You seem very bad understand what the tanks in the USSR)
T-80 is a tank with GTE, where the engine is Unit Stake Unit Stake T-72/64
T-80 is really a very expensive car. And it is very fancy.
However, it is possible to move inwards as T-72 and T-64. It's just a struggle between the plants themselves.
T-90A is really a continuation of the idea of the T-72 with a piece of equipment T-80B / C. But T-90MS is already much further. There's a new gun, new sights, and so on.
You know how much modification of T-80 exists? There is a joke that there are no two identical T-80
Roughly speaking: T-80 and its modifications expensive in every sense of the tanks that give small advantages for huge money.
T-72 simple and reliable machine capable of a bear all that wish nezdarovy mind designer.
T-64 originator of all, but with a huge list of problems that have been converted into the T-72 and T-80 in various ways.
If you wish, you can log in to the Russian forums, there is a description of all these tanks. But you have to spend a lot of time to understand that the difference between the T-72B in '82 and T-72B '89 / T 80UA T 80UE etc.
googltranslayte sorry)
5TD diesel for T-64. 6TD development 5TD
GTD 1250 very good engines, expensive but good.
T-80UD is the same as the T-80U, only with a diesel engine for a place 6TD GTD1250
They remove the gun from the old tanks. This is the same 2A46.
When we did the gun, but now, no.