The reason a stall converter works is simple; an engine at 1, RPM is only making about 80 or so horsepower on average , but at 2, RPM it could be making to HP, and obviously trying to get a car moving using HP is going to be much easier than one trying to launch at 80 HP or so.
Most performance engines don't make power until 3, or so RPM, hence why when you have a higher horsepower engine with a big cam, you need a higher stall speed so the engine is closer to it's "power band" when taking-off from the line, otherwise it will fall on its face and be a turd off the line. Some people believe that "stall" means the car won't start moving until the engine reaches that particular RPM, and that isn't even close to being true.
Most street performance cars running your typical "performance" cam should be running a stall converter in the 2, - 3, RPM area. For mild performance cars with something like a HP engine, a 2, - 2, stall is about right.
The basic rule of thumb is; if your engine "comes alive" at say, 3, RPM, then you want a stall converter with about 3, of stall to it. Most people tend to over cam their cars In this case, the slip in the Torque Converter is maximized. Also, the maximum torque multiplication that can be provided by a torque converter is called stall torque. One of the many parameters to consider in the design of a torque converter is the stall speed.
Stall speeds of torque converters vary according to the needs of each vehicle. In practice, this stall in a Torque converter only exists for a short time. Put that D or R into gear and start accelerating. After that, the turbine starts to spin. But torque converter stall speed directly affects vehicle performance, starting torque, and so on. Now the problem is with the Stall test.
As described earlier, the Stall test is used in the design of torque converters when performing performance testing and fixing on vehicles. This is done by practicing the aforementioned stall method and checking to see if the stall speed is set. There are several ways that stall tests can be performed. Many performance engines will not have good horsepower until around RPM, which is why when you have higher HP engine with a large cam, you will need a higher stall speed so that it will be closer to the power band when it takes off, otherwise it will fail and start out slow.
A car that has a RPM stall converter in gear and idling will roll along at about 10 MPH, just like any other car that is in gear with the brake off. Normally cruising into the pits, is done with the car idling and in gear. Stalling means that whenever the car is on the start line, and the trans brake is on, and you hit full throttle, the engine will rev up to RPM with the car sitting there, so whenever you slip that finger off the trans brake, and the tranny slips out of reverse, the car will launch at RPM, and a race motor at that RPM is close to hitting the peak torque curve when it leaves the line, which is why you get big wheelies or low 60ft times whenever the traction is good.
Race engines will have no power at a low RPM, and will be shy on torque, so the nastier that the engine is, the higher the stall speed will need to be. Gear ratio, cubic inches, weight, and torque have a factor in this. It is a pretty complicated process, so I will keep it simple. This is true for a RPM converter. Most mild race cars will use between to stall speeds on average.
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