There will be no impact on your result on the basis of boards marks. Board marks are not included in the raking. You should fulfil the eligibility criteria of the exam.

According, to the eligibility criteria you  should have passed class 12 or equivalent exam (from any state or central board of school education) with 75% aggregate marks in Physics, Chemistry and Mathematics/Biology. They also need to have secured 60% marks separately in each of the three subjects. Proficiency in English is another requirement for appearing in BITSAT.