I just got a header flange today from mazda trix to start fabbin up a custom header and mid pipe with one high flow cat for my car. The flange has 2 1/8" ID holes in it. The gasket holes are a bit bigger though. This header will be going on a stock port S4 NA.

Should I stick with the 2 1/8" ID tubing size or should I enlarge the holes and use a larger tubing for the two pipes coming out of the engine?

Also when I get rid of the old manifold and bolt this new thing up will I be able to reuse hardware or will I need to go pick up some studs and nuts?

 

Pick up new bolts, nuts, and lock washers for the exhaust. This is law, even if your old parts are in good condition. The studs may also pull out of the engine block with the nuts, so be prepared for that.

Has the engine been ported?

Why are you making your own header, instead of buying a proven preassembled unit?

 

it's a whole lot cheaper

 

tonybcrazy, do you plan on getting rid of the precats or just making a header that bolts to them?

 

I'm making my own because I have a have a great fab shop and want the experience plus I'm a poor college student. I want to use slip fit connections instead of flanges also. I will be ditching the stock cats in the process and going with one high flow one. I want to make a some what longish header that stays duals until right before its end.

 

you could fab up something like RB true duals, where each rotor exhaust is seperated up to the cat. also you could make a header then fab a mid pipe instead of having a 3' header

 

The RB flange is designed for 2" ID pipe. The pipe slips inside the flange. The resulting joint is stronger than a simple butt weld. 2" is a good size for primaries. No need to go bigger.

If you're going with a long primary system, and I highly recommend it, you'll need to rethink the cat situation. The cat should be as close to the front of the system as possible for effiecient operation. The long primary system with a single muffler will also be too loud for the street. We clock consistent 100-103dB ratings with our racecar and this setup. Far too loud for the street. If you're set on the long primary I'd recommend looking into using two RB pre-silencers. One in each primary. We race with a guy that has this setup on his ITS car. It's street car quiet and very fast with power equal to what we're making.

The RB catalog gives a rough idea of how long you'll want to make the primaries. It depends on where in the RPM range you want to tune the system. I recommend calling them, telling them what you're doing, and asking for a recommendation. They're very helpful with these kinds of things and their experience is second to none.

 

This was my home built full dual.
I learned later that I should have extended the flange pipe straight out further before making the bend to the rear.

 

I'm planning on trying to keep the rotor exhaust seperate until it hits the cat. I was figuring the one high flow cat will end up being about where the last cat now remains. I am sorta worried about the aftermarket cat getting to hot and burning up prematurely.

So my exhaust will go two into one then back into two. I have a racing beat cat back on there already. I am not to worried about sound because of this the racing beat mufflers seem pretty big and up to the task and my car is not loud by any means right now.