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2001 Impreza 2.5 RS
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Discussion Starter #1
Hello all.

Long story short I have the JDM Version 8 engine in my 2001 RS, which of course came with the twin scroll turbo. At the time of the build I had a rather large down pipe (Vishnu brand I think it was) lying around, so I had a twin scroll flange machined for it and bolted it to my engine. Anyways, here I am a couple of years down the road wondering if that may have been a mistake. For one thing it has a super hollow tinny sound to it, but more importantly, could it be affecting my performance? I know that usually a down pipe made specifically for a twin scroll will have either a divider at the mouth of the pipe, or sometimes even 2 separate pipes. Should I stop being so cheap and get a twin scroll specific piece? If so, any recommendations?

Thanks!

Matt
 

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2000,rs
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Unless I am missing something I believe the dividers/separate pipes that merge in later have nothing to do with single or twin scroll, they are to help reduce turbulence from IWG's.
 

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2000,rs
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Yes, the dp. in the first pic is divided to divert the gas from the iwg and have it merge later causing less turbulence right behind the hotside wheel. It is fairly common on aftermarket dpownpipes and has nothing to do with single/twin scroll turbos. But the general logic is that is does aid in performance (hence why it is a fairly common design).
 

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I personally don't think there is much to gain from divorced vs bell mouth, but Moore Performance in Pittsburgh has done dyno testing with their divorced twin scroll downpipe and claims to have some improvements. The one pictured above is a Tomioka and is not available w/a CAT but is probably the most affordable option in the US. Buy a vibrant high flow cat and have it welded in by a fabricator for about $150. Don't forget to have a bung welded in before the cat for your WBo2. This thread has all of the available options that I am aware of. Most need to be purchased outside of the US and cost a good amount to ship. Thus, most end up costing over $700. And all of a sudden, the Moore Performance downpipe is reasonably priced.

I had two 12" long vibrant resonators welded into my mid-pipe to knock down some of the raspyness. I had a Cobb axle back before going to the Varex, and it was still very loud but not as harsh. A CAT will also help knock down some of the honda fart can sound. But both CAT and resonator will affect performance (the CAT more so than the resonator). Now that I have a Varex, I'm considering removing my two resonators.
 

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2001 Impreza 2.5 RS
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Discussion Starter #6
Awesome thanks! The car sounds 100% better after changing the cat back but it's still just a bit too raspy, I'm thinking maybe from the giant size of the down pipe currently on it. When I first built the car I left the stock catted down pipe on there and it sounded amazing, however it has since been tuned for catless and I would hate to have to have it retuned just for the sound, or else I would put a converter back on it. Think it would still need a re-tune for just swapping to a divorced down pipe?
 

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I would second agentH's recommendation to check out Moore Performance, I have one of their early twin scroll kits and Ryan was always amazing to work with.
 

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I'm not sure if u would need a retune, but any changes to your intake and exhaust can effect the volumetric efficiency and thus throw off your tune. If you spend the cash on a moore performance pipe, I'd get it retuned so you can eek whatever benefit there is.

You might find that simply by installing 2 resonators in your midpipe as I have will significantly reduce rasp. Like I said, after moving to my 207 + twinscroll, I had to do something. It was just too much for around town and my exhaust had a cat. It was raspy and tinny, but did sound badass when whipping. The resonators made it sound less raucous but also less raspy.

A third option would be to keep what you have and install a varex muffler. Quiet when you need it, loud as sh*t when you want it.

I'd personally love a Moore pipe. They're very open to customizing a solution. I thought if I got one I'd have one made with a removable cat (sandwiched between 2 v-bands).




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