19hz - 16.78"/3"dia what would a 4" port be

 

There are many port caculators, www.the12volt.com has one for instance. Without knowing the VB of the box (volume) then it's hard to be 100% accurate,
Mark

 

i worked it out i need to do 3" or my port is to long

 

19hz is generally well below what most people tune their enclosure to unless you are running a stack of 15s and want to summon whales..

It's generally a bad idea to tune below the Fs of the driver too.

If you are still leaning towards SPL you will probably want to at least double that tuning frequency.