The Clamshell creates what I would call a Phantom port length. The port is calculated on port area and box volume. In my case 462 sq.in of Port and around 65-70 cubes. Nets us a tuning of around 28-32Hz give or take.. This is with 0" port length. The Clam itself is is around 40" long if we measured the angle/slopes. The distance from the end of the port to the back of the enclosure is 20" (it was 30" reduced it to raise tuning and stop the windsheild from breaking every time we went over 158+) So, in my case any port length drops the tuning. This is not to say it wont gain me, It might. The enclosure is far from finished. We have a bunch of tweaks to make and test with and without port length, try some phase plugs and 45s.. A combo of these or few of them will gain us, but this year was about having fun and making a noise.. we'll be working on things over winter for dB Drag next season.. So WATCH THIS SPACE