Dstretch33

How's everyone doing out there?
These r my exhaust questions before I buy a Dc sports header.
The dc sports jdm style header comes with a 2.5 collector, my exhaust Hks Hi-power is 2.25 on the pipping.
Will i need a cat that is 2.5 on one side/inlet, and 2.25 on the other side/outlet, to make the header and the exhaust match up???
I know you can buy bolt/weld-on cats, but do they make them with two different sizes(inlet/outlet)???

Am I even understanding this concept correctly??

I have searched, none of the information I found pertained to my questions. They were mostly "For Sale" and "Comparison" Threads.
O, and this is the exhaust set-up, I'm running for my minor N/A build...Bc3's, Omniman springs, retainers, ctr pistons w/miscellaneous accessories.
THANKS IN ADVANCE

93egSIboi

I'm pretty sure that a dc-sports header will bolt-up to the stock exhaust, but im not totally positive. Im not sure what the piping size is in the stock cat but i don't think its quite 2.25 piping, I think its a little smaller. The only thing that wouldn't work so well would be the change in airflow at the bottleneck from the 2.5 inch piping to the smaller cat piping to the 2.25 exhaust piping.

Padawan

First, your HKS has 60 mm piping, which is roughly 2.36", not 2.25". The 3-bolt flanges on a stock exhaust, your HKS, a 2.5" exhaust, etc. are all the same size externally. The difference is the size of the opening in that flange, which is normally the same size as the rest of the exhaust piping (although stock and some aftermarket systems retain a "bottleneck" here which is smaller than the main tubing). Because all the flanges are the same external size, you can bolt the 2.5" flange of a high-flow cat to the 2.36" flange of your HKS with no problem. You can therefore use a 2.5" inlet/outlet cat, but ideally you will also want to remove the stock flange on your HKS and replace it with a smoothly tapered 2.5" to 2.36" adapter flange.

Dstretch33

Thank You very much that was informative. So basically all the flanges or points the exhaust components connect are the same size. If so, does this mean i can retain my stock cat, until I decide to upgrade to a high flow cat?

Padawan

You can't keep the stock cat because the flared flange that connects to the header is too small to properly accept the 2.5" donut of the JDM header. Also, the JDM header is a different length, so it requires a cat of the correct length as well.

Dstretch33

okay so what do you recommend???
I was talking to kteller8 earlier, they were saying that the custom 2.5 to 2.25 or a regular 2.5 hi-flow cat will be fine. If not what length or size do people usually run??

Padawan

I always suggest SMSP. He has a number of options, and can provide you with a cat that will bolt right up.

Dstretch33

Does anyone know off hand what size/length the cat should be?
I'm asking now before all the parts r here, so I can get the cat, and install eveything at once. I have ordered a hks hi-power, and jdm Dc sports 4-1 header, and it's going on a 2000 si. Anyone know off hand??

white_jdm21

I have recently talked to SMS and he has told me he normally sold test pipes at 17 inch then he did an in house that measured 14 inch so you will be better off hanging the exhaust and putting your header on measuring your gap and then buying either the hi flow cat or test pipe.

Dstretch33

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by white_jdm21 &raquo;</TD></TR><TR><TD CLASS="quote">I have recently talked to SMS and he has told me he normally sold test pipes at 17 inch then he did an in house that measured 14 inch so you will be better off hanging the exhaust and putting your header on measuring your gap and then buying either the hi flow cat or test pipe. </TD></TR></TABLE>

Yeah, thats what I was trying to avoid. But, it seems like the only way to make sure it's the right length....thanks man

8krdline

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dstretch33 &raquo;</TD></TR><TR><TD CLASS="quote">Does anyone know off hand what size/length the cat should be? ...it's going on a 2000 si. Anyone know off hand??</TD></TR></TABLE>

wondering also bc i'm looking for a jdm itr header

00Red_SiR

It's been my experience that if you order from Kteller or SMSP and tell them what your car is and that you'll be running the DC JDM header, they can send you the proper length cat you'll need.

Dstretch33

I been told the only way to know for sure, is if you mount your exhaust. Then hang your header, to see how much of a gap you will have. I don't want to do this because it seems like a lot of useless extra work. Someone has to have this set-up, and tell us all what size cat to buy........

00Red_SiR

As I said in my post above, that specific size of the cat is a known thing by these manufacturers on a specific car with a specific header. All off the shelf cat backs bolt up to the cat at the exact same location on the car so it's not a factor. We (HT members) may not have that physical measurement easily available to us but those companies do so if you order from them and tell them what your configuration is you should be fine.

Dstretch33

well, i'll ask them to see what they say..Thanks again