Anybody who installed Ford Racing Shorty Headers and Bassani catted X-pipes experiencing hissing sound??

The hissing appears when the engine is cold and quiter after the engine is hot. I was told it's the headers and the Bassani Xpipes that flows better. Is this true? I had it check from two diffirent shops for leaks and they can't find any and they've given me the same conclusion. By the way this mod helped gain 3mpg (from average 16.5 MPG,before installing the headers and Roush pulley set to 19.6 MPG) and noticable torqe gain. For the HP gain, I still have to have it dyno tuned this coming weekend. Your input will be appreciated.

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Hissing sound is probably a leak in the exhaust some where. Should have went with a LT Setup anyway, just my .02.

 

LT was my first choice but since I leave in So. Cal with strict emission laws, I don't want to risk getting a big ticket or face an impound scenario. Besides, I already got stopped twice for the FRPP stingers in the past and got lucky for not getting caught witht the X-pipes which is also not legal in the state. I don't race that much so LT is not that necessary. Thanks for the suggestion, anyway. Anymore input out there?

 

Sounds like a leak (pun intended)