[W126 Coupe] auto tranny operation: torque converters

[Eddie]

I always thought you were supposed to manually rotate the driveshaft and
torque converter before and after a transmission oil change so the fluid in
the torque converter completely drains out, AND completely fills up before
the engine is turned over.

 

Edward

 

  _____  

From: mbcoupes-bounces at mbcoupes.com [mailto:mbcoupes-bounces at mbcoupes.com]
On Behalf Of Richard Hogarth
Sent: Thursday, April 05, 2007 11:34 AM
To: 'Mercedes Coupes Mailing Lists'
Subject: Re: [W126 Coupe] auto tranny operation: torque converters

 

I thought that everyone might find this interesting. 

-RPH

 


 


 


Torque Converters ExplainedTCI 8" race converter.

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
(torsional 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 an 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 get 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 tires 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 the
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 the vehicle at rest, and is affected by
car weight, amount of gear reduction and tire size, ability of tire to stay
adhered to ground and stiffness of chassis. (Does the car 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 rate 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 variables 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, the 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 our 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 in 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 tires
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 torque
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. TCIR uses an application
questionnaire to gather the pertinent information. TCIR technical
salespeople also spend a large portion of their day reviewing specific
customer applications and recommending torque converters for those
applications. 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. TCIR looks
at torque converter technology as an on going process of continuous
improvement.

We are in a more fortunate position when dealing with street and mild
off-road applications because there are greater numbers of similar vehicles
as compared to racing-oriented applications. This allows TCIR to perform
most of the particular design features on categories of torque converters
(i.e.: Saturday Night SpecialR, BreakawayR and StreetFighterR styles) rather
than have to set a unique combination for one particular torque converter as
we have to do quite often with the more uncommon race applications. This
also permits TCIR to provide training in the form of seminars, videotapes
and technical literature to the sales staffs of our leading warehouse
distributors and jobbers enabling the phone salesperson or counter-person to
recommend a street or street/strip application in the majority of cases.

Dependability concerns in choosing a torque converter - Regardless of the
reason or "want" for buying an aftermarket torque converter, an 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 behavior of the torque converter.

Needle bearings - properly selected and installed bearings withstand more
pressure and provide less internal drag (drag robs horsepower and increases
heat) than can be achieved with OEM style thrust washers. Thrust washers
also tend to flake off material adding to contamination in the system (the
transmission/torque converter hydraulic system.)

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.

TCIR hopes that this article has broadened your knowledge of this most
commonly misunderstood component allowing you to be a more educated
consumer.


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