Sounds nice when cold, boring when warm
 

I recently had a Milltek secondary catback installed, and enjoying the nice noise it makes most of the time.

At cold starts and when cold it sounds absolutely brilliant with a very nice rasp at blips and when revving, and a nice growl at idle.

When it warms up it sounds less exciting. No rasp, and less aggressive. Very nice idle tone though.

Why is it that is sounds different when cold?

Simple, air is more dense when cold, hence the difference in sound.

I think he's talking about the engine, not the air. Right?

Unless you're talking about the air being heated up by the engine on its way in.

Dosent the engine use port injection when warming up and switches to direct when warm enough?

I'm talking about the exhaust note.

When the engine is cold it sounds very nice, but mellows a lot as it warms up.

Well when the engine is not as warm, the air is colder and more dense, but when the engine is warm, nothing but hot air comes out thus the sound changes.

This is completely scientific rocket science. lol j/k

On startup the ECU alters the cam overlap to ignite the cat faster. So its just the sound of more aggressive exhaust cam timing. I believe some ECUTek tunes change the sound too.

Ah, that makes sense.

Oh, ok. I misunderstood what you were alluding to. I thought you were talking about air through the intake. Such as...

Colder air = denser air = larger quantity of air through exhaust = different sound

But yes, I believe your theory of nothing but hot exhaust air coming from a warm engine is correct haha.

I too have the milltek catback system and it sounded great during cold starts and slowly fell off when the car warmed up. I added a Motiv concept test pipe (2nd cat delete) and now it's too loud/raspy at cold starts and perfect/deep when warm. Works for me !

Also a cold cat/resonator does not work properly, a warm cat does. Sooo that explains a lot

The vast majority of the sound difference is from the ECU adjusting overlap/timing during warmup, you can hear it even with the stock exhaust.