PS. I have a single muffler splitting to exhaust pipes (vs 2 mufflers) if it matters.
Yes the Catalytic converters plays a major roll in your exhaust restriction, but the manufacture takes that in consideration and tunes your exhaust from there. That is why the Catalytic Converter from a Corvette and your Sierra are different and you do have a single exhaust with duel exhaust tips.
A true duel exhaust will have two Catalytic converters that is tunes to your exhaust flow rate with a X or H pipe to balance out the exhaust bank exhaust pulse. Once your have the proper exhaust flow rate to your preference of performance from your engine (Your major driving performance area), you can move on to the muffler. The mufflers will also need to be matched up to your exhaust flow rate and exhaust pulse, to small of pipe diameter will restrict the flow causing your engine to not breather causing you to loose upper rpm HP. Two large of pipe diameter will cause you to loose your lower end and torque and cause your engine to back draft (back Fire) when you take your foot off the accelerator which is just like a restriction your exhaust again. Remember your trying to get exhaust to flow out the tail pipe and not back in to the engine and out through the intake.
Difference between X and H pipe cross over. For any performance exhaust system, some type of crossover connecting the two sides of a dual exhaust system is important because it acts to balance the two banks of the engine.
The common H-style crossover (old school) is good at balancing sound pulses between the two halves, but does little to promote scavenging because the exhaust gases tend to follow the path of least resistance, which is straight through each pipe rather than taking the 90-degree turn through the H-pipe into the other half of the system.
In an X-pipe system, where the two sides of the system intersect. The exhaust gases have no choice but to intermingle as they pass through the X junction. This promotes improved scavenging effects by smoothing out uneven exhaust pulses from the engine’s firing order and also helps quiet down the exhaust, resulting in a mellower, less raspy tone like the old school Camaro's.
Scavenging effects: Is when the flow rate of the exhaust promote a negative pressure, which accelerates the exhaust gas flow and a negative pressure chamber (Muffler) which generates negative pressure in association with the acceleration of exhaust gases.
Now with know what you have learned, you can see why a single muffler tuned to your engine's exhaust flow rate is better then the true duel exhaust unless you have changed a few things like a different CAM to alter your intake flow rate. Think of it this way, your in the shower and the flow rate of the water is good and fast. Someone flush the toilet and you loose your water flow rate, until the water in the toilet is filled and the water valve turns off which will get your water flow rate in your shower the same again.
Pipe size for a duel should not be larger then 2.5" and then you can upgrade to a 3" after the X pipe and in to the muffler. Running a single duel chamber 3" muffler with a single out let would give you a mellower quieter tone, but you can feel it and rattle windows without the rice burner noise.
Going with the duel muffler, your not going to want to go larger at most 2.5" so your piping after he X pipe needs to stay 2.5" and your going to have a high popping tone, rather then that lower mellow tone that you can feel and rattle windows.
Once you have decided what type of exhaust you want or create, your going to have to get a free flow intake and Dyno tune or custom tune you PCM to take full advantage of your new intake and exhaust flow rate. A stock tune will only let your engine do so much as for timing and fuel injector rate and duration.
http://www.pcmforless.com/ ,
http://wait4meperformance.com/ are two of the best out in the market for tuning you engine. You can by a tuner you self, but they also have limits unless you get your self a HP Tuner like i did and play all year long with custom tuning your ride. But you really need a Dyno to take full advantage of the HP tuner.
