I was thinking about buying a plenum off of ebay and porting it myself. I can get one for less than 60 bucks. Is this worth it? Will I notice any hp gain?

 

Every little bit helps and adds up but dont expect to feel anything by itself. If you can DIY why not

 

It's good if it's done right but match it up with some other flow improvements to get the full benefit.

 

Could I just use a dremel tool from harbor freight? And I was going to buy some different runners and a plenum off of ebay or my local junkyard.

 

Just cut the divider out of the runner walls TB area if you like , over the hump there. If youre never going to port again just use a drill and a carbide. Dremels and those tiny bits youll get frustrated real quick. If you want to do more go get a good quality adjustable speed electric grinder ie makita mutabo etc. Dont need much speed at all really. Not those 50bazillion rpm 20 buck jobbies from Harbor. The ones with a long narrow snout are nice no need for a long carbide easier to control

 

haven't done any porting with it but I've got a variable speed dremel with a flexible shaft. Very useful and easy to handle and move around in tight spots.

 

Upper or lower plenum or both?

Not much you can do other then gasket match, and clean up behind TB.

If you get some bigger runners, you can open the lower up to match them. I have some pics in my build thread of matching mine.