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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