I want buy Tata nano I booked already bs3 version because tight budget

It is the pollution norms compliance difference. I think there is no power difference with the engines.
The Nano is available in 5 variants- Standard, CX, LX, CNG emax CX, and CNG emax LX. And all trims are powered by the same 624cc, 2 cylinder, MPFI engine, which delivers a max power of 38 PS @ 5500 ± 250 rpm and a max torque of 51 Nm @ 4000 ± 500 rpm.