A common request repair shops get is for a high stall converter that inevitably begins like this: “I need a 2500 stall converter for a TH400.” This is like walking into a shoe store and telling the clerk to “give me a pair of shoes” and expecting them to hand you the perfect fit and type of shoes. The clerk cannot know your size, style, intended use, or budget. Much like shoes, torque converters are not a “one size fits all” proposition.
There are several factors to consider when choosing a performance torque converter, and it is a lot like shoes; the better the fit, the more the customer will be satisfied. Although the customer may give some initial resistance, we should be steering the consumer towards what they truly need. The customer may not know enough to understand what they truly need. Before calling your torque converter supplier, it is worthwhile to take the time to ask a few important questions.
Let us start by defining stall speed. Stall speed is defined as the maximum slippage that you can achieve with a given input torque. The more torque you put into the converter; the higher the stall speed. Therefore, the same torque converter will have a higher stall with a 454 engine than when used with a 283. When asking for a particular stall speed, is the customer asking about Static stall, Brake stall, or Flash stall?
Static stall is the maximum slippage that can be achieved with a given engine with the input shaft locked. This is the preferred method, because it eliminates most of the variables. Static stall usually requires a transbrake, but sometimes can be done with brakes alone. Brake stall is similar to Static stall, but it is the stall RPM when the torque from the engine overpowers the brakes, and allows the wheels spin. This method of determining stall speed can be affected by final drive gearing, tire size, and vehicle weight. Flash stall is determined by letting the vehicle accelerate smoothly, and then punching the throttle while taking note of the instant RPM increase of the engine. This is the least preferred way to determine stall, because it is the most affected by the variables.
Why do we need a high stall converter? In general, factory engines typically start producing relevant torque around 1700 RPM and peak torque around 3500 RPM. They are matched with a converter that has enough slip that will allow the engine to come up to that 1700 RPM range. High performance engines are designed to operate at higher RPM ranges and need a converter to match the torque curve of the engine. If the converter stall speed is too low, then the car will be sluggish on breakaway or launch. If the stall speed is too high, then it results in wasted efficiency and more heat generation.
Another element to consider is the Stall Torque Ratio. This is the ability of the converter to multiply torque. As with most things in life; more is not always better. A high-powered engine in a lightweight car with small tires and limited suspension modifications is not a good match with a converter that has a high STR.
Yet another consideration is to identify what the vehicle is going to be used for; street and strip, strip only, road racing, rock hopping, and so on. There is quite a difference between a converter that is set up for drag racing and one that is used for circle track.
Most high-performance converter shops will use a specification sheet like the following example. This helps determine what information will need to be collected from your customers. This will include information on the engine. A dyno sheet can provide the best direction. But if that is not available, then cam specs, head flow, compression, and intake system information will guide you.
You will also need to know whether there are any power adders like nitrous, turbos, or superchargers. Additionally, you will need to know chassis information such as weight, final drive ratio, tire size, and suspension. You will be able give your customer a transmission and converter that will be the perfect fit for their application by taking a few minutes and gathering the needed information from you customer before ordering a converter.