TORQUE CONVERTERS EXPLANED.
Torque converter ---
a torque converter is a fluid-coupling device that also acts as a torque
multiplier during initial acceleration.
The Torque
Converter consists of four primary components:
Cover ---
the cover (also referred to as a front) is the outside half of the housing
toward the engine side from the weld line. The cover serves to attach the
converter to the flywheel (engine) and contain the fluid. While the cover is not
actively involved in the characteristics of the performance, it is important
that the cover remain rigid under stress (torsion and thrust stress and the
tremendous hydraulic pressure generated by the torque converter internally.)
Turbine ---
the turbine rides within the cover and
is attached to the drive train via a spline fit to the input shaft of the
transmission. When the turbine moves, the car moves.
Stator ---
the stator can be described as the
"brain" of the torque converter, although the stator is by no means the sole
determiner of converter function and characteristics. The stator, which changes
fluid flow between the turbine and pump, is what makes a torque converter a
torque converter (multiplier) and not strictly a fluid coupler.
With the
stator removed, however, it will retain none of its torque multiplying effect.
In order for the stator to function properly the sprag must work as designed:
(1) it must hold the stator perfectly still (locked in place) while the
converter is still in stall mode (slow relative turbine speed to the impeller
pump speed) and (2) allow the stator to spin with the rest of the converter
after the turbine speed approaches the pump speed. This allows for more
efficient and less restrictive fluid flow.
The sprag is a
one-way, mechanical clutch mounted on races and fits inside the stator while the
inner race splines onto the stator support of the transmission. The torque
multiplier effect means that a vehicle equipped with and automatic transmission
and torque converter will output more torque to the drive wheels than the engine
is actually producing. This occurs while the converter is in its "stall mode"
(when the turbine is spinning considerably slower than the pump) and during
vehicle acceleration. Torque multiplication rapidly decreases until it reaches a
ratio of 1:1 (no torque increase over crankshaft torque.)
A typical
torque converter will have a torque multiplication ratio in the area of 2.5:1.
The main point to remember is that all properly functioning torque converters do
indeed multiply torque during initial acceleration. The more drastic the change
in fluid path caused by the stator from its "natural" return path, the higher
the torque multiplication ratio, a given converter will have. Torque
multiplication does not occur with a manual transmission clutch and pressure
plate; hence the need for heavy flywheels, very high numerical gear ratios, and
high launch rpm. A more detailed discussion of torque multiplication can be very
confusing to the layman as high multiplication ratios can be easily considered
the best choice when in fact more variables must be included in the decision.
Remember, the ratio is still a factor of the engine torque in the relevant range
of the torque converter stall speed, i.e.: a converter with a multiplication
ratio of 2.5:1 that stalls 3000 rpm will produce 500 ft.-lbs of torque at the
instance of full throttle acceleration if its coupled to an engine producing 200
ft.-lbs of torque at 3000 rpm. However, if this same engine produces 300 ft.-lbs
of torque at 4000 rpm, we would be better off with a converter that stalled 4000
rpm with only a 2.0:1 torque multiplication ratio, i.e.: 300 x 2.0 = 600
ft.-lbs. at initial acceleration. Of course it would be better yet to have a
2.5:1 ratio with the 4000 rpm in this example (provided his combination still
allows the suspension to work and the tyres don't spin.) This is just a brief
overview as the actual scenarios are endless.
Impeller pump ---
the impeller pump is the outside half of the
converter on the transmission side of the weld line. Inside the impeller pump is
a series of longitudinal fins, which, drive the fluid around its outside
diameter into the turbine, since this component is welded to the cover, which is
bolted to the flywheel. The size of the torque converter (and pump) and the
number and shape of the fins all affect the characteristics of the converter. If
long torque converter life is an objective, it is extremely important that the
fins of the impeller pump are adequately reinforced against fatigue and the
outside housing does not distort under stress.
Stall speed ---
the rpm that a given torque converter (impeller) has to spin in order for it to
overcome a given amount of load and begin moving this turbine. When referring to
"how much stall will I get from this torque converter", it means how fast (rpm)
must the torque converter spin to generate enough fluid force on the turbine to
overcome the resting inertia of the vehicle at wide open throttle. Load
originates from two places: (1) From the torque imparted on the torque converter
by the engine via the crankshaft. (This load varies over rpm, i.e. torque curve,
and is directly affected by atmosphere, fuel and engine conditions.) (2) From
inertia, the resistance of the vehicle to acceleration, which places a load on
the torque converter through the drive train. This can be thought of as how
difficult the drive train is to rotate with vehicle at rest, and is affected by
car weight, amount of gear reduction and tyre diameter, ability of tyre to stay
adhered to the ground and stiffness of the chassis. (Does the care move as one
entity or does it flex so much that not all the weight is transferred during
initial motion?)
