Most easily describe as the engine RPM that can be reached when the vehicle is in gear with the brakes locked and the throttle wide open. Converters are often rated by a typical stall speed number, such as 2000, 2500, 3200 etc. If you have what is referred to as a 2500 RPM stall converter the engine torque will begin to overtake the holding capacity of the brakes at about 2400 or 2500 engine RPM. The key advantage of a higher than stock stall speed converter is it will allow the vehicle to accelerate much quicker. This improved acceleration will provide quicker elapsed times at the drag strip by as much as ½ second or more.
A typical torque converter will multiply the engine torque by 2:1 with some units producing as much as 2.35:1. The main advantage to the torque multiplication is better acceleration. As an example: If your engine produces 200 lb. ft. of torque at 2000 RPM the torque converter, at a 2:1 ratio, will spin the input shaft of the transmission with 400 lb. ft. of torque. This is quite an advantage under initial acceleration. When you take that to the next level when using a higher stall speed torque converter your vehicle will have tremendous acceleration capabilities.
One of the negative factors of a higher stall speed torque converter is it will produce more heat in the fluid. All higher stall speed torque converters will generate more heat regardless of the brand. Be sure to protect your investment, in a performance torque converter and transmission, with a supplemental transmission fluid cooler. There are many good coolers on the market available from Tru-Cool, Flex-a-Lite, Perma-Cool and B&M just to name a few brands.
A good cooler is a great investment for many trouble free miles down the road. Another way to keep the fluid cool is to install a high capacity oil pan onto the transmission. Often times high stall converters are based on a smaller converter core than the stock unit. This will decrease the fluid capacity in the transmission and converter as a package. A deep transmission oil pan will increase the fluid capacity by as much as 2 – 4 quarts. This will help in controlling the temperature as more fluid will be available to circulate between the transmission, torque converter and the cooler.
The camshaft design is mainly chosen to increase the torque and horsepower of the engine. Often times performance enthusiasts choose a cam shaft that has a high lift and long duration. This is great if you have a stick shift transmission and can just adjust the idle up a few hundred RPM’s to allow a reasonably smooth idle when the vehicle is stopped. However, when you have an automatic transmission, this may cause engine stalling when the vehicle brakes to a stop. A higher stall torque converter can help this situation especially if the vehicle is lighter than 3000 lbs., typically a street rod.
When choosing a performance torque converter there are several factors you should consider. Engine size, power adders (Turbo, Nitrous, Supercharger), weight of vehicle, rear-end gears and how you will drive the vehicle. If you are building a cruiser for visits to the burger joint or fairgrounds, a mild/entry level converter (Spec Series) would be a good choice. If you intend to drive with a heavy throttle and spin the tires often, or if it is a heavy (3200 lbs+) vehicle, you should choose a converter with a little more strength and durability (Silver Series features furnace brazed fins). If you are driving a big block or a supercharged small block or a plate style nitrous system, the highest level of strength should be your choice (Gold Series features anti-balloon plates and furnace brazed fins).
Determining the amount of stall speed you should choose depends on the performance level you desire and your driving style. Typically a 2500 RPM stall speed converter is a good all around choice as it stalls almost a 1000 RPM above a stock converter. This is a very street friendly converter as it will drive just like a stock converter and will provide the acceleration you are looking for when you want to accelerate quickly. If you have a cam that is real radical you may need to step up to a 3200 stall speed converter if you want the vehicle to idle when stopped at a traffic signal, especially if the vehicle weighs less than 3000 lbs.
If you have a high torque big block or a supercharged small block you may want to choose a 2500 stall converter as it will stall higher than the listed number as larger displacement and supercharged engines produce a lot of low end torque (below 3000 RPM). A rule of thumb is if your cam shaft has an advertised duration below 248 degrees a 2000 – 2500 stall converter is a good choice. If the cam duration is up to 268 degrees a 2400 stall converter is a good choice for a big block while a 3200 stall is a good choice for a small block engine. If the cam duration is 272 or above a 3200 or higher stall converter should be chosen.