need confirmation for ported box

Bring the sub up again in WinISD as a new project.  Input '2' as number of subs at the start.  Click through the dialogue boxes and let the program calculate box volume...then make your necessary changes for space consideration after that. 

What is making me uneasy is that changing the number of drivers from 1 to 2 makes no difference in port and box volume. Is this correct? I would have thought 2 subs would need a bigger volume than one sub?

You must have been changing from one to two subs after already bringing the project up with just one woofer.  The program is not going to continually alter the box size at this point, because you are now in design mode.  This program is a tool, not a dictator...it allows you to design the box as you want to.  If it kept changing things on you as you try to make changes yourself, you wouldn't be able to get anything accomplished!

Hint:  the program will normally show larger box volumes than you might have room for in a car.  It will seek the flattest response.  If 64 liters (2.25 cu ft) per sub is too big and bulky for the intended installation, work the program with smaller volume for the two subs to fit the space.  You will see WinISD alter the port size to compensate for the difference in box volume so that it still meets the intended tuning frequency.

Keep the length to width ratio of the port opening to 8:1 or less.  4 cm is a bit narrow in relationship to 34 cm.   With a smaller box volume, you will see that the port size will have to grow bigger, so increase the port opening width while keeping the port opening length at a size that will fit your box.

 

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.