Likely the VVT on the cam TF. Mine has a definite dip right around 5500RPMs as well. It even shows up in the TT Dyno software. It's not as pronounced as the one here, but I have a bit bigger cam which likely helped.Great midrange torque. I wonder why it really falls over after 5400? This is just idle curiosity, since I'm considering swapping the 6 psi pulley in my car for an 8psi pulley.