Performance Transmission Shift Improvement Kit Level I
"An Effective Approach To Transmission Performance"
When building a high performance 4L60E transmission the first question usually ask is, "what type Shift Improvement
kit should I use?". At Trutech Performance Transmissions our ASE Master Technicians backed by years of transmission
repair experience and training researched this issue. We have studied the operation of this transmission and
determined what modifications will actually improve shift performance and give a long service life. In most shift
improvement kits on the market two areas are commonly targeted for modification. These being a ccumulators and the
separator plate. Understanding the operation of the automatic transmission we knew that modification to these
parts would create a firmer quicker shift feel but would do nothing to improve the operation or increase durability of
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 driling 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.
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.
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 4L60E 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 Iits" 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.
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.