I figured I would make a separate post for this to get as many opinions as possible. I recently acquired a 2.25 inch test pipe. My exhaust is a 3 inch cat back and the collector pipe for my header I believe is also 3 inches. The holes in the flange on the test pipe are about 3/4 inch. The header is bolted into the cat via a flange but the exhaust on the other side is welded to the cat so a flange will need to be welded on to the exhaust (don't ask why). Will I have issues with fitment, and if so what are the easiest ways to get around them? NotARacist mentioned that I may need to be creative with donut gaskets but it should bolt up. Not that I don't trust his reason, I just want some more opinions. Any ideas?

 

Get a "3 gasket, vibrant makes a good one.

 

And with a 3 inch donut it should bolt up? The header already came with one for that side. Just need one for the other side.

 

why are you running a 3" exhaust? why use a smaller diameter test pipe?

 

My first answer would be WHY modify a test pipe in order to mate the flange at your header and your cat back system instead of simply CREATING a new "custom test pipe" ???

Now the better answer would be:

You really haven't given us enough information to get a usable answer from this group. First, I suggest you post a picture of the area of the exhaust system that you wish to modify... from just ahead of the header collector to just aft (behind) the cat/cat back junction. Also, "guessing" on any of the sizes of components is a mistake... I suggest you MEASURE the inside diameter of the header collector flange instead of just saying "3 inch I think". It is unlikely that you have a "B" series header with a 3" collector although not impossible. Also, sharing your engine build and HP output would help this body guide you as well. Normally, installing a 2.25" test pipe between a 2.5" header and a 3" cat-back is a bad idea because it becomes your bottleneck in the system, that is, unless you have an engine that doesn't make enough wheel HP to NEED anything larger than a 2.25" exhaust system... in which case, the header and exhaust may be too big for your needs and could be costing you some midrange power.

 

Alright the reason is the only test pipe I have access to right now happens to only be 2.25. I'm really tight on cash at the moment and my cat is falling pretty badly. It needs to come out. Its clogging my system and causing problems. I have an sir-2 jdm b16a, 4-2-1 equal length header, stock cat, and 3 inch cat back. There was some shitty exhaust work done before I bought the car, which makes it difficult to measure the diameter of the collector pipe. I'm not 100% sure on the power output. I know the sir-2 makes 170 crank horsepower. With modifications and accounting for power loss over the years (i have not done a compression test) I guess maybe 185 at the crank? I'm not sure. It does not have a tune. I have a brand new o2 sensor on standby and a defouler. If you want to read more about my problem, you can take a look at my other thread. But the cat needs to come out as soon as possible.

 

And before you all ask me why it has a three inch exhaust, I don't know. I bought the car that way and I can't afford to buy a new exhaust system. Talk to me about the drawbacks of having a bottleneck in the system.

 

Also just went to the website where I bought the header. The collector pipe is 63.58 mm, so 2.5 inches.

 

Than you should be fine to install the cat to the header, you just need a 3 bolt flange and gasket plus 3 bolts and nuts that are grade 8, take it to a muffler shop and tell them what you need done. Go home and treat the weld's with something to prevent rust.

 

Alright. And as far as sound, will deleting the cat make it sound like ****?

 

Considering the majority of this was because you had a bad cat and are now putting in some shitty test pipe with a defouler, you probably will. Muffler shop might have to create a 2.5 to 3 inch adapter. hmm...since you'll have to cut off the cat converter. I'd suggest a 2.5 3 bolt flange and have the muffler shop custom a pipe from the flange to the 3in exhaust pipe.

 

What makes a test pipe shitty, and what does having a defouler have to do with the sound?