But i've been starting to see the phrase "Torque Multiplying" more and more in regards to getting more torque to the ground. Please explain...

Thanks in advance.

Erik

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1966 Ford Mustang 2+2
Mine - Restomod in Progress

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Alex Denysenko
Co-Administrator and Moderator

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Torque Converters Explained
By Phil Parsons

Most hot rodders know what a torque converter does, but relatively few know how it does it. The converter is a sealed unit, so the classic "take 'er apart and figure it out" technique does no good. They are taken out of the box and installed on the car without ever learning all that much about it. This article should give you some insight.

The torque converter offers two main benefits. The first is hinted at by the name: torque converters actually multiply torque at low engine speeds, usually by a ratio of about two to one. That means that a car producing approximately 300 ft/lbs. of torque at the flywheel will put out approximately 600 ft/lbs. of torque at the rear wheels (less any parasitic losses from mechanical drag and the operation of the converter itself). The second is that a torque converter allows the car to stop and idle without disconnecting the engine from the rest of the drive train. Basically, a true automatic transmission would not be possible without the torque converter.

The torque converter consists of three main components: the impeller, the turbine, and the stator. The impeller is bolted directly to the flexplate, and rotates at engine speeds (the impeller is an integral part of the outer cover itself). The turbine is splined to the input shaft of the transmission (it is the shaft that protrudes from the back of the converter). The stator is sandwiched between these two, and is equipped with a one way clutch. All three components have canted blades that are somewhat similar to those of a fan. The converter assembly is filled with transmission fluid, which it shares with the rest of the transmission (as a matter of fact, the majority of the fluid in a transmission is contained in the converter). As you read on, refer back to the following illustration to keep track of the different parts:

As the engine spins the impeller, the blades on the impeller pick up the fluid, and force it towards the outer circumference of the converter. The high pressure fluid is then picked up by the blades of the turbine (which are canted in the opposite direction), forcing it (and the rest of the drive train) to turn in the same direction as the engine. If the resistance in the drive train is greater than the force of the fluid, the turbine stays still, and the converter "slips", allowing the vehicle to remain stopped.

At lower speeds (when the drive train(turbine) is spinning slower than the impeller(engine)), the fluid leaves the turbine at an angle, and enters into the stator. The blades of the stator accelerate the fluid, and then send it back into the impeller at a higher pressure, where it is then accelerated again, and sent back to the turbine at an even higher pressure. This is called vortex flow (the arrows in the illustration), and it is what creates the torque multiplication that gives automatic equipped vehicles such strong low speed torque. One way roller clutches in the stator prevent it from spinning backwards (and ruining the vortex effect).

As the turbine picks up speed to match that of the impeller, centrifugal force sends the fluid outward and prevents it from being returned to the impeller. At the point where the turbine is spinning at roughly 90% of the impeller speed, the fluid begins to hit the backs of the stator blades, unlocking the roller clutches and forcing the stator to turn in the same direction as the impeller and turbine. This is known as the coupling phase, and during this time, there is no torque multiplication. This is a brilliant design. Using this system, the torque converter automatically applies the correct amount of torque to the rear wheels. When the vehicle is launching (i.e. under load), the engine(impeller) is spinning, but the drive train(turbine) isn't. The difference in speeds creates the vortex flow through the stator, and applies full torque multiplication (2:1). While this multiplication phase makes tremendous power, it also generates tremendous heat and drastically reduces efficiency: if the converter were to stay in this phase continually (as it would if your stall speed was too high) the transmission's life would be greatly shortened, and efficiency would suffer. To adjust for this, as the vehicle picks up speed and the drive train (turbine) speed catches up to that of the engine (impeller), the converter gradually reduces vortex flow, until the entire assembly is turning at nearly the same speed in the coupling phase. This increases efficiency and reduces heat.

One traditional problem with automatic transmissions is that the converter never achieved a true coupling; that is, there was always a small difference in speed between the impeller and turbine. This was due to the fact that the converter uses fluid for power transfer, rather than a direct mechanical coupling. Thus, the high speed efficiency of automatics suffered, because some of the engine's power was always lost as heat from slippage in the converter. This problem was alleviated somewhat by the development of the lockup torque converter. This type of converter uses a clutch (similar to the type used in stick shift transmissions) to create the needed direct mechanical coupling, which eliminates slippage at highway speeds. The clutch is usually engaged at steady cruising speeds by an electrical solenoid in the transmission. Some drag racers use a switch to manually engage the clutch during runs, shaving some time off of the 1/4 mile.

The basic design of the torque converter is so good that it has barely changed in over 40 years. Most of the modifications that have been made center around the design of the components themselves in terms of blade angle, etc. Also, certain converters use a second stator to achieve higher vortex acceleration rates, improving responsiveness.

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This was borrowed from,
http://www.calgarydragracing.ab.ca/index.html

I don't think they'll mind.

