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2000, Impreza RS 2.5
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75 Posts
Discussion Starter #1 (Edited)
I recently bought a Tsudo HFC ( wrapped it and sealed it with high temp silicon) and my car now sounds like King Kong farting in a can and runs like dog s**t, it’s also throwing the p0420 code as I knew it might but thought the 02 extender would prevent that. Now before everyone explodes on me with the “ hfc causes raspy exhaust” and “of course it can throw a p0420” remarks let me preface this with my entire exhaust setup.

I’m running a 1320 4-2-1 UEL T304 stainless steel header (high-temp silicon painted) and a custom 2.25 cat-back w/ 14” resonator and a magnaflow oval style straight through muffler. It sounded absolutely badass before with the stock cats but I wanted to increase the diameter of the cat pipe to 2.25 and somehow missed the fact that Tsudo is 2.5”. So ... my question is would getting a Stromung 2.25 high flow cat pipe sound and perform better? What do you guys think about taking the Stromung HFC pipe and welding in a second cat to straight section right after the 90 degree turn after the first 02 sensor? I was thinking that by doing this I could prevent the p0420 and the rasp at the same time while maintaining a 2.25 piping instead of the 2.5” Tsudo or 1.85” stock.

The only other option I can find is the magnaflow direct performance upgrade which is identical to the stock pipe but does have 2.25 inch mandrel bent stainless steel pipe, however it also cost an absolute FORTUNE $1000. Please enlighten me.

The rest of my mods are rather simple (doubt they matter), NGK wires, coil and plugs, SPT intake with homemade heat shield and cold air duct from bumper and scoop on right vent of hood, light weight flywheel, kartboy short shifter and bushings. Soon to Reflash with JEC DELIOUS via ECU swap out.

NOTE will be selling my TSUDO HFC which is heat wrapped and silicon sealed, as of now it’s only been on my car since Sunday and has had about 100 miles put on it, long enough mostly to get the heat wrap to stop smoking and the silicon to fully cure up. Shoot me offers if interested, I live in Texas right next to DFW airport for reasons of shipping costs consideration, I will be pulling the pipe off this weekend.
 

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Super Moderator
2000 RSTi
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595 Posts
I wonder if the Delicious tune could delete the code for cats? I've heard that the larger the exhaust pipe, the louder the noise. Not 100% sure if thats accurate. Downsizing a 2.5 to 2.25 is childs play for a welder, so don't worry about that.

With a 14" resonator already, I bet you need to find a different muffler to make a difference. I like Borla hush, and subaru's SPT, those are both really quiet till you get on it, then you have that rumble. I've heard Maddad mufflers are awesome sounding too, also quiet till you don't want it to be.
 

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Registered
2000, Impreza RS 2.5
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75 Posts
Discussion Starter #3
I wonder if the Delicious tune could delete the code for cats? I've heard that the larger the exhaust pipe, the louder the noise. Not 100% sure if thats accurate. Downsizing a 2.5 to 2.25 is childs play for a welder, so don't worry about that.

With a 14" resonator already, I bet you need to find a different muffler to make a difference. I like Borla hush, and subaru's SPT, those are both really quiet till you get on it, then you have that rumble. I've heard Maddad mufflers are awesome sounding too, also quiet till you don't want it to be.
Well it’s not a matter of fitting the 2.5” to my 2.25” that actually bolted right up surprisingly. I just do not want a 2.5” cat pipe to begin with as I believe it’s way too large for our motors without some aggressive cams or forced induction. I’d prefer 2.25” or 2” cat section then 2.25 the rest of the way back. Something tells me having the largest section of my exhaust being the cat pipe (where you really want the most heat and pressure) is part of what’s making it sound so bad because it gets reduced again back to 2.25”. It would probably sound a lot better for someone running 2.5” exhaust through to the muffler
 

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Super Moderator
2000 RSTi
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595 Posts
I guess it's worth a shot, for the noise. Don't think that a 2.5" section in a 2.25" exhaust is going to flow much different or affect performance. It still gets constricted back to the smaller pipe.
 

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Registered
2000, Impreza RS 2.5
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75 Posts
Discussion Starter #5
I guess it's worth a shot, for the noise. Don't think that a 2.5" section in a 2.25" exhaust is going to flow much different or affect performance. It still gets constricted back to the smaller pipe.
Yeah I get what your saying in regards to performance I would agree that having a 2.5 cat pipe then reducing to 2.25 will likely have little effect on power. My thought is focused mainly on the sound here, because I think the header sounds perfectly tuned to handle the flow velocity but with it dumping into a 2.5” which is more than double the CFM flow rate of a stock 1.85 pipe and having a single High flow 200 cell metallic cat vs two regular 400 cell ceramic cats of the stock cat setup is causing the exhaust pulses to slow down and loose a lot of heat causing the exhaust flow to back up on itself just before the reduction back to the 2.25 cat back... my theory is this is causing the raspy sound and the power drop which is reverberating down the rest of my exhaust tube as a nasty resonance of buzzing and ticking
 
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