I could of swore the 700r4 has a better first gear than a th350? Maybe iim thinking of a 200r4 being nicer than a th350?
Transmissions most definitely do slip, and original transmissions before lockup conerters never stopped slipping either. The clutches were different materials and the heat load of the transmission was different.
Also, GM quite literally invented the PWM TCC lockup which under 99.99% of driving conditions will slip 5-150rpm
AS DESIGN, they also invented the ECCC.
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im more inclined to think a feeling of slipping on a truck with a 700r4 in 4th or while still locked is experiencing chuggle or a misfire under load