Could you create a version of Base91 that exactly map to 13bits?

[DonaldTsang]

I would like to have a B91 system where each 52b maps to exactly 4 char in base91. Is it possible?
2^13 in base91 is 90:2, that means everything from C" (90:2) to "" (90:90) should be invalid pairs.

I though Base91 will sometimes put 14bits into 2 characters due to "higher compression", and i wanted a version of base91 that WILL NOT switch between 13bit and 14bit blocks.