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Old 04-14-2015, 02:35 PM   #1
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Ghost Knock

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.

Rattle gone.

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.
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Old 04-20-2015, 09:32 AM   #2
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Update: So it's been about a week and I"ve noticed 2 things. Gas mileage seems to have gone up approximately 2-3mpg and the surge/bucking while taking off after a cold startup (yes, I give it time to warm up first) has ceased to exist.
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Old 04-20-2015, 11:57 AM   #3
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Interesting.

That's why I'm pretty meticulous when doing installs. I go back and recheck each bolt when tightening to unspecified specs.

Unless I was using a torque wrench and keeping track of each of them.
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Old 06-26-2015, 08:25 AM   #4
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Video of the rattle:
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Old 06-26-2015, 08:58 AM   #5
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Mine makes a similar noise usually at start up. Sometimes you can hear it when you take off. I also have a magnaflow true x. But I too have checked all my bolts several times and they are tight. I never dropped my cats when I did the install so I know they are tight. But I haven't noticed and performance issues like you mentioned


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