Hi Im Mike and on this installment of Summit
quick flicks, we are going to talk about selecting the right torque converter for your vehicle.
Selecting the right torque converter for a vehicle can be critical to a vehicles performance
and overall acceleration as an end result. It is difficult for a lot of customers to
select a proper torque converter because they don't understand how a torque converter
operates. What we are going to talk about today is the function of a torque converter
what it does and how to go ahead and select the proper torque converter for an application.
So before we go about selecting a torque converter for an application it is important to understand
how the torque converter operates and what a torque converter actually is. A torque converter
is a fluid coupling device that is going to go ahead and transfer power from the engine
to the transmission and it is also going to multiply the torque being created by the engine.
What this does it is it gives us the ability to leave the vehicle in gear and come to a
complete stop at the same time. Now what makes this possible in the torque converter is three
components that it is made up of. The torque converter as a whole is going to look like
a fairly simple device and internally it is actually fairly simple as well. What you are
going to is an outer shell that is going to attach at the back side to the flexplate assembly
but as we dissect this torque converter you're going to come to find that a torque converter
mainly operates off of fluid motion and centrifugal force. What we are going to start with explaining
is the fluid impeller and that is going to be the front shell of torque converter assembly.
This is the only part of the torque converter that is physically attached to the engine
and is in constant motion with the engine because it is because it is welded via this
seam to the compete assembly. Which is going to go ahead and operate off of flexplate motion
as the engine is rotating. What the impellers job is to create fluid motion inside the torque
converter to create that transfer of power. These fins will catch the fluid that the converter
is filled up with then It will go ahead and send that fluid through the torque converter
assembly itself. The next part of the torque converter we are going to discuss is the stator.
The stator is a part that goes ahead and redirects fluid flow towards the impeller for a quicker
reaction of the torque converter for better torque reaction in general in the torque converter
itself followed up by the turbine assembly. The turbine assembly is the part inside the
torque converter that actually transfers the motion between the engine and the transmission.
This is the part that engages with the input shaft internally and what it's doing is
catching the fluid motion created by the impeller that is being forced against the turbine and
then transferring that power torque to the transmission assembly. We can go ahead and
alter torque converter stall by changing the fin angles the size of the torque converter
which is going to go ahead and affect the amount of fluid flow throughout the torque
converter assembly itself. This in turn affects how high a torque converter stall, how quickly
it reacts how slowly it reacts and this is what makes it such a critical component when
making engine changes, when changing camshafts in an engine and determining how the vehicle
is going to go ahead and accelerate from a dead stop. So now that we understand how the
parts inside a torque converter actually operates and what it actually does, we have to understand
what stall speed really is. Stall speed is basically the term that refers to the RPM
in which the torque converter is transmitting all of its torque throughout the transmission
assembly. There is point in which the converter is going to be slipping prior to this that's
going to go ahead and let the engine reach its RPM band at a quicker rate of speed to
bring it up to point in which it makes power. That is why a torque converter must have stall
in order for it to operate correctly in an automatic transmission application. Now torque
converters do have two different types of stalls they can produce, a lot of times there
is a little confusion between two and the accuracy of one compared to the other? The
first type of stall that most people tend to measure a converter by at home, let's
say they have their vehicle at home they put a new converter in and they want to test the
stall range rating of the converter, they're going to go ahead and test it via foot brake
stall and what this is essentially is , they're going to go ahead and put the converter in
the vehicle, go out start the car up, get in the driveway, hold the brake pedal and
the gas to the floor at the same time. Now one of two things is going to happen at this
point, either one, the engine is going to quit making RPM's way before its max RPM
point, or two, it's going to break the tires loose and override the brakes at some point
in time in the RPM band. This is what's known as foot brake stall. Foot brake stall
is actually a very inaccurate way to measure torque converter stall, it's always going
to happen much sooner than the other type of stall that a torque converter is going
to produce. It is actually harmful and dangerous because what it does is overload the torque
converter creates a lot of excessive heat and a lot of excessive wear and tear on the
torque converter itself and will actually shorten the torque converter life. So when
you do something like a torque brake on a vehicle with an automatic transmission what's
probably going to end up happening is damaging the torque converter in the long run. The
other type of stall a torque converter is going to produce is what is known as flash
stall. Flash stall is the actual stall of the torque converter that the torque converter
is rated at that you are going to see advertised either on our website or in the catalog and
what you will notice about the stall of a torque converter is its always listed in range.
