So has overpressure been turned off for the M551?

I smacked a VIDAR in the rear of the turret with the M551’s HE, and nonpenned, and just shot a Leopard 1 in the back of the turret, and did nothing but break his radiator, with the HE, and in the same game i shot a DF105 in the UFP and only broke is transmission. Where TF is the overpressure?

Vehicle parts tend to absorb explosions and when separated from the crew compartment you’re very likely to just break whatever you hit.
You could take a M113 which is just an aluminum box, it gets overpressured very easily by mere HEAT shells. Shoot it on the left side and it probably won’t survive, but if you shoot the right side (where the engine is) you might just take out 2 crew and the engine.