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IHRA-Tech e-Bulletin 9. 21st February 2020
A torque converter is a fluid-coupling device that also acts as a torque multiplier during initial acceleration. The Torque Converter consist of four primary components.
COVER: The cover (also referred to as a front) is the outside half of the housing towards the engine side from the weld line.
The cover serves to attach the converter to the flexplate (engine) and contain the fluid. While the cover is not actively involved in the characteristics of the performance, it is important that the cover remain rigid under stress (torsion and thrust stress and tremendous hydraulic pressure generated by the torque converter internally).
TURBINE: The turbine rides within the cover and is attached to the drive train via a spline fitted to the input shaft of the transmission. When the turbine moves, the vehicle moves. 
STATOR: The stator can be described as the "brain" of the torque converter, although the stator is by no means the sole determiner of converter function and characteristics. The stator, which changes fluid flow between the turbine and pump, is what makes a torque converter (multiply) and is not strictly a fluid coupler.
With the stator removed, however, it will retain none if its torque multiplying effects. In order for the stator to function properly the sprag must work as designed   
  1. It must hold the stator perfectly still (locked in place) while the converter is till in stall mode (slow reaction turbine speed to the impeller pump speed.
  2. Allow the stator to spin with the rest of the converter after the turbine speed approaches the pump speed). 
The sprag is a one-way, mechanical clutch mounted on racers and fits inside the stator while the inner race splines onto the stator support of the transmission. The torque multiplier effects means that a vehicle equipped with an automatic transmission and torque converter will output more torque to the drive wheels than the engine is actually producing. This occurs while the converter is in its "stall mode" (when the turbine is spinning considerably slower than the pump) and during vehicle acceleration. Torque multiplication rapidly decreases until it reaches a 1:1 (no torque increase over crankshaft torque) 
A typical torque converter will have a torque multiplication ratio in the area of 2.5:1. The main point to remember is that all properly functioning torque converters do indeed multiply torque doing initial acceleration. the more drastic the change in fluid path caused by the stator from its "natural: return path, the higher the torque multiplication ratio, a given converter will have. Torque multiplication does not occur with a manual transmission, clutch and pressure plate; hence the need for a heavy flywheel, very high numerical gear ratios, and high launch RPM. A more detailed discussion of torque multiplication can be very confusing to the layman as high multiplication ratios can be easily considered the best choice when in fact more variables must be included in the decision. Remember, the ratio is still a factor of the engine torque in the relevant range of the torque converter stall speed, i.e.: a converter with a multiplication ratio of 2.5:1 that stalls 3000rpm will produce 500 ft/lbs of torque in the instance of full throttle acceleration if its coupled to an engine producing 200 ft/lbs of torque at 3000rpm. However, if this same engine produces 300 ft/lbs of torque at 4000 rpm, we would be better off with a converter that stalled 4000 rpm with only 2.0:1 torque multiplication ratio, i.e.: 300 x 2.0 = 600 ft/lbs at initial acceleration. Of course it would be better yet to have a 2.5:1 ratio with a 4000rpm in this example (provided this combination still allows the suspension to work and the tyres don't spin). This is just a brief overview as the actual scenarios are endless. 
IMPELLER PUMP: The impeller pump is the outside half of the converter on the transmission side of the weld line. Inside the impeller pump is a series of longitudinal fins, which drive the fluid around its outside diameter into the turbine, since this component is welded to the cover, which is bolted to the flywheel. The size of the torque converter (and pump) and the number and shape of the fins all affect the characteristics of the converter. If long torque converter life is and objective, it is extremely important that the fins of the impeller pump are adequately reinforced against fatigue and the outside housing does not distort under stress.
STALL SPEED: The rpm that a given torque converter (impeller) has to spin in order for it to overcome a given amount of load and begin moving this turbine. 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. Load originates from two places:  
  1. From the torque imparted on the torque converter by the engine via the crankshaft. (This load varies over rpm, i.e. torque curve, and is directly affected by atmosphere, fuel and engine conditions).
  2. From inertia, the resistance of the vehicle to acceleration, which places a load on the torque converter through the drive train. This can be thought of as how difficult the drive train is to rotate with the vehicle at rest, and is affected by vehicle weight, amount of gear reduction and tyre diameter, ability of the tyre to stay adhered to the ground and stiffness of the chassis. (Does the vehicle move as one entity or does it flex so much that not all the weight is transferred during initial motion?).
Note: While referring to the resistance of the vehicle to move while at rest, the torque converters stall speed and much of its characteristics for a given application are also affected by the vehicles resistance to accelerate relative to its rate of acceleration. This resistance has much to do with the rpm observed immediately after the vehicle starts moving, the amount of rpm drop observed during a gear change and the amount of slippage in the torque converter (turbine rpm relative to impeller pump rpm). A discussion involving how resistance to acceleration affects a torque converter involves more theory than fact and must involve all the dozens of other variables that affect rpm and slippage. The primary thing we want 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. The higher this combined load the higher stall we will observe from a particular torque converter, and conversely, the lower the load, the lower the stall speed. Naturally, if the engine is not at wide open throttle we will not expect to observe as high stall speed as we would under a wide open throttle.
