Soviet tank crew survival.

[Marcello]

"In game, if you have two tanks with armor 50, one with surv. 4 and other with 5, and if you hit them with weapon with penetration 51, tank 2 will have better chance to survive than tank 1."

This is more or less what we have been told repeatedly.

The point which you and others made was that T-XX with autoloaders should be more vulnerable due to their armor storage and others factors and should deserve a lower survivability rating with its consequencesater chances of going boom with even only limited penetrations, lower chances of the crew getting out and whatever.
I disagree, to an extent, for the reasons exposed in the previous posts (at least as far as T-72 goes, I do not have detailed manual drawings for the T-64 and the T-80).

"My point is that T-72 armor is not solid.For example maximum thickness of T-72M1 turret is around 430mm vs KE (corners of the turret), minimum 290mm(upper front turret,weakened zone around the gun - mantlet).There is good chance to hit this weaker armor instead heavy. Same for all versions of russian tanks (T-90 max. turret armor 80cm KE min 35-45cm KE)"

So now the issue becomes armor ratings.There is no easy answer for that.The T-72 armor scheme is supposedly designed according to hit probability,with the area most likely to be hit being given the maximum protection and viceversa.Or so I have heard at any rate.That simply cannot be captured accurately in the game.ERA in the game is a mess.Several compromises have been implemented at various times to deal with these issues.