i tell a lot of people this:
the stock auto can hold a lot IF you know how to drive it right. it's usually slipping of the lock up clutch in a constant manner (pulling up hills while it's slipping... not one quick slip for 1 second) that will quickly fail the transmission. warpage of the lock up clutch happens, then comes shuddering which just follows down the line, lots of debris flowing through the transmission, and then it's dead. so *RPMS versus Torque versus throttle position* (which equals the line pressure inside the transmission... and RPMs too) needs to be taken into consideration when rolling down the road. i'm guessing that i'm at about 300hp and 715 foot pounds of torque now with only upped line pressures and a converter stock/UNlock switch (redynoing in May).
although the internals need to be updated for serious hp (2nd band strut, # of clutches, etc. ), if you drive it right, IT CAN hold a lot of power!
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i don't tow heavy, but when i do tow period, i have to often wind the rpms up high locked rather than running in OD locked with lower rpms (so as to not slip the lockup clutch). only 2000miles per year?? as long as you don't slip it, then the transmission should be fine. however, to be *reliable*, an aftermarket transmission needs to be taken into consideration.
Tom