Why do all old torque converters act like new ones ?

In any car generally pre '80s, the torque converter is usually rather stiff. Like when shifting from a neutral non driving gear to a driving one, the rpm should drop a bit instead of just 20 rpm as it currently does in beam. The worst offender of the this current behavior is when the car is on the move; The converter should not allow the rpm to lower so much when moving at speed, and give it a second or so it drops completely to idle (depending on speed/ final drive). In reality it it shouldn't create a noticeable drop after like 20mph~ (again depending on final drive). Why is it that older converters aren't simulated ? is it the added complexity of keeping more things up to date (the same issue with non sycromesh 'boxes), is it that no has noticed ? or something else ?

 

Three possible reasons.
1. In BeamNG, automatic transmissions in classic cars upshift too early. In the Bluebuck the engine idles at 600RPM, the 3AT upshifts at 850 and downshifts at 700 when not kick-down, this is only a little bit higher than idle.
2. Every vehicle has DFCO (Deceleration fuel cut-off), including classic carbureted cars. Thus, at 1000 RPM where the engine torque is negative with zero throttle in BeamNG but positive IRL.
3. The "stiffness" of the torque converter in BeamNG depends on the engine RPM. For example at a given RPM, let's say 2000, and when transfer 200Nm of torque, the slip is 500RPM(output shaft 1500RPM), if the engine is at 4000RPM and the torque transferred is still 200NM, the slip will be 250 RPM(output shaft 3750RPM). An extreme example is to stop your vehicle at top of a hill, shift to 1(L or M1), then release the brake and coast down hill, the vehicle can reach 60mph when the engine is still at idle(!) RPM and the gear is in 1st. If you then tap the gas the engine will overrev(and explode) suddenly.

 

A good example of a more "correct" behavior for the torque converters would be the Oldsfullsize mod, with those transmissions notice once moving the rpm never really drops even when there is no throttle application, they are very stiff. However i believe the TH350 in that mod needs a little shift point tuning because they seem to be quicker to downshift than upshift, which doesn't seem right, but thats not the point