For the following reasons, TBMSport felt that it was necessary to rid the GMT360 platform of the 4 speed automatic transmission, known as the 4L65/4L70, with GM's latest and truly greatest 6 speed automatic transmission, the 6L80/6l90.
A) 4L65/70's just cannot repeatedly stand up to the power and weight of these vehicles under hard use. This will result in repeatedly rebuilding the transmission.
B) 4 speed autos drop the engine RPM out of its torque band during shifts, causing anything from too much shock to the gears/shafts, too much work load to the engine, to too much torque converter heat build up.
C) The 6L80/90 comes from the factory behind engines making more power than GMT360's and vehicles weighing more than GMT360's
D) The 6L's are clutch trannys that automatically, with the computer control, adjust the clutches so you maintain the same consistent shifting as the clutches wears
E) For the most severe applications, the only upgrade necessary will be stronger clutch discs; which are available
F) The transmission swap cost is equal to or less than the cost of rebuilding the factory 4L65/70 (w/ torque converter) and will last longer
G) Keeping current axle gear ratios will result in improved acceleration and fuel economy
H) 4L80/85 transmissions are expensive, reduce 1st-3rd gear acceleration through less torque multiplication, (giving the impression of sapping a lot of overall engine power), and increase fuel consumption
I) To swap the 6L80/90 transmission into these means less money in the long run with other mods, from forced induction to cam to exhaust. Do a transmission mod ONCE.
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