after swaping my d16z engine to a b16a1 last summer i only used it fore some weeks before parked it for the winter, so i did not change the exhaust from 2,25" to 2,5" i think the dim is. would i loose much hp on running the 2,25 from the header and back ( i have no cat)

 

what type of power do you want?
2.25 you'll get more low end power and 2.5 you'll get more top end power.
i'd stick with 2.25 b/c it'll be a little bit quieter and you wont have to spend the extra money for new exhaust