substitution not working properly

edit

I have the following code and I want to substitute all B^2=17.

B,x,y= var('B x y')
eq=81*x^16*B^6 + 40662*x^15*B^3 + 14353281*x^14*B^2;eq
eq1=eq.subs({B^2: (17)});eq1

and I get

81*B^6*x^16 + 40662*B^3*x^15 + 244005777*x^14

I am not sure why the substitution didn't bother to simplify B^6=17^3 and B^3=17*B

I also tried using this code which was suggested earlier :

B,x,y= var('B x y')
eq=81*x^16*B^6 + 40662*x^15*B^3 + 14353281*x^14*B^2;eq
eq1=eq.subs({B: sqrt(17)});eq1

which yields

397953*x^16 + 691254*sqrt(17)*x^15 + 244005777*x^14

and I have this sqrt(17) terms which is not written as B. I need it to be written as B because later on I need to collect the coefficient of B. Perhaps I should use something like while{B^2 : 17} but no idea how to do that.