if the rains an issue you have a couple options.
A flapper like the big rig exhaust or a metal check valve.

 

a flapper would work, but i think it would probably screw up the look of the fireball these guys are after. Maybe just a small hole at the elbow to allow drainage would suffice.

 

I would be more concered at how loud it would be hahah. If you are really worried about rain when its parked just make a little metal cover and put it over when you park.

 

Hello,

I just recently got my Evo and has an O2 Dump out the hood. I have also been concerned with it when it rains here in NC, but after reading and researching I have came to the conclusion that I have to be more proactive with the weather. When I am at work, I just put a bag over the dump (of course wait until it is cold) and it does fine. Did you do it yet? what are your thoughts?

New to the Evo game.

-Jose

 

No need to worry about covering it, I've had my hood dump since September and never cover it. It's kinda fun when it gets full of water because it blasts out at WOT. My friends call it the blowhole!

 

AR Design, it bolts up beautifully

 

Extremepsi.com sells them. Priced pretty well also.

 

Got one just debating waiting until summer to install it. Don't know if I can wait that long though.