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The torque converter also replaces the clutch that is
required in a manual transmission; this is how an automatic
transmission vehicle can come to a stop while still being in
gear without stalling the engine.
The torque converter also acts as a torque multiplier, or
extra gear ratio, to help the car get moving from a stop. In
modern day converters this theoretical ratio is anywhere
between 2:1 and 3:1.
Torque converters consist of 4 major components that we need
to concern ourselves with for the purpose of explanation.
The first component, which is the driving member, is called the impeller
or "pump". It is connected directly to the inside of the
converter housing and because the converter is bolted to the
flex plate, it is turning anytime that the engine rotates.
The next component, which is the output or driven member, is
called the turbine. The transmission's input shaft is
connected to it. The turbine is not physically connected to
the to the converter housing and can rotate completely
independently of it.
The third component is the stator assembly; its
function is to redirect the flow of fluid between the impeller
and the turbine, which gives the torque multiplication effect
from a standstill.
The final component is the lock up clutch. At highway speeds
this clutch can be applied and will provide a direct
mechanical link between the crankshaft and input shaft, which
will result in 100% efficiency between the engine and
transmission. The application of this clutch is usually
controlled by the vehicle's computer activating a solenoid in
the transmission.
Here's how it all works. For the sake of simplicity, I will use the
common analogy of two fans which represent the impeller and
the turbine. Let's say that we have two fans facing each other
and we turn only one of them on- the other fan will soon begin
to move.
The first fan, which is powered, can be thought of as the
impeller that is connected to the converter housing. The
second fan- the "driven" fan can be likened to the turbine,
which has the input shaft splined to it. If you were to hold
the non-powered fan (the turbine) the powered one (the
impeller) would still be able to move- this explains how you
can pull to a stop without the engine stalling.
Now imagine a third component placed in between the two, which
would serve to alter the airflow and cause the powered fan to
be able to drive the non-powered fan with a reduction of
speed- but also with an increase of force (torque). This is
essentially what the stator does.
At a certain point (usually around 30-40 mph), the same speed
can be reached between impeller and the turbine (our two
fans). The stator, which is attached to a one way clutch, will
now begin to turn in conjunction with the other two components
and around 90% efficiency between the crank and the input
shaft can be achieved.
The remaining 10% slippage between the engine and transmission
can be eliminated by connecting the input shaft to the
crankshaft through the application of the lock up clutch that
was mentioned before. This will tend to lug the engine, so the
computer will only command this in higher gears and at highway
speeds when there is very little engine load present. The main
function of this clutch is to increase fuel efficiency and
reduce the amount of heat that is generated by the torque
converter.
Another term that may be unfamiliar is that of a "high stall"
torque converter. A high stall converter differs from a stock
converter in the sense that the rpm is raised at which the
internal converter components- the impeller, the stator and
the turbine start to turn together, and hence, stop the torque
multiplication phase and begin the coupling phase. The point
at which engine rpm will stop climbing with the drive wheels
held stationary and the throttle fully opened is referred to
as "stall speed".
The idea behind a higher stall torque converter is to allow
the engine to rev more freely up to the point where the power
band begins, and therefore, enable the vehicle to accelerate
from a stop under more power.
This becomes increasingly important when an engine is
modified. Engine modifications such as ported heads, bigger
cams, bigger turbos (in some cases), bigger intakes, etc. tend
to raise the point where the power band begins. For best
performance, the stall speed needs to be raised accordingly to
work optimally in conjunction with the given vehicle
alterations.
In simple
terms, for best performance, the stall speed should be raised
at least to the point where the torque curve is heading
towards its peak. As a rule of thumb, the stall speed should
be set to match the rpm at which the engine is making at least
80% of its peak torque for a street driven vehicle.
As you can imagine, a vehicle that can accelerate from a stop
with 80% of its peak torque will easily outperform an
otherwise identical vehicle that can only launch at 50% of its
available torque.
For a performance or "high stall" torque converter to produce
maximum gains, it needs to be configured to the specific
vehicle in which it will be installed.
Factors such as engine torque and the rpm at which it is
greatest, differential gear ratio, vehicle weight, camshaft
design, compression ratio, type of induction- forced or
naturally aspirated, and a host of other variables all need to
be taken into consideration.
Be aware that the "off the shelf" type performance torque
converters sold by some manufacturers are very unlikely to be
optimized for all vehicles and their unique requirements.
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