What most people are looking for in transmission strategy is a quicker, firmer shift, or more technically a shorter shift duration. Shift duration is how long it takes for a shift to occur, or the amount of time that the clutches are slipping. A shorter shift duration has a performance feel, and generally is better for the clutches, because the wear on the plates happens during this slip. Shortening duration also will reduce heat, which is responsible for most automatic transmission failures. The way to reduce shift duration is to increase the engagement pressure of fluid to the clutch during the shift.
One way to increase pressure in an electronically controlled automatic is to have the PCM (by an aftermarket chip) command the EPC (Electronic Pressure Control Solenoid) to increase pressure. Doing this will give the desired result, but at a cost. Chips that raise and maintain Line Pressure, do so at the expense of either the Lubrication circuit or overworking the pump assembly. By the time symptoms appear of the damage this can do, the damage is done and the repairs can be very expensive. If the lube circuit is starved in order to maintain pressure to the accumulator, the first obvious indication could be when the output shaft welds itself to the rear case bushing, a catastrophic failure that would essentially destroy the transmission. Also, some transmissions that have used software to raise Line Pressure have shown longer term (12-18 months) damage to the pump assembly caused by the extra load placed on the pump.
The alternative to raising Line Pressure with software is to use a mechanical 'shift kit' to raise Modulated Pressure or more simply, a set of valves and springs in the accumulator valve body. The term 'Shift Kit' is a trademark of a transmission aftermarket company, so I avoid using it when I can. Since the accumulator is hydraulically 'downstream' of the EPC, a mechanical shift kit does not interrupt any lubrication fail safes and since it is using modulated pressure boost (through the Line Modulator Boost Valve) it does not place any additional load on the pump. There are 3 basic variations of mechanical shift improvers, a 'Line Pressure Mod Valve', 3 springs and a valve, and a full accumulator.
An accumulator is a fairly straightforward assembly. In the 4R100, there are 3 shift bores, and 1 line pressure/throttle bore. The 3 shift bores are identical, and different springs are set in them to control the reaction of the accumulator piston to hydraulic pressure, this reaction translates into shift duration on the shift controlled by that bore. All of the bores are fed pressure by the Line Modulator Boost Valve, which reacts to the Throttle Valve, controlling this circuit boosts line pressure into all of the shift bores.
One popular mod over the years was to install a boost valve. If you change the Line Modulator Boost Valve only and raise line pressure throughout the body, the individual shift bores are no longer 'tuned' properly and the reaction in the shift bores will be out of calibration for the elevated line pressure. For this reason, you need to recalibrate the shift bores, both in the top circuit (above the accumulator piston) and below (the 321-310 circuit). Also, I have found that increased pressure in the lower circuits caused the 321 valves to 'side load' or bind up due to asymmetric application of hydraulic pressure during the shift, resulting in a 'bang shift'. A Line Mod valve fails to address these conditions.
The second popular option is the 3 springs and a valve shift kits. Using this type of kit recalibrates the upper circuits to the elevated line pressure, but still ignores the side loading of the lower control valves.
For the Factory Tech Accumulator Valve Body, I start with an Accumulator Valve body, a Genuine Ford Part, I add a Line Modulator Boost Valve, recalibrate the upper circuit with different springs and upgrade the lower circuit springs and use a valve we machine to cycle more freely in the bore to prevent both side loading and binding. My lower circuit valve is designed to preload the clutch at less than line pressure, taking the variability out of the shift control. Although it looks very much like the kits available elsewhere, the control of the shift is much more precisely controlled this way, accounting for the much better performance you get from a Factory Tech Accumulator.
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