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.