As Dumhed has said,the back half of the system can be fairly random,as long as it flows,up front is where you make the most difference to power.
Muffler size,design,and placement make the most difference to the noise level and pitch of the output.
You'll see a lot of cars with a small resonator (or nothing at all) in the middle after the cat and then often a simple cannon or straight-through slight offset muffler at the rear.
This setup is great for overall flow,but lousy at reducing the output volume,and will likely have a drone to it on cruise.
Adding a decently-sized slight offset path muffler in the middle of the system can remove a LOT of the drone,and harsh raspiness from the note,without a drastic decrease in overall flow.
If you check out the design of most aftermarket systems for silvias,you'll see that the quieter types will have a mid muffler as part of the cat-back section.
On an NA car,you need to more careful with system design than on something turbocharged,as the flow of the system depends on gas velocity to promote scavenging,and in extreme engines,a full charge of the cylinder on intake.
You'll need proper extractors of a design and diameter to flow best in your ideal powerband,and to maintain that diameter until at least the outlet of the cat. After the cat,diameter is less critical,but you must maintain a consistant flow rate through the system as well as the layout of the car can allow. The gas will be cooling and expanding less and less as it moves along the lngth of the system,so the further back along the car you go,the less critical piping diameter is.
As an example,an NA 1.6l 4AGE will make it's power in the top end only,so it will run large diameter extractors of a 4-1 design. These will be 40mm+ at each cylinder,before merging to a 60mm or so collector just before the cat. Catback aftermarket systems generaly run 50mm or 60mm piping post-cat,depending on application.
On a turbo car,you want to get the gas out of the engine and into the turbine as smoothly and as fast as possible,so short but equal length runners that merge at the turbine inlet are most desirable. After the turbine outlet,everything is a restriction to flow,so you want to minimise that as much as possible.
A smooth dump pipe,that does not allow the wastegate gas to merge immediately with the outlet of the turbine is most desirable. Everything you do after that is to minimise noise and backpressure. Noise for legal reasons,and backpressure to allow best response of the turbine.
Split dump pipes with a separate wastegate pipe to the main turbine outlet are popular because they work best. Blended designs were popular in the 90's,before we knew better.
Changing a rear muffler alone on a car will give a small gain in overall flow,but is primarily for added noise. The real gains are made in the front half of the system.
Justin...
_________________ STI - Made in Texas,not Japan.
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