I replaced my system with a cat-back flowmaster kit. The loss in low in torque was terrible. IMO the engine is so well tuned for the amount of backpressure, freeing up the exaust resulted in a loss of performance. It did run better in the 4500-5500 range. I ran tests with my G-tech before putting a stock system back on to verify the seat of pants opinion. Tests before and after with and without the tuner confirmed.
I don't really think that any muffler change will result in much performance increase IMHO. With that being said, if you want noise like I did, you can get it without losing low in TQ. I replaced the 2 in 2 out factory with a flowmaster 2in and 2 out. The flowmaster is smaller diameter inlets so we followed with the smaller pipe straight out the back. From what I have read, 2 1/4 pipe for duals is large enough. Mine is loud but once at cruising rpm not bad. I don't feel that I lost any low end, but cannot prove that, but with the wester's I can break the tires lose easily.
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