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Discussion Starter · #1 ·
I acquired a SPT exhaust (P/N D441SVA000) for 2015-Up STI. After doing some research I found that the difference between to D441SVA000 (STI) and the D441SVA100 (WRX) is only the resonator that is added to the STI version. I bolted it up to my WRX and ran it about a year. The sound was very similar to the stock WRX except maybe slightly deeper tone.

Recently I was on a kick to get a new car, so I put my stock WRX system back on. Interestingly, I seemed to notice that an acceleration flat spot was gone and it seem peppier with the stock system. Well the new car idea wore off and decided to take the SPT exhaust system and remove the resonator to make it like the version for the WRX. I got 2 feet of 2-3/8" stainless pipe to splice in where the original resonator was. Now keep in mind that intermediate pipe diameter is bigger than the stock system. But I got the big surprise when I cut about 1-1/2" back from the resonator outlet and found two diameters of pipe. While the outlet diameter is 2-1/2" OD, there is actually a choke tube that's about 2" long that only has a 1-5/8" ID. :eek:

So while the resonator equates to a baffled glass-pack with 2-1/4" inlet and 2-1/2" outlet...there's no getting around the 1-5/8" choke tube. Then the rest of the system is large diameter, free flowing radii pipe. Which I beg to ask why if there's such a small ID midway in the system. I guess it's hard for Subaru to justify $1200 without giving the impression of bigger pipe and mandrel bends.

It makes me wonder if the WRX version (D441SVA100) has any choke tube installed, or if the stock systems have it either. Needless to say, when I'm done the system will be no smaller than the 2-1/4 inlet at the intermediate flange. I'm also interested in seeing how it sounds and performs.
 

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While interesting, realistically it probably isn't doing much. Adding backpressure, but shouldn't cause a huge ( if any) noticeable loss of power on a dual exit system.
 

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Discussion Starter · #3 · (Edited)
The choke point early in the system where the gases are hotter and restriction more effected.
Just really surprised that they'd push all the exhaust thru one 1-5/8" pipe. (60% less than 2-1/4 :()

STI version
STI Exhaust Diagram.JPG

WRX version
WRX Exhaust.JPG
 
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