im looking to use a set of wet stainless risers that mount at the transom so I dont have to ditch my already mounted exhaust tips and internal flappers, but I have a cam and would like to keep the exhaust as "dry" as possible.

my thought is to weld up most of the end of the riser and install a bung for water to dump through a transom bung, yet still leave a portion open to allow some water to flow through and keep the tips cool. Any idea how much i should leave unwelded?

I don't know the answer to your question but you can get silicone exhaust that will take up to 500 degrees. There is some info on this site that might help you figure it out.

http://www.mesamarine.com/Trident.html#Trident

You will melt it for sure, if you run it dry. If your determined to run it dry, try to butt your tail to your tip, and use hose to just seal....... been there done that....

What are the cam specs?????

I don't have the answer either, but I'm about to do the same thing. I'm going to have my dry tails go all the way up to tips going through the transom. I can't have my dry tails go through because of interference with my swim platform on the outside. My dry tails are gil and they have a bung welded on for dumping over the transom, but also have two small little holes drilled at the very end of the tail. One on top and one on bottom. From what I've found it's to keep from having air pockets at the tips, but have read that it I'll be more thn enough water to cool the flappers. The holes on mine are only 1/8 at most. I figure the water is under 20+ psi so a decent amount of water will make it out of the holes to cool my internal flaps. I will know in the next week.

They make metal butterfly flappers.. Most people dont like them because they make a funny noise. But, If you are running a mostly dry exhaust then it will be loud as **** and you wont be able to hear the flappers..

Chris

http://www.cranecams.com/product/car...detail&p=24308

Thats a a fairly big cam, but it also depends on the engine ci. Its not much bigger than 525efi cam.
You shouldn't need to run competely dry. Normally the holes(3) I've seen in risers are about 3/4" diameter for wet exhaust. I would think (3) 1/4 holes should cool the hose enough and then run an overboard dump also.

its an aluminum headed 496