TL;DR If you have an aftermarket exhaust, make sure those bolt clamps are tight!
I'm convinced I was an unknowing victim of ghost knock.
Several months after my exhaust was installed I developed a really bad rattle. Long story short, I tracked it down to one of the bolts on the X-Pipe clamp that needed to be tightened.
However, at some point within the last year I also noticed a "flat spot" had developed. I couldn't pin point exactly when it happened... and it most likely developed gradually. But it seemed like a my low end "ooomph" wasn't there... pretty much anything below 3k RPMS seemed lacking.
Over the months of reading several hundred forum posts, and watching multiple mustang related youtube videos I discovered the existence of Ghost Knock. I didn't really think much of it...until this weekend.
Since I was under the car to install the Cervinis Lower Grille I went ahead and checked all the bolt clamps and gave them a few tugs to make sure they were snug.
To my surprise I noticed an immediate difference in the low end response.
Unfortunately I don't have any logging data to back any of this up, but I know it's not a placebo.. there is a definite difference in how much harder the car pulls now under 3k. The "Flat" spot is gone.
I'm convinced the pipes were causing a rattle or knocking I couldn't hear, which the knock sensor was interpreting as knock and retarding the timing.
2014 Mustang GT Premium (deep impact blue)
Too many mods to fit in my sig.
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. (still updating mods installed so far)