effective cross sectional area of port

[Benny212]

http://www.partsexpress.com/pe/showd...number=260-387
using pvc to port is kindof a pain in the ***, so I've decided to use one of these things because I've heard good things about parts express ports.
I'm making an ABC box and my question is this. this part says 2 inch diameter. But since it is adjustable I assume that it is thinner on one end than on the other, so which measurement is responsible for the "effective cross sectional area" of the port? I need to know EXACT diameter here cause I hear that ABC boxes are extremely sensitive to careless designing/building. For example if the port is 1.98 inches in diameter on one end and 2.02 inches in diameter on the other which do I use in port calculations?

I couldn't find a 2" port on this sight anywhere that was long enough to fit my purposes except this adjustable thing. What do all you guys use for your ports? because all the stuff on this sight is really short and not very practical for smaller boxes.

thanks for reading my post. Please answer if you can because I am eager to learn. dont want to be a noob forever.

[Trixter]

Don't quite have an answer for your Q. But have you looked at the precision ports on that site? They have a tube between the flared ends and you can cut it to length. Also they have their own formula to find it's length. (due to it's flared ends)

If it helps you to see if one would work for you, here is the formula:

Lv ={[(8466.4 x R^2) / (Vb x Fb^2)] - (1.463 x R)} - 5

Lv = length of vent
Vb = Volume of box (in cu')
Fb = tuning freq.
R = radius of port

[Benny212]

yeah that would work, but come on... 10-12$ a port? thats ridiculously high for a plastic tube. If it comes to that I'll just use PVC and find a way to make it work.

[Trixter]

Those ports blow PVC away. I have used them before and they are worth the $$. I used 2 4" ones in a box for my 15" Brahma. They sounded awsome and moved enough air to do a 146 on lin-x. I now have a box with a slot port that is more area than the other set up, but the precision ports sounded better so I'm going to make another box using them again.