I just bought a cobb dp and the flange for the donut gasket seems to reduce less than 3 inches, will this restrict air flow because the buschur racing dp doesnt have the flange but you have to get their 02 housing to make it fit right.

Also, is it hard to drill a hole and weld a nut on for an 02 sensor

 

I would recommend the megan racing downpipe. I have it and its 3" mandrel bent ss polished with warranty. Cheaper than most.

 

but dont all donut gasket flanges taper down to the same size to fit the same gasket?

 

Yea, for stock the doughnut gasket to fit they all come down to about 2.75 (if I remember correctly). Some higher end O2 housings are a true 3.0 and use a differerent gasket. Unless you are going for 500 whp it shouldn't be a factor though.
As far as the O2 sensor bung, all you need is a drill now days;
http://www.motoiq.com/magazine_artic...sor-mount.aspx

 

so does that mean a true 3" exhaust is useless because of the 2.75 taper for the donut gasket?

 

I received my Cobb downpipe today and i went to a good welding shop and the guy did it in 10 minutes and cost me $15.

That aem thing is a bit ridiculous ....

 

i just got mine welded but he didnt weld it in a spot with a lot of room so it barely fits! lol its facing the oil pan, but it works

 

downpipes are a pain