First, I would like to apologize for this thread. I have searched, and given the sheer number of people who have upgraded exhaust systems, I have been unable to find a definitive answer for my question.

My Evo is as near as makes no difference stock and has ~33.5k miles. I have determined that my stock cat is toast, and rather than put the stock tune and air box back in the car to have it warrantied, I figured I would go blow a few hundred bucks on replacing it myself to save the hassle.

For the time being I have no intention to replace the full exhaust system, however I figure if I have to replace the cat, i might as well put a HFC in. I have my eye on the Mil.Spec 3" (with the Evo IX O2 bung) however I don't know if the 3" flanges will seal properly on the stock exhaust flanges. Before I spend the money I'd like to be sure it'll work. Alternately if it's a bad idea, does someone have a recommendation of a HFC that'll mate up nicely with the stock exhaust (and won't require modification (like the O2 bung)).

Thanks for reading, and sorry for my general noobiness.

-Bryan

 

I don't think the stock exhaust system is near 3" piping so 3" HFC is wasting money. Just get another stock cat if you don't want to spend money on a full 3" exhaust system.

 

I assume you want a hfc for a little more power. I'd suggest a dp and this in the mean time.http://www.maperformance.com/invidia...e-evo-vii.html

 

contact mil.spec

 

You could always put in some reducers to match the stock exhaust size. I'm running a 3" HFC with a stock cat-back and 3" front pipe. The HFC has a 2.5" reducer welded on and goes into 2.5" piping to match the stock piping, so there's no overlap at the flange.

 

A 3" flange won't perfectly mate to a stock-sized flange. So perfectionists won't be happy. That being said, it can work. I think your best bet (if you're interested in our cat) would be to get a 3" catback as well. If you want to save money, you can find a used cat back.

 

You can get a used cat for cheap, but how do you know your stock cat is bad? There's also the option of pulling off your stock cat, drilling it out, and then getting one of those o2 fixes to avoid the CEL.

 

I've ran a 3" test pipe on stock exhaust before

 

Thanks for the feedback everyone.

I ordered one, I figure if it doesn't really fit properly as is, I'll lop the ends off and have reducers and flanges welded in so it will fit.

I am reluctant to buy a new exhaust just because I'll never be satisfied that I found the "right one" and my OCD will have me constantly second guessing whatever I do buy and I'll probably never be happy with it. Not really a good solution, but that's my strange reality.