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Toyota Modified Valve Body

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As many Toyota and Jeep owners know, the AW4 / Toyota 340, A340,
and A341E series of transmissions are generally very well made
and demonstrate excellent durability in unmodified applications.
The framework is present for an extremely strong transmission
that is to be used in a high performance or extreme duty
application. The problem is that the original calibration is
engineered for driver comfort rather than ultimate component
strength. The soft, sliding shifts that are part of the original
design are not appropriate for increased horsepower
applications, towing, off road use, racing, etc.

These calibration inadequacies quickly manifest themselves as
extremely poor shift quality, and more often than not, severe
damage to the gearbox is soon to follow. One of the most common
symptoms of this is the engine stuttering or hitting the rev
limiter during a full throttle upshift. Without going into too
much technical detail, the factory shortcomings can be addressed
through modification and recalibration of the control valve
assembly, a.k.a. the valve body. The valve body is a component
that is comprised of valves, solenoids, an orifice separator
plate and an intricate series of passages- it is the most
complex component in the most complex part of your vehicle- the
automatic transmission. The function of the valve body is to act
as the “brain” of the automatic transmission- it directs
hydraulic pressure to the appropriate clutches and bands at the
right time to initiate upshifts, down shifts, selection of
reverse, converter clutch application, etc. As well as
controlling shift timing and shift quality, it is also
responsible for directing hydraulic pressure to the cooler and
the lubrication circuit. As you can imagine, the transmission’s
operational characteristics can be drastically altered and also
customized to the given application through modifications to
this component.

Because there are no commercially available shift kits for these
transmissions, we began working on valve body modifications that
were appropriate for Supras, Jeeps with the AW4 transmission,
Toyota Tundra, Tacoma and 4Runner, and also Lexus SUV’s and rear
drive passenger cars- especially those that needed to handle the
additional power that accompanies the installation of a
supercharger, turbo or nitrous oxide injection. At the risk of
oversimplification, there are a few things that are done in
concert to create much more favorable operation of the gearbox.
The first thing that needs to be done is to increase the
hydraulic operating pressure of the transmission- this pressure
is known as “line pressure”. All hydraulic functions of the
transmission are based on this pressure- what is especially of
concern for these purposes is the clamping force which is
applied to the clutches and bands to get them to hold against
engine torque.

In simple terms, increased engine output is complemented by
increased line pressure and increased “clamp” on the clutches-
this can be likened to a performance clutch with a heavier
pressure plate spring in a manual transmission equipped vehicle.
The idea is to raise this pressure only slightly at light
throttle but increase it by 30-40% at full throttle- where it is
really needed. The effects of this are shifts that are not
overly uncomfortable at lower throttle openings, while at
heavier throttle, firm shifts with much shorter clutch
application time and increased clamping force can be achieved.
An additional benefit of this is increased flow through the
transmission’s cooler and lube system. Secondly, hydraulic
pressure is normally routed through an orifice in a metal
“separator plate” that resides between the two halves of the
valve body before it gets to its intended destination. By
altering these orifices, we can increase the volume of hydraulic
oil that is used to apply the various clutches and bands. The
final part of modification is the alteration of the accumulator
circuits. These are hydraulic circuits that are parallel to the
components that are used for shifting. Their function is to
absorb or “accumulate” some of the hydraulic pressure that is
intended to apply a clutch pack or band. By limiting the action
of what is essentially a “shock absorber” for each shift, we are
able to further reduce clutch lock up time and shift lag at wide
open throttle. The end result is that shift time is reduced by
30 to 80%, depending on the amount of acceptable shift feel and
the intended application. The clamping force that is required to
apply the clutches and bands is increased by 30 to 40% at full
throttle. Flow through the cooler and lube circuit is increased
and the converter clutch application time is also reduced.

Transmission and valve body upgrades are not only for American
made vehicles anymore. Increased performance and more efficient
shifting is now available for vehicles of almost every
manufacturer thanks to the few companies that are willing to do
the research and development work required for this rapidly
growing segment of the automotive aftermarket.


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  • Posted On May 26, 2006
  • Published articles 283513

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