Having different exhaust piping sizes?

I’m looking at a new I/H/E setup, and I understand it’s important to keep the piping sizes consistent.
the DC Sports 4-2-1 header has a 2" outlet, which mates perfectly to the 2" inlet of a Magnaflow High Flow Cat. The Magnaflow has a 2.5" outlet, but the Greddy SP2 catback exhaust has a 2.36" piping size. Will there be an issue with fitment or power/flow? Thank you.

Fitment should be fine, and thats probably not enough size difference to notice any power difference. My Skunk2 is the same size and im using a Magnaflow Cat with no fitment issues.

EDIT: Oh and this thread probably would have been better placed as a reply in your other exhaust thread right below this one considering you asked almost the same question in that thread. Not being a jerk, just trying to keep topics together for others searching.

A size difference isn’t necessarily a bad thing, it’s the location of those different sizes. In general, if there are going to be different sizes you’d want to have the larger diameters closer to the engine and the smaller ones further away - partly because exhaust gas volume decreases as it cools and it cools as it gets further from the engine.

If you are running a stockish engine, then I wouldn’t worry too much about it. But if you’re planning on actually building the engine (and not just bolt ons) you would want to be more aware of those exhaust sizes. And, depending on what engine you’re running, the header and cat look like your main problem. For some engines a larger exhaust itself would be ideal, but for most 2.36" is going to be fine. But you’d be wise to pair that with a header and cat which are at least that diameter. I’m currently running a 2.5" header, 2.5" cat, and 2.36" exhaust.

you may have trouble finding a donut gasket with a 2" ID and that’ll be big enough to seal for a 2.5" ID flange on the cat… im not sure…

why not go with the 2.25 cat?

i should stick to a maximum size of 2.25" if i’m just doing minor bolt-ons. so i preferably don’t want to move into 2.5" piping unless i don’t have a choice. i try to buy recommended parts/valued parts so i’d like some suggestions in keeping a relatively quiet/rasp-free setup. the greddy sp2 is a definite even though its a 2.36". i can only find the dc sports 4-2-1 headers in a 2" sizing. i see people posting about getting a 2.25" magnaflow hi flow cat, but the magnaflow site is only showing one with 2’ inlet and 2.5" outlet. where are they getting their’s from?

There shouldn’t be a problem with 2.5" header, 2.5" cat and 2.36" exhaust, and it should flow better and likely produce better numbers than with a 2" bottleneck at your header. The only problem is that 2.5" headers which clear the stock crossmember aren’t all that easy to find (I don’t think).

How strict are your smog regulations? Another option is to go with an SMSP cat, you can get it in basically any size/shape you want: http://sms-products.com/Converters.html The cat’s I’m seeing on Magnaflow’s page ar 2" in and 2" out: http://www.car-sound.com/02product/displaydirectfit01.asp?catalogid=40&directfit=22637

i understand i can also run a 2.5" header and cat but it seems that most exhaust piping guides say to stay with 2.25" if you’re under 190hp. i have a non-vtec b18 and don’t intend to go beyond the i/h/e upgrade. i’m not worried about emissions since i have a buddy that can handle that for me but i prefer to stick with a high flow cat to lower the potential noise, rasp, and smell. i stand corrected about the magnaflow being 2" in and out. i can’t find the 2.25" magnaflows that people are saying they have, though.