Subaru WRX and STi Exhaust general info:
There is no irrefutable evidence that one diameter is better than the other. Purchasing a larger exhaust than needed for your level of modification has no disadvantages other than cost and possible less ground clearance issues.
What other differences are there with exhausts? The other differences are as follows:
1. High flow cat(s) or catless.
2. Exhaust pipe length. Some manufacturers have proprietary exhaust component lengths. This means you have to use all of their exhaust piping or some of it with a custom piece. As well, JDM specification exhausts are different length and require all JDM specification piping or some of it with a custom piece. Additionally, some manufacturer’s components may take the place of two OEM components. This generally occurs with the downpipe section of a TBE where the “downpipe” is actually a downpipe + midpipe and these are commonly referred to as “long downpipes”.
3. Flanges/mating surfaces. Exhaust components are either equipped with a flat flange or have a donut gasket designed to flex during movement. Flat goes to flat and donut goes to donut, ensure you double check prior to purchase which you have and what your new exhaust has. Some companies do make adapters of various kinds to mate say diameter A to diameter B piping and/or flat flange to donut gasket.
Which exhaust provides the least back pressure? No one is really sure, but theory suggests that larger piping is better than smaller, baffle-less mufflers are better than baffled mufflers, and no cats/freer flowing cats are better than OEM/poorer flowing cats.
Which exhaust has the best sound? This subject is up to the individual. It is highly subjective and can lead to trouble with local noise abatement ordinances. That great sounding exhaust may be the cause of some expensive and/or time consuming noise abatement tickets. A smart move is to find someone in your local area with the exhaust component(s) you are interested in to hear it in person.
Which exhaust is the quietest? This question can be answered by listing hard data. By using hard data, you remove human judgment and don’t get stuck with a recommend “quiet exhaust” from someone that in your opinion is now a loud and expensive nuisance. Keep in mind that these quiet exhausts only protect you from the chance encounter in areas with strict noise abatement laws. At wide open throttle (WOT) any aftermarket exhaust is considerably louder than OEM and even the quiet ones will not protect you from folly.
How do I quiet down an already loud exhaust?
1. Install an aftermarket silencer. This is an insert that is fitted into your tailpipe.
2. Install a resonated exhaust tip.
3. Have an inline resonator welded into your exhaust.
4. Install an auger style muffler or vortex insert cone.
One special note about silencers. While they do quiet down your exhaust, they generally work by restricting your exhaust flow down to a 1 1/2″ hole. Strongly consider what this does to your backpressure and the effects of backpressure on a turbocharged engine.
Which exhaust has the best gains? There has not been a 100% conclusive test of every aftermarket exhaust to prove or disprove best gain level. Suffice it to say though that theoretically, the exhaust with the largest pipe diameter along with a 100% baffless muffler would be better than a smaller diameter and/or baffled muffler.
Are there any downsides to exhausts? There have not been significant amounts of problems with exhausts. For STi and some 2.5L WRX owners, there have been reports of overboosting issues with some TBEs.
Where do I buy an exhaust? Every Subaru/Import performance store sells exhausts. For purchasing, support your favorite Subaru blog by buying through amazon or with your favorite vendor.