Its not just where it exits... its the inner volume of the system that matters... Look at it in another way.

Take a garden hose.... Stick it into a pool, suck on the other to get the water flowing out end and drop it. The Water does not stop coming out... and you can literally leave it until the entire pool is drained.

Look at the pool as the "CYLINDER" the water as the "EXHAUST" and the hose as your "EXHAUST SYSTEM"

The movement/velocity of the water pulled the water from the pool... You want your exhaust system to work similar to this method. You dont want your piston to PUSH the exhaust out... you want a VACUME to be present behind the exhaust valve so when it does open... the exhaust is sucked out.

So look to your HEADERS, PRIMARY collectors and "Y" pipe Inner diameter and apply that to the amount of realistic HP you should be making and where you want to make it... to determine your exhaust systems requirement.

There are plenty of calculators on line to help chose the right ID of your exhaust system.

SPEED SAFE, AIR RAM

 

Just don't cry when you fry a valve..lol

 

The powerband on my Mustang changed when removing the stock converters and installing an offroad pipe. The power band went up. Less low end and throttle responce, but more power above 4000rpm.
I now have 2 high flow 2.5" converters and a 2.5" Magnaflow cat-back. It is the best of both worlds for power and keeping the exhaust clean smelling.

When the funds allow, I will keep the factory H-pipe on the 08 RAM and use a Magnaflow Cat-back with the dual tail pipes exiting under the rear bumper. (That is is they ever rerelease the correct 08 kit due to a different cat supplier for the 08 Rams and no front resonator like the 06/07)

 

I'm so glad I now have a single 3" stainless steel system with custom muffler, exiting just before the rear tyre after reading this.
Al.