Actually with a y pipe where it separates into 2 pipes into two separate mufflers it will free up the exhaust gas flow and be less restrictive than the single muffler since it will have two outlets. Similar to the exhaust on the last gen Eclipses with the 2.4 liter 4 cyl, truth be told their exhaust sound nice for a factory set up on a naturally aspirated 4 banger. Just make sure to get the correct side muffler since the hangers, inlets and outlets are offset accordingly. Running a pair of mufflers in parallel will not restrict the flow, whereas running them in series (aka "in-line") will restrict it. That's why people are saying to remove your factory resonator, because that will free up the flow in your exhaust, the only down side to it is that you get that annoying rice rocket sound. A solid solution to getting a higher flowing exhaust without making it sound like a cheap fart can is to not install a canister style muffler and to keep your resonator. If you want it to flow a little more than the stock unit and still have a great sound, then switch your factory resonator out for a high flow resonator like the MBRP or Vibrant. The factory resonator body is about 15 1/2" long, so you will want to go with the 16" length high flow resonator. The 16" body resonator has a total length of about 20" including the collars, so it will fit. The factory pipe diameter is 2.25" (2 1/4"), so just get that size high flow resonator and get the Y-pipe and the driver side muffler and hanger insulators and your pretty well set then. It will fit like factory, look clean like the V6 and sound burlier and throatier than the stock 4 cylinder exhaust did, I think you will be happy with that set up, especially since it isn't loud enough to make the cops stare at you like a street racing ricer. Also, it will free up some horsepower and torque. So all in all, you basically want it to look like the V6 exhaust from the outside which means the 2 mufflers, and you want some performance gain, and you don't want it to sound like a raspy rice rocket, the layout I described will achieve this.
1. High flow Resonator 2.25" diameter piping and 16" body length so it fits in factory location.
2. 2.25" Y-Pipe to run the second muffler
3. Driver side factory muffler
4. Muffler hangers for the 2nd muffler
5. Competent exhaust shop to do the install
The high flow resonator will only run you about $70-$75
The 2nd muffler currently $189 on HondaPartsCheap
Y-Pipes vary, but plan on $30 or so for decent ones
Hangers are cheap, like less than $10 for all needed
Install in my area (San Jose, CA) for work like this is usually around $150
So figure somewhere around $450 total, all parts and labor.
One more thing, on a technicality, you may want to get the drivers side heat shield for your muffler, it's a protection item for gas tank safety, just a thought.
Hopefully that helps ya!
It depends on your driving style. If you hammer the throttle then you won't gain any fuel milage. But, if you drive normal, then you can get a slight increase in fuel economy, not much, maybe another 20-50 miles at best from a tank. The other thing is if you go with too open of an exhaust you will lose low rpm torque and that usually means people push the gas pedal a little harder to make it move the car like how it accelerated stock, so thereby you lose gas mileage and power with a too open/loud set up, plus the loss of back pressure if too much makes your lose torque and makes it accelerate slower. It's a 4 banger, it needs back pressure to make torque in the low end. Even V6's suffer from torque loss with too open of an exhaust. The exception to this is forced induction, especially turbos really benefit from running full open since they can spin faster/quicker/in less time, hence faster throttle response sooner full spool and quicker to generate peak power. Superchargers not quite as much as turbos, unless exceptionally high boost loads and large CFM's of total air are being moved, but really with 4 bangers you don't need much more than the stock exhaust to flow sufficiently for normal forced induction (like less than 12 psi on a turbo and 6 psi on a supercharger), but you will definitely get noticeable gains in power on forced induction motors when you get the right balance on your exhaust flows, like pipe diameter, smooth bends/turns, correct velocity & pressure, hence the tuned exhaust systems costing more money, research & development and testing are costly when correctly done to achieve proven results. But the basic principles apply to any exhaust, you need balance for it work optimally. As far as intakes go, CAI is not good for rainy places, or running your car through the machine car washes, please be aware of the car wash as some may hydrolock your motor if you run a CAI, just food for thought. It doesn't take submerging your engine to hydrolock it when there's high pressure sprayers involved like in a car wash. That's one of those key details that people tend to overlook. If you get enough water on the filter and try to start it, you could destroy the MAF sensor, foul other electronics, hurt the throttle body. and worst case suck up just enough water to hydrolock the motor, which can happen if enough water makes it's way into your intake, then all kinds of shit can go wrong. Best advice, if using a CAI, wash your car yourself, and run a water repellent pre-filter casing to protect it during washes and rain, which the pre-filter can be left on year round. Less worries with a Short Ram Intake for as far as water damage goes, but people always talk about heat soak. One way to fight off heat soak is to get the SRI tube Silver Ceramic Coated on the outside only, that way it reflects the heat from the engine bay and doesn't absorb it into the tube, technically any intake tube would benefit from being ceramic coated, whether it is a CAI or an SRI. But that's your best bet short of making a custom insulating barrier for the passageway from the front bumper where the factory intake resonator sits and plumbing it to the inlet from the fender to the engine bay and sealing around the underside of the hood and making a heat shield for around the air filter.