got a powerflow back box and thinking of doing centre section next, how many extra horses will it give me and will it still pass m.o.t if i dont touch downpipe?

dont know about how many horses but it will sound better and still pass the mot as long as you have a cat in the downpipe, i used to have a my97 which passed its mot with a scoobysport back box and centre
mark

Will pass MOT no problem...............may not see many more horses but throttle response will improve no end................so will noise! ;)

centre section it is then! just thought that if the existing centre had a resonator box, than that would be a little restrictive and that if you got rid of it then it would add a little more horsepower