I've written and deleted this post several times over... for what it's worth I did search for that piping in a 14" length and didn't come up with anything.
~300 cfm through a 2.5" ID pipe is doing something like 100 ft/s. The ratio of (time spent in contact with the surface)/(surface area) is next to nothing in comparison to an actual air to air or air to water intercooler. I even did the maths before I realized A) I know you know this isn't going to be a game changer, and B) if you want to do it at least partially for looks, my numbers aren't going to change anything anyway.
Your numbers are in the ballpark as far as the air velocity - and I agree that the time in contact with the pipe is minimal at that velocity. But it's the thermal mechanics going on at the molecular level that makes it work.
The air molecules in contact with the pipe are upwards of 350 degrees and heats the aluminum almost instantly, transferring its BTUs into the pipe and in turn pulls BTUs from the adjacent air molecules in the air stream. Air flow in the center of the pipe will be little affected, but the outside layer will be, so even in that short length of pipe there will be some cooling effect on the air passing through it. Giving the aluminum pipe more surface area by having fins to dissipate that transferred heat at a quicker rate will allow for additional cooling of the air over a smooth turbo pipe. Employing a finned turbo pipe is my personal decision and living in the desert any increase in cooling capacity, even if it's a minor one, is definitely a plus.
This is a project that will take some time to figure out and get it made (I seem to have a lot of those). But having cool looking turbo pipes on my Goblin doesn't hurt either and it will be more convincing with one of these stickers.
GREAT RAT ROD DECAL STICKER 4in. dia.JUST PEEL AND STICK TO ANY CLEAN SURFACE.
www.ebay.com
