Before we can answer what a Hi Stall is we first need to know what a Torque Converter is, Wikipedia defines a Torque Converter as a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. It is usually located between the engine's flexplate and the transmission.
The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. Some of these devices are also equipped with a temporary locking mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.
Not a bad explanation to be honest, the only problem is that most people wouldn’t understand any of it. Essentially the easiest way to describe what a torque converter does is to think of a clutch on a hobby Go Kart, as the engine RPM increases, the clutch begins to engage and drive is achieved. Now a Go Kart uses a mechanical torque converter or a centrifugal clutch while the torque converter in a modern automatic transmission uses transmission fluid being pumped by the impeller and flowing through the turbine to achieve drive.
So in your modern automatic transmission equipped car, the engine is connected to the impeller and the turbine is connected to the transmission which eventually sends its drive to the wheels. In between the impeller and the turbine is the stator, once the oil has passed through the turbine, the oil then passes through the stator and is redirected to the impeller. By passing through the stator and having the direction of the flow altered by the stator, the torque converter is able to work more efficiently than other types of fluid couplings.
Another aspect that makes a torque converters operation unique and far superior to a centrifugal clutch or a fluid coupling is the fact that a torque converter acts like an infinitely variable ratio transmission and thereby multiplies torque. What this means is that an automatic transmission that is equipped with a torque converter needs far less gears than a manual transmission with a clutch to achieve the same performance. The amount that the torque converter multiplies torque by is referred to as the Stall Torque Ratio (STR for short).
So what is stall speed? Well if we mechanically locked the turbine and opened the throttle wide open the engine would rev to the Stall Speed for that given design of converter. There are several factors that affect Stall speed but the essential thing that you need to understand is that the RPM achieved on the foot brake while “Stalling Up” is not the stall speed and has little to do with the stall speed of the converter and more to do with the STR of the converter, your differential ratio and the efficiency of your rear brakes.
So what does the stall speed do that makes having a Hi Stall so important in a high performance car? First and foremost, drivability, engines that are equipped with larger than factory camshafts tend to make very little vacuum at low RPM and this will tend to make them stall as soon as any load is applied. In a manual car you simply depress the clutch at a higher RPM and the effects of the low manifold vacuum are overcome, but in an automatic car, as soon as the gear shifter is moved to drive and the load of the torque converter is felt by the engine, the engine may (or will) stall. A higher stall speed will correct this ailment.
So to stop your car stalling when you put it in gear is one reason to fit a Hi Stall, but we are just scratching the surface. As I explained earlier, the true stall speed is the RPM the engine achieves when the throttle is wide open and the turbine is mechanically locked, the most common time you will see this is when a Trans Brake is used. A Trans Brake locks forward and reverse so that the vehicle will not move, Trans Brakes are not for the faint hearted and are certainly not common on the street but they are almost a necessity in a serious modern drag racing automatic.
Every engine has an optimum operating RPM where the engine will achieve its best average HP, the key to optimizing your performance on track is keeping the engine within this RPM range. The dyno sheet shows a typical 550Hp small block that makes its peak Hp at about 6300 RPM. If I was setting this engine in a car with a 2 speed (Powerglide) transmission for drag racing use, I would best looking for the best average 1200RPM. With a 3 speed car we could go as tight as the best average 800RPM, in some instances we may even use a tighter sweep again.
In the above example the engine seems to make its best average HP between 5800 and 7000RPM, so I would recommend a Hi Stall with 5800RPM stall and advise the driver to shift at 7000RPM. This will guarantee the best average Hp and in returns it will guarantee the quickest acceleration.
A lot of you are probably thinking that a 5800RPM stall means that the car will not move until it reaches 5800RPM, well you’re completely wrong. As I mentioned before the drive line needs to be completely locked and the throttle needs to wide open to achieve 5800RPM stall. When you launch the car (at the drags) and assuming that you have 100% traction, you will notice that the tachometer will go directly to 5800RPM and then on the gear shift it will drop back down to 5800RPM, but when you drive the car around with light throttle the car will move at a much lower RPM.
The amount that the car moves is determined by the STR for that given converter, the STR also has to match the weight of the car, the differential ratio and the tyre size. For example a heavy car with 3.0 : 1 differential ratios and a 28” tyre will need a very high STR or the engine will make a lot of noise but not go anywhere and the oil in the torque converter will eventually boil. A light vehicle with 4.11 : 1 ratio’s and a 26” tyre will need a very low STR or you will not be able to stop the vehicle; as I explained earlier, a torque converter has the ability to multiply torque, the amount that it multiplies torque in called the STR (Stall Torque Ratio) and if the converter multiplies torque too much for the weight of the vehicle and the differential ratio and the tyre size, it will feel very pushy and hard to stop, in some cases you won’t be able to stop it once it is in gear.
If your car has had modifications including differential ratio change, larger camshaft, extractors, exhaust, tune etc then it will more than likely need a Hi Stall, if your car is standard but you read in a magazine or one of your friends said you need a Hi Stall (in these scenario’s the magic 3500 or three and a half seems to be the common Stall that is asked for) then you probably don’t need one!
See you on the street