Hey guys, I've had my 3" magnaflow system on for about a week/week and a half now.

I've really only put about 80-100 city miles on it so far, and no long duration drives. Probably irrelevant, but throwing it out there in case it's worth while for you to know, regarding the "break in" period some systems go through.

For the first few days, it actually did this longer then just start up, it did it at certain RPMs. That resolved itself after I put enough miles on it, so I am hoping this might as well.

When I first start it, it sounds pretty bad. Droning might not even be appropriate, sounds like a slight rattle/vibration in the rear, muffler area. The install is flawless and I had a mechanic give it a quick look just to make sure. In all fairness I should probably shoot a quick video of the sound to give a better idea - I will probably do that on Wed or Thurs.

once it's warmed up, it no longer does that through any part of the RPM range and it sounds kick ***, very happy with it. It's just the intial 30 secs to 1min where it's happening.

Anyone able to give me some insight as to what's going on? I am hoping this will go away after I've put more miles on it / go on a longer drive a few times to help settle it in, it's confusing that it only does it from a cold start and for a short duration though.


other random possibly relevant info : stock headers, stock cats, 3" catback (single, dual exit, http://www.planetmagnaflow.com/pdf/16784.pdf ) and it's been about 50*f lately.

 

The engine speed is normally higher than normal idle after a cold start... Its probably just that rpm range like 1100-1300, after it warms up(and idles around 600 i think) try to make it do it by applying the throttle. IDK, thats my thought on it all.

 

even as I drive it out, this noise persists until it's warmed up for a little bit. it doesnt happen at a *very* specific range but it is more dominant in the lower rpms, but that's kind of a bad statement in itself since I never gun it while the oil pressure is high shortly after start up.

usually after I've gone a couple of blocks it fades and is normal.

 

Its not all that surprising really.

An exhaust system is a tuned resonance system. Tuning frequency is determined by the speed of sound of the exhaust gas. And the speed of sound of the exhaust gas is determined by its molecular weight and temperature.

When you start up the car the exhaust system is very cold so it cools off the exhaust gas flowing within it. The tuning frequency of the system therefore starts off low and goes higher as the system heats up.

Apparently, the lower tuning frequency brings it to a sweet spot where it is easy for the system to resonate like a low note on a pipe organ.

That's basically what it comes down to.

 

My MR2 exhaust does the same thing - when I first start it, it sounds much louder than it does once it warms up. I don't know the physics of why, I just know it's normal in my case.

 

ahh you are a valuble asset to this community. thanks for the logical explaination.

 

man my nismo exhaust does this

the only difference btwn our situations is i LOVE that sound. with this, you could actually hear that i have an exhaust while driving lightly. Once it warms up, even stock exhaust is louder (light driving i mean)

 

Video???