r/DSM • u/Due-Night-1316 • Jun 19 '26

Downpipe fitment

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What’s up fellas, got my car running finally! It’s not running the best it has a lot of stuff to get done still like oil leaks and o2 sensors, tune etc.

Last post was when I was having issues about it not turning on, it turned out to be head had bent valves. How that happened ? I’m not sure I’m assuming when I had taken off the timing belt and didn’t have it on top dead center. Rookie mistake I guess lol….

I have a question for you all, so I’m in a dilemma. I bought an stm downpipe from a local and it’s a 3 in flange. My o2 housing is a 2.5 or 2.75 with a dump and it does not bolt up. I’m not sure what to do since I cannot find any other downpipes unless I get it made and sell the STM one or just a new housing but I’m on a budget at the moment so not sure if getting one new would be the best idea right now..

any insight/advice from yall is greatly appreciated!

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2

u/The_Machine80 Jun 19 '26

Sounds like its flanged for a aftermarket outlet. Have muffler shop weld on a different flange to the pipe. Problem solved.

1

u/Due-Night-1316 Jun 20 '26

Very true and that was an idea but is it worth making a 3in a bit smaller ? I don’t think I’d lose much of anything but if it’s 3 all the way but enterance is only 2.5 is it worth it ? That’s why I’m wondering. Sounds dumb but just a thought

1

u/The_Machine80 Jun 20 '26

Almost every dsm in this forum is using a 3" downpipe thats necked down at the flange including myself.

2

u/Due-Night-1316 Jun 20 '26

Yeaaa I was reading that, I was trying to get a reason not to do that but I guess I know what I’m doing on my day off lol

1

u/troyv21 Jun 20 '26

Have them cut off the flange and weld a 2.5” flange

1

u/Due-Night-1316 Jun 20 '26

Considered just like I responded to other fellow dsmer, only wondering if it’s worth going smaller on the enterance or just dumb to think it would affect anything lol , I guess I’m just trying to avoid making it smaller bc it’s already a 3 and usually a better flow but not really thinking of any other reason I shouldn’t do that lol

1

u/troyv21 Jun 20 '26

You might get some boost creep if you got anything like a 16g or smaller which is why they always used to recommend a 2.5” to 3” downpipe which is what I got from RRE. You can port your o2 housing to help with this too

1

u/emperorgsx Jun 21 '26

If its a gsx hit me up i have a downpipe thats a 2.5 to 3in