Got what might be a silly question. I have an 07 GT, it has a K&N cai, Bassani X pipe & mufflers, & long tube headers. When I first start the car when cold it sounds very loud & awsome! Within a minute, it slowly quiets down, even though rpm's remain unchanged. I'm sure it's something computer controlled but it makes no sense, the exhaust is open, why the sound change???

 

It is the Idle Air Control ( IAC ) valve. It is allowing the engine to run when it is cold for emission control. It replaced the choke on carbed engines. Mine does it as well. The throttle body plates are closed on start up. The IAC valve opens to allow combustion air in, but at a reduced rate. This is causes a slightly lower vacuum which is the cause of your sound. When the IAC closes, it increases your vacuum and your exhaust tone changes again. That may be completly wrong, but, that is how it was explained to me. If I am wrong, someone PLEASE correct me so I do not sound like a fool again.

 

Sounds good....I call it "open loop" until the car warms up.

 

All the S197's do it. My FRPP Stingers sound absolutely awesome on startup and then tone down a good bit.

 

Ok, I figured it was computer, emissions, etc. It sounds great all the time, but for that first minute or so it's awsome. Thanks for the help.

 

I am not sure, but I think that it does not reach closed loop (thanks for reminding me of the proper term daredevil95) until the engine reaches normal operating temperature. BTW, you should hear my Bronco when I first crank it up!! WOW