I know the flexible aluminum drier vent ducting is easy to work with, but if you can use PVC or some other smooth walled ducting you'll have faster airflow due to the smooth inside surface. As an example, I've read that highly corrugated drain pipe has 9 times the flow resistance of smooth walled drain pipe (I'm assuming this is for water since it is drain pipe. Even with air though, a 3" corrugated duct can flow as slowly as a 2" smooth duct.) The pipe in effect flows air, or water, as if it was narrower than it actually is due to the increased resistance due to turbulence. The PVC can also easily be flared at the open end which has a big effect on the ease at which air can enter the tube (see my post on airflow in ducts with different shaped openings.) Here are some articles about making cold air intakes which mention these things:

http://www.autospeed.com/A_0009/article.html

http://zhome.com/Care/airhorns.htm

 

of course you will see more airflow. MIND YOU this will net you maybe .5 -1 more horsepower, and PVC weighs a bit more than the flex tubing so that negates the horsepower advantage.

I'd stick with flex stuff since it's so lightweight, easy to use etc. and it kind of looks neat when done perfectly.

 

The universal dryer tubing (flex alum) is 4" wide no matter what... it's universal. So its kind of like having 3" smooth. no big deal right? Now if you use flex tubing thats only about 3" diameter, I think youre cutting off air flow.

 

My ducting to my MAF is 3", as is the snorkle on the air box, so I just continued it to bring in some cold air. I had to use flex duct too in order to route it past all the obstructions. To make the inside of the flex duct smoother I bent it to shape, then poured in a bunch of Elmer's glue and let it coat the entire inside surface, filling in the grooves. It was a little tricky keeping the coating even. After it dried I put 2 more layers. They were easier. The inside surface only has a slight ripple to it now. It may not produce a measurable difference, but I like knowing it's better this way. This is the kind of little extra job I like to do to get the most out of the small projects I do.

Every little improvement adds up. Like keeping bends to a minimum. In a 3" diameter pipe, a 90° bend causes the same pressure drop as 5' of straight pipe. The longer one has to route their tubing, and the faster the airflow, the more important these little things become. I'm not saying it's going to make or break the project, it's just some info I felt like passing along.