Another point concerning engine torque is that we are only concerned with what we'll call the "relevant range" of the engine torque curve when discussing initial stall speed. This means if a particular torque converter chosen has a design that should produce a stall speed in a range of say 2000 to 2600 rpm given the application then we would refer to this as a relevant range of our interest in the engines torque curve for this particular torque converter. In other words, only the torque characteristics of the engine torque is this rpm range will affect the amount of stall speed we actually observe. if we are using  a high horsepower / high rpm engine that does not make much torque before 3000 rpm if we are trying to use the torque converter in this example because its relevant range is 2000 - 2600 rpm and we would expect to see poor stall (2000rpm or less) due to the poor torque produced by the engine in this range.
CHOOSING THE CORRECT APPLICATION TORQUE CONVERTER: The buyer of a performance torque converter normally has a very specific "wants" to be filled, namely: They want to improve the performance of their vehicle. This can mean they may want the new torque converter to help the car run quicker, run faster, idle in gear better, leave from a stop harder, "chirp: the tyres on the gear change, or pull a steeper hill". The buyer may be looking for any or all of these performance improvements.
They want to improve the dependability of their vehicle meaning they want to get rid of existing drive train failures they are currently having with either OEM or competitors products such as short life (to what they perceive is a proper life), "trash" related transmission failures, overheating, hard part breakage, engine problems that they may believe is caused by torque converter and general unreliable performance.
They may have been told by friends, sales people, advertising, technical articles, etc. That their particular application needs to have a "stall" converter. This is particularly true of first time performance camshaft purchasers where the sales person or the camshaft catalog, will recommend a higher than stock stall speed torque converter.
A torque converter does not function in a void by itself. The converter is an integral part of the total vehicle combination. While many vehicle combinations and applications are very similar and it may seem obvious what the best torque converter selection is, it is normally a wise step to take a look at the intended application and choose the best torque converter for the particular application. Most converter manufacturers use an application questionnaire to gather the pertinent information. There is no "black magic" formula that the variables can be plugged into resulting in a definitive torque converter choice.
Torque converter choices are made based on accumulated historical knowledge of performance in various applications and the use of all or several basic charts and ratios derive through this historical information. As with many other automotive performance parts, torque converter design and construction is a dynamic art and can not be patented on the results of a "plug in" formula or solely allowed to follow the historical applications.
Dependability concerns in choosing a torque converter: Regardless of the reason or "want" for buying an aftermarket torque converter. and educated buyer should look for several features in the product his is considering purchasing in order to assure that he can reasonably expect to receive dependable result and long life from the purchase.
FURNACE BRACED FINS: Greatly improves the strength characteristics of the fins. The furnace brazing causes the housing and fins to move and act integrally as one unit. This greatly reduces the amount of flex, which caused fins to bend and break. the more rigid the fins stay while under pressure, the more consistent the behavior of the torque converter. SERVICE and TIME PROVEN MANUFACTURER: Ask for recommendations from leading car enthusiast in your local area or check out what the racers are using. 
DRIVABILITY CONCERS IN CHOOSING A TORQUE CONVERTER: A performance torque converter should not compromise one aspect of car performance to achieve another. When investigating a converter purchase ask whether the particular torque converter being looked at may improve initial take off at the sacrifice of top end mph or other similar results, questions, etc.
With the technology and product available today a buyer very seldom needs to sacrifice one area of performance to gain in another. However, without proper selection assistance or guidance (and with many under engineered products on the market today) it is unfortunate that many buyers end up with a product that does not best suit his needs or expectations. Too low a stall torque converter will not benefit the customer. I the user has an application which requires at least 3000 rpm stall and they purchase a 2000 - 2500 rpm stall range converter, it will normally not even give them 2000 rpm stall. it will act very similar to the stock torque converter they just removed...why? Because the engine needs to operate in its optimum rpm range and since the chosen torque converter is below that range, it is not getting enough load from the crankshaft side to operate as designed. Symptoms include engine stalling when in gear at a stop, low stall speed, hesitation when going to full throttle, a "bog" when leaving from  at wide open throttle. Too high a stall range torque converter will not benefit the customer. You will see this situation most often when the customer does not have sufficient gear ratio for the converter stall range or the engine is not capable of the appropriate rpm range (too small a duration camshaft, inadequate valve springs, too low compression, etc.) Symptoms include high "revs" to pull away from stop, "marsh mellow" acceleration feel when driving at part throttle, transmission and possibly engine overheating, and a pronounced engine rev when nailing the throttle from a cruising speed.
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