Subaru Impreza Sti Spec C Twinscroll Twin Scroll Downpipe Down Pipe Project on 2040-parts.com

I decided to sell my twinscroll downpipe project.

This is suitable for someone that has a GD body, 2002-2007 Subaru Impreza WRX Sti and has an exhaust that is not JDM. If you have a USDM or any other than JDM midpipe and muffler (catback), then the length of this downpipe will be suitable.

All the components here, except for the hanger are unused, new. The hanger is in an as-new condition.

The reasons I decided to begin the project are:

-The downpipes available for twinscroll applications are beyond ridiculously expensive

-The diameters of these downpipes, many of which are over $500 are not even close to 3". When they describe them as 3", in fact that may or may not be the outer diameter. Sometimes they forget giving you the diameter completely, but remember to charge you $800 for the cost of ... a pipe.

-Some of the more affordable downpipes neck down to that factory donut, so as t "fit to the exhaust with no modifications". Why have a $500 pipe, if you restrict it's diameter back to stock, at the far end?

-Some of the remaining offerings are JDM length, so you have to extend them 4" to make them fit a USDM car, ridiculous after you pay that much.

-Nobody seems to have counted how many oxygen sensor bungs they should give you: The Twinscroll EJ207 has the main oxygen sensor AFTER the turbo, so that takes the bung on the bellmouth housing. Then there is the rear bung for the rear oxygen sensor. But where to install the wideband? You don't get a viable answer, after spending a ridiculous price for this pipe.

So I set out to make my downpipe, by buying a USDM length single scroll 3" stainless downpipe. This has a 3" outer diameter, but right there is better than most twinscroll offerings.

I also bought a kinugawa cast bellmouth twinscroll unit. While this looks good, it turns out to have unbelievably thick walls. This may be good at heat retention but necks down the inner diameter to less than 2.6", at the area where you need it to be the largest. Also, the shape of the opening of the bellmouth has to be gasket matched, as it's not close. I did this.

I came up with cutting this bellmouth in an area where it's cross section is still large and slightly oval. I think the equivalent oval cross section came in slightly over a circular 2.7". This is now close to the inner diameter of the remaining 3" pipe, from the USDM downpipe. I then flared the neck of the downpipe, to fit over the bellmouth outlet. Very difficult with stainless steel, to flare it out, basically hammered it.

I got an additional bung welded at the far end.

I bought an OEM hanger, I cleaned it up and fitted it, mocked it up on the pipe (but never had this welded), since the manufacturer of the ebay generic single scroll downpipe "forgot" to install a hanger.

I got some rough welding done for the bung and the neck and then having the unit finally in one piece, was able to test fit it to my 2002 WRX body. It fit!

At this point, I decided to make this into a catalyzed downpipe. So I bought a 100cpsi converter in a 4" unit, the biggest flowing unit available. I began fitting this to the downpipe and cut a section of the downpipe, to make room for it.

FYI: All the big names, especially a Zerosports, etc, if they are "Jasma compliant" have a quite dense converter, that proves to be a restriction, as high as 400 cpsi. Even an APS converter has more than 200 cells per square inch. Godspeed? 200. To go to a brand name with 100 cpsi, you have to go to a Milltek or H&S.

I since threw out the section I cut, but still have a small section I had cut from the bellmouth.

Anyway, I never fitted it successfully after this, because it seemed I had cut 1/2" too much at one end.

So, this can be easily put together by someone that has welding skills and a welding machine available. I did not have any, had to use a meineke and pay for every welding session.

If you don't want to make this catalyzed, you can buy this kit cheap, and get a section of 3" pipe welded instead.

Here is what is included:

-the bellmouth, kinugawa welded to 3" stainless downpipe section.

-the rear of the downpipe, with the extra bung welded

-the gasket for the far end flat flange, where the midpipe connects.

-the stainless bolts that go with these

-The OEM hanger bracket, that needs to be welded to the pipe

-The 4" 100cpsi catalytic converter, metal substrate

The welds that I had done (paid for) to the bung and bellmouth are low quality and need to be gone over, by a welder that has the time to put into these.

With this said, I will begin this at a really low price, because these are taking up room in my garage. By the time everything was added up, new downpipe, new bung, new bellmouth, OEM used hanger and welding, my cost was over $300, before adding the expensive catalythic converter.

There is no warranty, the sale is final, there are no returns.