Ever notice that your exhaust sound changes...

As your engine and exhaust system reach their fully warmed/stabilized temperature? I've noticed that every car I've had with "louder than stock" exhaust system, as the operating temperature reaches fully warmed, the sound becomes a little louder and the tone becomes a little bit more full and mellow sounding. Anyone else ever notice this? For years I've wondered, "Why does it do this"?

All the time.  I think part of it (maybe all of it?) is that exhaust cools off and therefore contracts as it goes through the system.  As the system heats up, the exhaust cools off less since it can lose less heat to the metal, so it contracts less, so all of the pressure waves and such that create sound will be different.

Heat expansion, incidentally, makes creating a tight-clearance exhaust system tricky.  I found that a stainless exhaust can grow 3/4" on a stock horsepower LS1 GTO from moderate-use exhaust heat.  Finger clearance at the rear valence when cold, hard contact if you whomped on it a bit.   This made me cringe a bit when I saw the Bad Obsession Motorsports guys make an exhaust with super tight clearance in the back on their Mini.  That sucker's probably going to grind and vibrate the floor when they start caning it.

3/4 as in total length? I read that as 3/4 in diameter and thought, "Holy Hell!"

Yep, exhaust is definitely quieter when the engine isn't warm yet.  Especially in the winter.  But it's not noticeable at idle, as the engine usually puts out more noise / exhaust volume at cold idle than hot idle, so cold idle is usually louder (even if it's quieter off-idle).  

Also fun- on certain motorcycles, the engine noise, especially from the top end,  changes drastically based on how much fuel is in the tank.  You can literally hear whether it's full or not.

I guess that depends on the vehicle. My disco has an electric fan built into the exhaust to help light off the cats. When I first start her up, the exhaust pulses hit up against this pressure and the truck is VERY loud. After a minute when the fan shuts down, she gets quiet again. Yes, I am running a full SS exhaust, one from Magnaflow

rslifkin said:

Yep, exhaust is definitely quieter when the engine isn't warm yet.  Especially in the winter.  But it's not noticeable at idle, as the engine usually puts out more noise / exhaust volume at cold idle than hot idle, so cold idle is usually louder (even if it's quieter off-idle).  

Compared to?

Basically every car for the last 15+ years is typically way louder on cold start up. The cars dump in more fuel and retard iginiton timing to help light off the cats.

There are many new performance cars, where people have removed cats/aftermarket exhaust, that specifically change the cold start settings in the tune so the cars aren't so loud on startup.

I spent 20 minutes looking for a video I remember and came up empty. It was a gopro mounted to the rear bumper showing the stainless exhaust growing almost an inch as it heated up and it changed surprisingly quickly with engine load.

Anyone remember what I am thinking of?

In reply to z31maniac :

Cold idle compared to warm idle.  Even when I ran a tune with no timing retard on cold start and no high idle when cold on the Jeep, it was still louder at cold idle than warm.  It took a bit more throttle to maintain idle speed when cold.  More air = more noise.  

In efi cars you are getting the same air and more fuel when cold.  I am betting it has to do with the fuel mixture causing a change in sound.  

Glad to hear that I haven't been imagining this for last 31 years  My Miata with the RS3 exhaust, in particular, sounds way moar awesomer after full warmup.

Still wondering about the tone deepening/smoothing/ more rumbly, less crackly. as it warms. Ideas?

less un burnt fuel in the exaust and the explosions are of a different frequently due to the speed of the explosion being different due to a richer mix.

dean1484 said:

In efi cars you are getting the same air and more fuel when cold.  I am betting it has to do with the fuel mixture causing a change in sound.  

I've watched the IAC position.  It definitely feeds more air when cold.  Fuel doesn't burn as well right on a cold start, so it needs a bit more throttle to keep idle RPM.  If you unplug the IAC with the car at warm idle, let it cool and then start it up cold, it'll idle noticeably low and slowly come up as it warms (due to not having any extra air supply).