Note:
While referring to the resistance of the vehicle to move while at rest,
the torque converter's stall speed and much of its characteristics for a given
application are also affected by the vehicle's resistance to accelerate relative
to its rat of acceleration. This resistance has much to do with the rpm observed
immediately after the vehicle starts moving, the amount of rpm drop observed
during a gear change and the amount of slippage in the torque converter (turbine
rpm relative to impeller pump rpm.) A discussion involving how resistance to
acceleration affects a torque converter involves more theory than fact and must
involve all the dozens of other variable that affect rpm and slippage. The
primary thing we want to remember about torque converter stall speed is that a
particular torque converter does not have a "preset from the factory" stall
speed but rather its unique design will produce a certain range of stall speeds
depending on the amount of load the torque converter is exposed to. This load
comes from both the torque produced by the engine and the resistance of the
vehicle to move from rest. The higher this combined load the higher stall we
will observe from a particular torque converter, and conversely, thee lower the
load, the lower the stall speed. Naturally, if the engine is not at wide open
throttle we will not expect to observe as high a stall speed as we would under a
wide open throttle.
Another point
concerning engine torque is that we are only concerned with what we'll call the
"relevant range" of the engine torque curve when discussing initial stall speed.
This means if a particular torque converter chosen has a design that should
produce a stall speed in a range of say 2000 to 2600 rpm given the application
then we would refer to this as the relevant range of our interest in the
engine's torque curve for this particular torque converter. In other words, only
the torque characteristics of the engine torque is this rpm range will affect
the amount of stall speed we actually observe. If we are using a high
horsepower/high rpm engine that does not make much torque before 3000 rpm, it
does not matter that the engine makes excellent torque over 3000 rpm if we are
trying to use the torque converter in this example because its relevant range is
2000-2600 rpm and we would expect to see poor stall (2000 rpm or less) due to
the poor torque produced by the engine in this range.
Choosing the correct
application torque converter --- The
buyer of a performance torque converter normally has very specific "wants" to be
filled, namely: They want to improve the performance of their vehicle. This can
mean they may want the new torque converter to help the car run quicker, run
faster, idle in gear better, leave from a stop harder, "chirp" the tyres on the
gear changes, or pull a steeper hill. The buyer may be looking for any or all of
these performance improvements.
They want to
improve the dependability of their vehicle meaning they want to get rid of
existing drive train failures they are currently having with either OEM or
competitors products such as short life (to what they perceive is a proper
life), "trash" related transmission failures, overheating, hard part breakage,
engine problems that they may believe is caused by torque converter and general
unreliable performance.
They may have
been told by friends, salespeople, advertising, technical articles, etc. that
their particular application needs to have a "stall" converter. This is
particularly true of first time performance camshaft purchasers where the
salesperson or the camshaft catalog, will recommend a higher than stock stall
speed torque converter.
A torque converter does not function in a
void by itself. The converter is an integral part of the total vehicle
combination. While many vehicle combinations and applications are very similar
and it may seem obvious what the best torque converter selection is, it is
normally a wise step to take a look at the intended application and choose the
best torque converter for the particular application. Most converter
manufacturers use an application questionnaire to gather the pertinent
information. There is no "black magic" formula that the variables can be plugged
into resulting in a definitive torque converter choice. Torque converter choices
are made based on accumulated historical knowledge of performance in various
applications and the use of all or several basic charts and ratios derived
through this historical information. As with many other automotive performance
parts, torque converter design and construction is a dynamic art and can not be
patterned on the results of a "plug-in" formula or solely allowed to follow the
historical applications.
Dependability concerns in choosing a torque
converter: Regardless of the reason or "want" for buying an aftermarket torque
converter, and educated buyer should look for several features in the product he
is considering purchasing in order to assure that he can reasonably expect to
receive dependable results and long life from the purchase.
Furnace brazed fins ---
greatly improves the strength characteristics of the fins. The furnace brazing
causes the housing and fins to move and act integrally as one unit. This greatly
reduces the amount of flex, which causes fins to bend and break. Also, the more
rigid the fins stay while under pressure, the more consistent the behaviour of
the torque converter.
Service and time proven
manufacturer --- Ask for
recommendations from leading car enthusiasts in your local area or check out
what the racers are using.
Drivability concerns in
choosing a torque converter --- A
performance torque converter should not compromise one aspect of car performance
to achieve another. When investigating a converter purchase ask whether the
particular torque converter being looked at may improve initial takeoff at the
sacrifice of top end mph or other similar results, questions, etc.
With the technology and product available
today a buyer very seldom needs to sacrifice one area of performance to gain in
another. However, without proper selection assistance or guidance (and with many
under engineered products on the market today) it is unfortunate that many
buyers end up with a product that does not best suit his needs or expectations.
Too low a stall torque converter will not benefit the customer. If the user has
an application which requires at least 3000 rpm stall and they purchase a 2000
to 2500 rpm stall range converter, it will normally not even give them the 2000
rpm stall. It will act very similar to the stock torque converter they just
removed...why? Because the engine needs to operate in its optimum rpm range and
since the chosen torque converter is below that range, it is not getting enough
load from the crankshaft side to operate as designed. Symptoms include engine
stalling when in gear at a stop, low stall speed, hesitation when going to full
throttle, a "bog" when leaving from stop at wide open throttle. Too high a stall
range torque converter will not benefit the customer. You will see this
situation most often when the customer does not have sufficient gear ratio for
the converter stall range or the engine is not capable of the appropriate rpm
range (too small a duration camshaft, inadequate valve springs, too low
compression, etc.) Symptoms include high "revs" to pull away from stop,
"marshmallow" accelerator feel when driving at part throttle, transmission and
possibly engine overheating, and a pronounced engine rev when nailing the
throttle from a cruising speed.
