Detailed Information
Performance Transmission Shift Improvement Kit Towing
"An Effective Approach To Transmission Performance"
In the automatic transmission an accumulator's function does as it's name implies, it accumulates or absorbs fluid in a clutch apply circuit and normally uses a spring loaded piston to cushion the apply of a clutch or band. Without an accumulator in the apply fluid circuit, the rapid buildup of fluid pressure would cause the clutch or band to apply very quickly and create a harsh or "bang" shift. The separator plate has small orifices that regulate the rate of apply fluid into the apply circuit. This orifice diameter has a direct effect on shift timing of the apply components. The larger the orifice the more fluid that will pass creating a quicker apply. It is easy to undertsand how modifing these two areas can change shift feel. At Trutech we looked beyond shift "feel" to determine what modifications would improve transmission performance and durability. Issues we have found with drilling holes too large in the separator plate is that it creates a shift overlap of apply components at certain RPM ranges. Shift overlap occurs when two apply components are applied at the same time. Shift overlap greatly decreases the apply component life because the components work against each other. Blocking or modifing fluid passages in the accumulators also have an adverse affect, the rapid apply of the apply component causes a jolting effect on both the apply components and the hard parts inside the transmission. If severe enough this jolting can cause broken internal parts.
The 1-2 accumulator piston and spring setup was redesigned by the factory in 1994. This was done because the accumulator spring normally broke or caused damage to the separator plate. The redesign by switching piston and spring postions was an inexpensive solution to the problem. The Trutech Shift Module contains heavier accumulator springs to slightly speed up the rate of apply to the friction components. We also have you convert the 1-2 accumulator piston back to the pre-1994 setup and we supply a heavy duty plate to use between the spring and the separator plate to protect it from damage. This arrangement provides a performance shift feel without the damaging effect of a "bang" shift. We provide heavier springs for the 3rd accumulator in the servo and 3-4 accumulator but we do not recommend any modification that will make the accumulators inoperative. Our Modules contain new accumulator pistons to restore hydraulic intregrity to your transmission. The pistons are common wear areas that allow fluid bypass. The later model 1-2 "plastic" accumulator piston is replaced with the more durable aluminum version. In our Performance Shift Module we upgraded with two pinless dual seal accumulator pistons.
At Trutech we were interested in improving the holding ability of the apply components. In an automatic transmission the diameter of the servos determine in a large part how well a band will hold. As with any hydraulic component the larger the apply area the more efficent the component. In other words, larger servos mean better band holding efficiency. On the other hand, in the 700R4 the servo size plays a large part in shift timing. The larger billet "super servos" are recommended for "racing applications only" by the manufacturer. These "super servos" depend a large part on the servo return spring to release the band when changing to the 3rd gear range. Racing applications are operated at wide open throttle and rarely downshift, these racing transmissions benefit from the larger servos. The 4L60 type transmission hydraulic design does not exhaust an apply circuit once applied as it shifts through all forward gear ranges. The servo diameter and servo return spring determine timing of band release in the 2-3 shift when the 3rd accumulator circuit is filled. Modifications to the circuit apply orfices in the separator plate are good modifications as long as down shifting and low RPM operation are not factors. Racing transmissions have a certain amount of "shift overlap" at lower RPM ranges. Racing recommended servos and "shift kits" should not be used in street driven transmissions because this lower RPM overlap will cause early transmission failure. Our shift module has a performance servo combination that works and provides excellent band holding ability in a street driven transmissions. The increase in servo size also has an effect on shift feel. Our servo combination provides a firm crisp shift. There is a direct relation between the 1-2 accumulator, accumulator valve and the 2nd servo. We have included a performance accumulator valve in our Performance Shift Module.
We also provide a performance calibrated separator plate with gaskets. This plate has a performance factory shift calibration. Because a common failure in the 700R4 transmission is the 3-4 clutch we have modified the apply orifice to the 3-4 apply circuit, allowing fluid into the apply circuit at an increased rate. To prevent shift timing issues in the 2-3 or 3-2 shift we have included a metered check valve for the 3rd accumulator circuit. Removal of the valve body will be required and the use of an inch pound torque wrench will be needed for reassembly.
The transmission line pressure is increased with the addition of a large diameter boost valve and heavier pressure regulator valve spring. Increased line pressure will provide higher pressures for better apply component holding capability. The line pressure increases as the accelerator pedal position and engine torque increases.
With the Trutech Shift Modules we took a completely different approach to transmission performance than that of other manufacture's shift improvement kits. We do not claim to correct problems caused by wear. The only correction for a wear related issue is part replacement or the part must be remanufactured back to factory specification. Our Shift Module works with the factory engineered design. The parts provided in this Shift Module are of the highest quality. No special tools are required and installation can be preformed with the transmission in the vehicle.