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, downshifts, 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, Toyota Tacoma and Toyota 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 a 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 the 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 the 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 are 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.