Minimizing the effects of a clutch/torque converter for direct drive

I'm trying to create a powertrain as close to a direct drive as possible. From my experience, neither manual or automatic transmissions do a very good job. A manual transmission with 1 gear is as close as I've gotten, but to make it work properly, I need to change shifting to realistic and turn off throttle/clutch assistants. With automatic transmissions, the torque converter has a tendency to lock the motor at a single RPM up until around 70mph where the motor's speed starts rising linearly with the car's speed.

I've messed with as many settings as I've been able to find, but nothing has worked well enough that I'd consider releasing it. I need either the stiffest torque converter that will not destabilize the powertrain or a clutch that is either in or out - nothing in between.

I've considered creating my own powertrain device, but I don't have enough experience with lua to comprehend the way the game works.