There is no such thing as a specific thirty five hundred stall converter, it's going
to stall around thirty five hundred RPM depending on certain attributes of that vehicle, whether
it be the vehicle weight, the gear ratio the tire size that's used on the vehicle the
amount of power that is being produced by that engine. Those are all going to have an
effect on the stall point of that converter. The flash stall rating is the only true rating
of that torque converter that is the only true stall rating you're going to find on
the torque converter. This is typically much higher than what the foot brake stall is going
to be and it is going to be much more accurate and it is actually much harder to figure out
at home after the torque converter has been installed in the vehicle. It is essentially
is the point from a dead stop when you are going full throttle without the brakes engaged
at the point in which the tachometer needle is going to jump and then the transmission
engages and the vehicle accelerates. Catching this and figuring this out is kind of a tough
task at home it leads to a lot of confusion when people try to go ahead and look at the
stall that is being produced by the converter at home for personal use in a specific application.
As we understand flash stall we now begin to understand why it is difficult to select
a torque converter for an application because of all the different variables that can have
an effect on the stall range and stall rating of the converter in a specific application.
There are a lot of contributing factors to determining what stall is converter is going
to be right for our vehicle and selecting the right one is critical to a vehicles performance
and how it accelerates especially in a drag race application in which 60 foot times are
very important and can have a considerable effect on the end result as far as quarter
mile times go. The main contributing factor to this is determining what converter stall
is right for an application is the camshaft that is installed in that engine. Every camshaft
is going to have an RPM starting range in which it starts to make power now this can
be affected by the cubic inches displacement of the engine as well as the vehicle combination
as a whole. Mainly what we will suggest to most customers and what we suggest that most
of our sales representatives do is they look at the starting arm band of that camshaft
then we will go ahead and select a converter that is 500 RPM higher in stall range than
the starting RPM of the camshaft being used in that engine. Without this critical information
about the camshaft it becomes difficult to select a torque converter accurately for a
vehicle and engine application for any customer. One other thing that was mentioned was that
the gear ratio, tire size, cubic inch displacement of the engine can also have an effect on the
torque converter stall. All of these are not going to have a great effect but they will
have a small or slight effect on the way a torque converter reacts in a vehicle application.
Some things to kind of consider is if you're putting that converter behind a big block
in comparison to a small block, typically it will stall about 300 RPM higher behind
the big block application in comparison to the small block application and this has a
lot to do with the amount of torque production of a big block type engine. It produces much
more low end torque so the converter has a harder time keeping up with that torque multiplication
that's happening so it inherently makes it stall at a higher stall speed it's not
going to be greatly noticeable but its significant to know. This is part of the reason that torque
converters are via range instead of a specific number things like gear ratio, the numerically
higher the gear ratio is in that vehicle it's going to stall sooner because the vehicle
is going to have less load on it to get it to move forward, it's going to motivate
itself to move forward much easier, same thing goes with tire size. Tire size is going to
have an effect on the gear ratio of that vehicle. So if you had let's say a set of 373 gears
and you change from a 28in tall tire to a 26 inch tall tire now what you may not notice
is the torque converter is now stalling at a sooner point than it did previously because
the vehicle has an easier time moving from a dead stop. The other consideration is that
you want to make sure your torque converter stall range is going to be below the cruise
RPM of vehicle on a street application. This is another area where gear ratio comes into
play because if the torque converter stalls higher than what the cruise RPM is on the
highway at 70 miles per hour what's going to happen is it's never going to into full
engagement point or stall. What that means is that the torque converter is continually
going to be slipping not completely, it may be slipping at percentage let's say 20 percent
or so. But that 20 percent is creating friction inside the torque converter as well because
the components are essentially not meshing up with one another, they are not rotating
at the same speed so now what you have is a turbine that is spinning at a different
rate of speed than the impeller of the torque converter. What this does is it creates excessive
heat that excessive heat creates ware and what can happen is you may break a weld on
the snout or you may damage the internal components because of the excessive heat which inherently
is then going to damage the internal components inside the transmission because the transmission
has a certain RPM operating range that it wants to operate at temperature wise which
is typically anywhere from 170 to 210 degrees much like your engine. So what are some symptoms
of having the wrong torque converter for a vehicle? Stall is too low, the engine shuts
off when put into gear, very poor acceleration, the vehicle lunges and tries to override the
brakes essentially when put into gear. If the stall is too high the engine will have
a very small operating range. The converter never reaches stall at the cruise RPM or the
transmission will over heat or burn fluid through the converter never stalling. So in
summery the general rules of thumb for selecting a torque converter for a street application
are: Select a converter that's stalls roughly 500 RPM higher than starting RPM of the camshaft,
remember converters installed in big block applications typically stall 300 RPM higher
than a small block application and always choose a converter that will below the highways
cruise RPM. This can be determined knowing the gear ratio and tire size, if this is now
known the torque converter life will be shortened considerably.
