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Toyota Modified Valve Body - DOC

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					Toyota Modified Valve Body

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807

Summary:
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 a...


Keywords:
modified valve body, upgraded, valve body


Article Body:
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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