3" downpipe to 2.5" catback?

[zd0gg]

Hello,

I have a 2.5" catback on a stock t25 turbo on my gst. I am looking into getting a dowpipe and was not sure if I should go with the 2.5" in / 3" out downpipe, or 2.5" downpipe. My goal is to get a big 16g, maybe 300-350hp. I dont really do that much racing.

I was gonna get it from ebay, and the cheapest downpipes are the 3" ones, and so I was like why not just get the 3" since its cheap (I cant seem to find 2.5" dowpipes on ebay, only 3").

I will get a high flow cat, and get it welded onto the downpipe with the original dp flange put onto the end of the high flow cat.

So, my main question is, if a 3" dowpipe (flanges i guess..) will fit onto the 2.5" catback?

I also noticed some 2.25' Inlet / 2.75' Outlet downpipes. Is that better than a 2.5", and will it fit onto the catback?

If there are any other things to take into consideration, please let me know.

Thanks.

 

[e85_4g63]

It's generally frowned upon to use a larger down pipe with a smaller cat back. If money is the main issue, I would advise you to save more money instead of just buying a piece of junk like they sell on eBay. Here is a link to a down pipe you may like. It replaces the restrictive o2 housing on our cars, and it also has an open end (meaning no flange). You can have any muffler shop weld a reducer onto in and then onto your cat. It's a little pricey, but remember you get what you pay for..

ForcePower Products Page (it's almost at the bottum of the page, it's called an FPM/J series down pipe).

here's a link to a 3" to 2.5" reducer for the end of the down pipe also... Dynomax Performance 41973 - Dynomax Universal Exhaust Connectors and Reducers - summitracing.com

 

[pboglio]

You are doing it backwards. A proven combination for at least 450 h.p. is a 2.5" downpipe and 3" catback exhaust. Just save yourself the headache and get a 2.5" downpipe and call it a day.

To answer your question, it depends on which 3" downpipe. I have a 3" Megan downpipe that would never fit on my 3" Thermal catback without some cutting and welding.

 

[zd0gg]

alright, thanks for the replies. I think I might just go with the 2.5" and keep things simple and effective.

 

[ryguy86eklipse]

As stated, you want to stick with the 2.5" down pipe to 2.5" cat-back. If your goals are to hit 400+ hp I would definitely just change over to a full 3" exhaust, or at least a 3" catback with 2.5" dp as it will lower your chances of dangerous back pressure. In case your wondering why, the exhaust gases are extremely hot coming out and are more dense. As it flows out to exit the exhaust system it's cooling and expanding, hence why you want to have an equal or larger sized cat-back section as the down pipe.

 

[bmylez]

You're right about heating and cooling, but wrong about denser and expanding. Everyone knows that hot air is *less* dense than cold air. Cold air "shrinks" and hot air expands.
It is pointless, however, to have a less restrictive pipe leading into a more restrictive pipe (i.e. 3" running to 2.5"). Like everyone said, 2.5" downpipe to what exhaust you have now, or just full 3" exhaust somewhere down the road.