Technical exhaust question
 

MY18 BRZ with ACE350 headers, HKS (60mm) dual resonated frontpipe and OEM catback.
I have a spare used Milltek 2.5" (63.5mm) catback that has wider and shorter pipes than the OEM 2.25":


https://dihdn14x1fl5t.cloudfront.net...SSXSB043_1.jpg


Which is better to install for low end torque, the Milltek catback as it is, or to adapt the OEM 2.25" midpipe to the Milltek backbox so I'll keep some backpressure and exhaust gases speed due to thinner Subaru midpipe?
Or leave it as it is cause in every case I'll loose some low end torque due to Milltek backbox?

If you want to use the Milltek to change the exhaust sound, you're probably best to use it as-is. As you've surmised, in theory, you'll lose a small amount of low-end torque with the 2.5" vs 2.25" piping due to reduced exhaust gas velocity, but whether it would be enough to be noticeable on the road is debatable, and if you're looking for more sound, you'll get more enjoyment out of *that* than the minimal difference in torque.

Since you have both cat backs on hand, your best bet is just to try them both as-is and see which you prefer the sound and feel of on the road - I.E. which gives you more enjoyment when driving. Go with that one...but maybe keep the other one on hand just in case.

Because of the significant difference in layout between the OEM and Milltek (with the latter's central inlet to the backbox), Frankensteining the two together would be an irreversible change to the OEM midpipe. That would also likely cost you just as much as having a custom 2.25" midpipe fabricated to match the Milltek midpipe's design. Either way, it's an expense that will almost certainly not be worth the miniscule difference in performance.

I say this as someone who spent thousands of dollars on several different iterations of customized exhaust setups in search of the "perfect" sound, torque and power compromise on my previous car. ;)

You will gain none or barely any performance swapping catbacks on an NA setup. All your realized performance pretty much ends not much further from where all your exhaust streams are collected coming out of the engine. That's why people tend to focus on headers, overpipes, and front pipes.


I prefer to think of catback only from a sound enjoyment point of view. From that standpoint, running complete systems is probably better for sound and build quality. Mixing and matching from a sound perspective is a gamble.

The ACE350 headers and HKS dual resonated frontpipe I have, give pretty much normal sound, except some "resonance" around 3000-3500rpms if I accelerate with higher gears 4th-5th-6th.
I guess if I keep the OEM midpipe and add only the Milltek backbox it will be even better.
I am planning to change exhaust hangers also with the uprated STI ones that reduce the minimal drone/resonance or whatever it is.

I tried changing exhaust hangers (STI ones)

I am happy with the results for expecding $52, it seems the more time pass, the rubber get harder, and the results are better.

https://www.ft86club.com/forums/showthread.php?t=143514

Interesting, such small mod has such big impact on exhaust sound, actually the hangers are from another model the Legacy and not STI but doesn't matter