This is not a true statement. Check link.

http://www.the370z.com/intake-exhaus...sults-3hp.html

3 whp is well within the range of dyno variance between runs. The OP of that thread did two runs with the muffler, and one without. That's not enough to determine any gain or loss. It's impossible to tell whether a 3 whp gain is from a mod or just the natural variance of the dyno. He would have needed to do several runs with the muffler and average the numbers, then do several runs without the muffler and average the numbers. As it stands, you can't determine a gain from his post.