for my box the port is going to end up being 5 inhes in diameter and about 17 inches long, this is going to be too long for my box, can i make two ports that are 5 inches in diameter but 8.5 inches long, or do i have to totaly change the measurments? thanks for any help.
change the port diameter to 4" and re-calculate. the port should be shorter.
using two ports will actually make each port longer than using one single port.
basically, just find the volume of the port that you want use. then you can any number of ports as long as they have the same volume. just remember not to make the opening of you port too small or you will get port noise.