BTO performs a seven-step...
BTO performs a seven-step process to clean the case, leaving it spotless. Shot-blasting adds additional surface hardness, while tumble deburring reduces stress risers and gives the cases a semipolished look that's immediately discernible from painted or stock cases.
Increasing Line Pressure
BTO's second priority is to eliminate slippage and wear by revising the hydraulic system to increase line pressure quickly as power is applied, as well as eliminate high-rpm pressure drop-off. To that end, the pump is completely blueprinted, beginning with a new, hardened stator. Both sides of the pump halves are then CNC-machined flat, indicated from the new stator. This ensures the stator is perpendicular and creates precision mating of the pump halves to eliminate internal pressure leakage.
Special hardened-steel pump-guide rings are installed to prevent pressure drop above 5,500 rpm and allow the driver to control the shift points manually without ill effects. Then BTO installs a new high-performance, 10-vane pump rotor, and upgrades to oversized 0.500 low-boost and 0.290 reverse-boost valves.
Teflon-coated front bushings are used from the later 4L60E, and new heavy-duty stator support bushings are also installed. The pump-regulator valve is modified to increase flow and is installed with a stiffer pressure-regulator spring. Finally, the pressure curve is recalibrated for quicker response off idle, which reduces slip with high-torque engines.
Decreasing Internal Slippage
Friction and durability issues within the 700-R4 are tackled next. The 2-4 gear band assembly is upgraded to a wider 3/8-inch Kevlar band with a reinforced anchor to help eliminate slippage on shifts under full power.
A hardened stator is pressed...
A hardened stator is pressed into the pump for improved durability; one can be seen here, both installed and sitting next to the pump for illustrative purposes. The double-lip premium front seal is also shown. A pump-regulator valve (already installed) is modified to increase flow and was installed with a stiffer pressure-regulator spring. The cover for the valve is visible here.
Next are a heavy-duty, 29-element input sprag clutch to withstand the solid shifts and a modified governor to raise automatic WOT shift points to the 5,200 to 5,600 rpm range (dependent on combination). The 3-4-gear clutch pack is upgraded to nine friction and nine steel plates, along with the BTO-designed special spring kit for quicker 3-4-gear clutch release. Premium-quality Borg Warner high-performance friction plates are installed throughout all clutch packs.
BTO then installs its custom-machined, oversized servo and cover assembly. The reverse drum is measured to ensure concentricity and trued if necessary; it's then installed with extra-width bushings to prevent rocking and absorb the greater loads of more powerful engines. A hardened input shaft and reinforced stock input drum and sun shell are then installed for optimum strength for engines over 400 lb-ft of torque (Level 3 and 4), and a 30 percent larger Fourth-gear servo piston is added.
Recalibration
The part-throttle, throttle-valve accumulator circuits in the valvebody, and the spacer plate passages are then recalibrated for improved shift timing, firmness, and more precise pressure regulation. BTO considers these mods proprietary, so we don't have specifics on them.
The line-bias system is also modified. All these factors must work in concert with the TV cable actuation, which controls line pressure to make the transmission shift at the right time and with the right firmness for each throttle position and rpm range. BTO also upgrades front and rear seals to an aftermarket double-lip style, using premium one-piece seals throughout to minimize internal and external leaks.
Deflect The Heat
Once assembly is complete, the new torque converter is prefilled with ATF before it's plugged into the front of our sparkling-clean unit. The whole assembly is raised under the GTA, bolted back into place, topped up with ATF, and checked for leaks.
After seeing-and smelling-the burnt ATF inside our stock trans, we opted for the BTO transmission-fluid temperature gauge ($29) and a Hayden transmission cooler ($49 at Pep Boys).
Quality Control
An interesting feature of BTO's transmission quality control is that the company installs the transmissions and road tests them in a specially equipped '73 Chevy Stepside pickup behind a GM Performance Parts Ram Jet 350 to evaluate function and feel before they're shipped. This road-testing policy has resulted in warranty returns of less than 1 percent, saving everyone a lot of frustration.
Both halves of the pump are...
Both halves of the pump are precision machined to be perpendicular to the stator to improve internal sealing and eliminate "short circuiting" of pump pressure due to warped pump halves, a common problem in stock pumps.
The Corvette servo on the...
The Corvette servo on the left has a larger fluid reservoir area compared to the standard passenger-car servo on the right. This design will apply more hydraulic energy at a quicker rate to the band, thus making for quicker, more positive shifts and reduced slippage, wear, and heat. You can also see the premium seal on the Corvette part.
BTO makes this CNC-machined...
BTO makes this CNC-machined servo cover with a 30 percent larger surface area to firm up the Fourth-gear shift.