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
