s ability to produce power and the torque converter’s ability to hold the RPM back.
Changes to either the engine’s power output and/or the converter will change the balance and in turn will change the “stall” speed. The RPM at which stall occurs within a converter is a function of engine peak torque. This is why the same high stall converter will not “stall” the same when interchanged between two different engines, producing vastly different power outputs.
When referring to "how much stall will I get from this torque converter", it means how fast (RPM) must the torque converter spin to generate enough fluid force on the turbine to overcome the resting inertia of the vehicle at wide open throttle.
The primary thing to remember about torque converter stall speed is that a particular torque converter does not have a "preset from the factory" stall speed but rather its unique design will produce a certain range of stall speeds depending on the amount of load the torque converter is exposed to. This load comes from both, the torque produced by the engine and the resistance of the vehicle to move from rest. For best drag strip results use a converter that stalls close to peak torque. Some people advocate a bit above, others a bit below peak torque RPM. Again, close cooperation with the converter manufacturer is the best approach.
So what determines Stall Speed of a Torque Converter?
Careful selection of the right components should result in a torque converter that can maximize your race car’s performance. The elements that affect torque converter stall are:
Efficiency of Torque Converters
Keeping in mind that the pump assembly and the turbine assembly spin at different speeds, every torque converter has a different rate of slippage between the two. The amount of slippage is what determines efficiency. This is why auto manufacturers created lockup or ‘sprag’ torque converters. Rather than having a torque converter that is, let's say 92% efficient, they mechanically link the engine to the transmission, giving it a 1:1 ratio or creating 100% efficiency. This lowers the RPM of the engine, therefore increasing fuel efficiency.
All the variables for converter manufacture are affected by differences between a stock drive-train configuration and any performance modifications you may have added. Just as different stall speeds are recommended for different engine torque ratings, items such as rear differential ratio, body weight, tire size and compound used, type of racing you are participating in (or if you are just looking for better street performance or better low-end torque for towing situations) and transmission shift ratios all affect construction of a torque converter so that it will do what you want it to do.
Stall speed is basically an rpm rating that rates a converter's performance. Numbers can range from as low as 1,800 rpm to as high as 5,800 rpm in some race applications. A converter rated at 1,800 rpm means you should be able to foot-brake-stall the converter at 1,800 rpm which will give you more engine rpm to launch from a dead stop and/or to be used to accelerate through the gears. In many cases, a good aftermarket converter will give you 1,000 extra rpm to put the power to the ground with more authority.
For most street and street/strip, you probably want a stall in the 2,500-3,500rpm range. But do not buy an off-the-shelf converter thinking it will give you the advertised stall unless it has been proven to do so in an identical setup. Speak to the manufacturer first to be sure you are getting what you need for your particular combo.
Performance engines are modified to produce more horsepower and torque. It is essential to know what the peak torque of your engine is and to match the stall speed ratio of the torque converter with the engine’s power curve. This match will give the optimum performance or “launch” of the vehicle. There are no closer-tied components in any vehicle than the engines’ camshaft and the torque converter.
Sprags and Spragless Racing Converters
Simply put, a sprag is a one way clutch, functioning just as a ratchet does in your toolbox. It is locking in one direction and freewheeling in the other direction. The sprag assemblies are often damaged during the initial tire shake as the racer goes through the burnout. The sprag is rapidly loaded and unloaded during the tire shake, causing the sprag to wear unevenly and eventually break. A worn sprag is actually worse than a broken sprag as the racer has no idea why the vehicle is so inconsistent and will spend countless hours checking everything but the converter. When the sprag eventually breaks, a racer will see the car slow down about .5 seconds and spend hundreds of dollars to repair the sprag until it cracks or breaks again. If the sprag slipping occurs on the starting line, the converter in turn will "load" the engine just for a moment (i.e. a couple of hundredths) and the car will roll through the stage beam at an RPM less than normal stall speed.
If the sprag in the stator of the converter slips on the shift change, the RPMs will fall back to something lower than the normal stall speed of the converter. Recent advances in technology, materials, and manufacturing techniques have resulted in the availability of stronger sprags and roller clutches. However, the horsepower and torque levels that these components are subjected to have also increased. Other factors such as burnout technique, transmission condition (particularly the converter charge circuit), etc., also play a role in sprag durability.
The major advantage of a "spragless" converter is that it does not have a sprag to malfunction or break. *A bracket or ET Eliminator type race car must repeat, round after round, to win a race and the "spragless" racing converter offers this huge advantage which is needed for this type of racing.
*This denotes bracket and ET Eliminator type race cars raced with electronics, example, throttle stops, RPM limiters, etc.
So in an all out bracket or ET Eliminator type setup, a spragless is a good idea for repeatability. But in a street car it will not free wheel, thus creating more heat and reducing mileage.
Selecting the Right Racing Converter
In the case of selecting a torque converter for your race car or street/strip vehicle, it is important to follow a ‘logical’ process to help in selecting the right components. It all begins by determining your performance ‘target’.
The key is the engine’s peak torque figure. Once you have determined that rpm figure you should select a torque converter with a stall speed 300 to 400 rpm higher than that number. Upon the initial starting line launch with race cars running a trans-brake, the rpm drops about [300 to 400 rpm] so the stall speed should compensate for that change. You never want the rpm to drop below that peak torque figure.
Size matters; even with torque converters! Torque converter size can also be confusing. Basically, the smaller the converter, the less fluid has to be pumped through it. Less fluid means less drag on the converter internals, which allows it to stall at higher speeds. In general, you want to avoid small converters on a typical street car due to the much higher stall speeds (usually 3,000 rpm and up). If you are adding a lot of nitrous (over 200 horsepower), running high blower pressure (over 12 psi), or use a trans-brake, you will need a converter built to handle the extra stress. The extra torque generated can cause a converter to “balloon", or expand in diameter.
Some guidelines are:
When ordering a torque converter for your race car, it is almost impossible to give your converter builder too much information. By supplying the correct information, you greatly enhance your (and the converter builder’s) opportunity for success in getting you the right converter for your application. The things that the builder must know include:
The "one size fits all" theory doesn't work for performance torque converters. No matter what your buddy tells you, or what advice you’re given by the speed shop counter guy or the Internet, we recommend you use a converter suited to your specific application.