Selecting The Right Torque Converter
By Phil Parsons

The torque converter (TC) is probably one the most misunderstood pieces of equipment in all of automotive. Hot rodders (even experienced ones) often brag about having a "3500 stall speed converter", without even knowing what that implies (as you will find out soon, a 3500 stall speed converter would generally be useless on any street car short of the most radical of Pro Streeters). Some even boast of having just a "stall speed converter", which displays a stunning ignorance, because ALL automatic transmission equipped vehicles have a "stall speed converter" - it's the number that makes the difference. The whole idea behind different stall speeds is to allow the car to launch at or just below the point where the engine makes the most torque. That way, the engine doesn't have to build up to the peak RPM point - all of the power is right there, on tap. If you've ever been to the drag strip, you have probably noticed that the cars rev way up before the light turns green - this is because most racing engines don't make substantial power until they are spinning over 3000 RPM. If these cars were using the stock stall speed, the tires would break loose long before the engine reached it's optimum RPM. A higher stall speed converter allows the engine to rev up to this optimum point without breaking the tires loose.

So what is stall speed? This is another widely misunderstood term. In the simplest of definitions, stall speed is the engine RPM level at which the torque converter "locks" and overcomes whatever resistance is present to turn the wheels. This resistance is the weight of the vehicle, combined with any other factors (i.e. if you have the brakes on). The old definition of stall speed used to be the engine RPM at which the brakes can no longer hold the wheels still at full throttle. This is not exactly accurate, due to the variations in brake holding power from vehicle to vehicle. In other words, in two cars that are exactly the same weight, horsepower, etc., the one with weaker brakes will display a lower stall speed, even if it really isn't, because the brakes will lose their grip at a lower RPM. The most accurate method for determining actual stall speed on your vehicle is to launch the vehicle at full throttle, and note the rpm at which the car actually takes off (this generally requires a partner watching the tach). This will be quite low on stock vehicles - around 1500-1800 RPM, slightly higher if the engine has been modified.

The next question probably goes something like, "If I already know the stall speed (i.e. what was printed on the box), why would that number change in my car?" The answer is that even though all converters have a rated stall speed (based on a fixed set of torque and weight figures), there are variables that affect this figure, mainly vehicle weight and engine torque. If you are really sharp, you may have already figured out why these two variables affect the stall speed. Weight affects the stall speed because it changes the amount of resistance that the converter has to overcome. A lighter car produces a lower stall speed because the amount of resistance (weight) has been decreased. By the same token, a more powerful engine also lowers stall speed because in the simplest terms, increasing engine power has essentially the same effect as decreasing vehicle weight.

Obviously it is extremely important to know what your vehicle's weight and peak torque is before ordering a torque converter. One of the worst mistakes in all of hot rodding is to buy a converter with a stall speed that is too high. This usually results in a car that is not only slower than it used to be, but also gets horrendous fuel economy and eats transmissions. This is because the converter is slipping all of the time, absorbing power and passing it along as heat to the rest of the transmission. If Joe X. tells you that his TPI equipped 305 Camaro has a 3500 stall speed converter, and you're reasonably sure that he's telling the truth, challenge him to a race and bet large amounts of money on it. Why? At 3500 RPM, TPI in stock trim is pretty much at the end of it's torque curve, meaning that most of the engine's usable torque is absorbed by the converter and passed along as heat. The same exact car with a stock converter would destroy Joe's car off the line, because the stock converter is designed to take advantage of the TPI's excellent low end torque output by using a stall speed of under 2000 RPM, right under the peak torque.

Probably the most important factor to consider when selecting a torque converter is the camshaft. The connection may not seem obvious, but the fact of the matter is that the camshaft basically dictates the RPM level at which the engine will produce it's peak torque, which will in turn dictate the optimum stall speed. If your camshaft has a duration of 220-230 degrees (@ 0.050" lift) or more, you definitely want to think about a higher stall speed converter, probably about 1000 RPM over stock, because the engine will probably make peak torque at well over 2000 RPM. A general rule of thumb is that most stock small blocks (especially TPI equipped) are designed to make most of their torque at low RPM, while small blocks with high horsepower generally lack low RPM torque. Does this mean that you shouldn't bother with an aftermarket converter if you don't have a radical cam? No. Performance converters are usually designed to accelerate more aggressively than stock, so an aftermarket converter with the same stall speed rating as stock will often be more responsive than the stock unit. You just want to be very careful about the stall speed that you select.

Here are some general guidelines for selecting a converter:

According to B&M, the stall speed should be rated at about 500-750 RPM under your engine's peak torque RPM. If you don't know this figure, be conservative in your estimate. You don't want to end up with a converter that has too high of a stall speed. Don't be too conservative, though - it is possible to get a converter with too low of a stall speed, which will have roughly the same effect as too high of a stall speed.

Know your camshaft specifications. If your cam has less than 220 degrees duration (@ 0.050" lift), which most street machines do, you make most of your torque down low in the RPM range, and you probably won't need more than a 2500 RPM stall speed, if even that much.

Have a good idea of your vehicle's weight. Remember, lighter vehicles will lower the rated stall speed; heavier vehicles will have the opposite effect.

High stall converters generate a lot of extra heat. The installation of an external transmission cooler is mandatory with a higher than stock stall speed converter. Actually, you should have one in there anyway. Heat is the number one killer of transmissions - 85% of all trannies die because of inadequate cooling.

The best advice I can give anyone buying a converter is to talk to the manufacturer. They know torque converters better than anybody, and can help you to select exactly the right converter for your combination. This article was designed to give you some insight into what is needed to determine the right converter, and to make you familiar with the terms and what you need to know to speak intelligently with the experts.

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