^^X2, as well as having too open of a muffler. The 4cyls just aren't flowing that much air to need a large/open exhaust, system. The high flow exhausts work great above 4k rpm's, it is just getting there that is an issue.
I would redo the tube in 2-2.25" to the muffler than 2" or smaller after the muffler if the Series 40 is an open style.
