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Toyota Modified Valve Body
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By John Lombardo
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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 madeand demonstrate excellent durability in unmodified applications.The framework is present for an extremely strong transmissionthat is to be used in a high performance or extreme dutyapplication. The problem is that the original calibration isengineered for driver comfort rather than ultimate componentstrength. The soft, sliding shifts that are part of the originaldesign are not appropriate for increased horsepowerapplications, towing, off road use, racing, etc.
These calibration inadequacies quickly manifest themselves asextremely poor shift quality, and more often than not, severedamage to the gearbox is soon to follow. One of the most commonsymptoms of this is the engine stuttering or hitting the revlimiter during a full throttle upshift. Without going into toomuch technical detail, the factory shortcomings can be addressedthrough modification and recalibration of the control valveassembly, a.k.a. the valve body. The valve body is a componentthat is comprised of valves, solenoids, an orifice separatorplate and an intricate series of passages- it is the mostcomplex component in the most complex part of your vehicle- theautomatic transmission. The function of the valve body is to actas the "brain" of the automatic transmission- it directshydraulic pressure to the appropriate clutches and bands at theright time to initiate upshifts, down shifts, selection ofreverse, converter clutch application, etc. As well ascontrolling shift timing and shift quality, it is alsoresponsible for directing hydraulic pressure to the cooler andthe lubrication circuit. As you can imagine, the transmission'soperational characteristics can be drastically altered and alsocustomized to the given application through modifications tothis component.
Because there are no commercially available shift kits for thesetransmissions, we began working on valve body modifications thatwere appropriate for Supras, Jeeps with the AW4 transmission,Toyota Tundra, Tacoma and 4Runner, and also Lexus SUV's and reardrive passenger cars- especially those that needed to handle theadditional power that accompanies the installation of asupercharger, turbo or nitrous oxide injection. At the risk ofoversimplification, there are a few things that are done inconcert to create much more favorable operation of the gearbox.The first thing that needs to be done is to increase thehydraulic operating pressure of the transmission- this pressureis known as "line pressure". All hydraulic functions of thetransmission are based on this pressure- what is especially ofconcern for these purposes is the clamping force which isapplied to the clutches and bands to get them to hold againstengine torque.
In simple terms, increased engine output is complemented byincreased line pressure and increased "clamp" on the clutches-this can be likened to a performance clutch with a heavierpressure plate spring in a manual transmission equipped vehicle.The idea is to raise this pressure only slightly at lightthrottle but increase it by 30-40% at full throttle- where it isreally needed. The effects of this are shifts that are notoverly uncomfortable at lower throttle openings, while atheavier throttle, firm shifts with much shorter clutchapplication time and increased clamping force can be achieved.An additional benefit of this is increased flow through thetransmission's cooler and lube system. Secondly, hydraulicpressure is normally routed through an orifice in a metal"separator plate" that resides between the two halves of thevalve body before it gets to its intended destination. Byaltering these orifices, we can increase the volume of hydraulicoil that is used to apply the various clutches and bands. Thefinal part of modification is the alteration of the accumulatorcircuits. These are hydraulic circuits that are parallel to thecomponents that are used for shifting. Their function is toabsorb or "accumulate" some of the hydraulic pressure that isintended to apply a clutch pack or band. By limiting the actionof what is essentially a "shock absorber" for each shift, we areable to further reduce clutch lock up time and shift lag at wideopen throttle. The end result is that shift time is reduced by30 to 80%, depending on the amount of acceptable shift feel andthe intended application. The clamping force that is required toapply the clutches and bands is increased by 30 to 40% at fullthrottle. Flow through the cooler and lube circuit is increasedand the converter clutch application time is also reduced.
Transmission and valve body upgrades are not only for Americanmade vehicles anymore. Increased performance and more efficientshifting is now available for vehicles of almost everymanufacturer thanks to the few companies that are willing to dothe research and development work required for this rapidlygrowing segment of the automotive aftermarket.
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