Why does my 996TT sound so mean at cold start. Stock everything.

 

Cuz you woke it up! I'd be pissed too!

 

Ha ha

 

Secondary air pump will make the exhaust louder briefly on cold start

 

There has to be something more to it than just that. Maybe its the 1200rpm warm-up idle.

My secondary air system is deleted and my car sounds the same!

 

The DME supplies an excess of fuel in the mixture which is results in rather poor combustion with the left over unburned fuel intended to be burned in the converters. The air necessary for this burning being supplied by the secondary air injection pump.

Additionally to lower emissions the DME switches on the low intake valve lift feature.

As a result my 996 Turbo engine has always sounded "mean" -- I think I use ragged or rough -- from a cold start.

However, as the engine idles and gets a bit warmer the engine smooths up some and when the DME has the converters hot enough and can go closed loop the engine idle speed drops to near hot idle levels and the engine smooths right up. It is still not sewing machine smooth -- no 3.6l engine tuned to deliver 420hp will be -- but it is quite smooth compared to when it was first started.

 

Makes sense!!

 

Most cars sound a bit harsher on cold start up. This is noticeable on the cars that have more aggressive/louder exhausts, or ones that have been opened up with aftermarket exhausts.

The primarily reason is because the exhaust plumbing (read metal) is cold. The cold and stiff metal produce a harder edged sound as the sound waves are reflected around more intensely. Once the metal warms up, it tends to soften, and hence soften the sound as it absorbs more of the high frequency energy.

 

that's true, but a cold motor means that fuel doesn't evaporate when it hits the back of the intake valve, so you need extra fuel (cold start enrichment) until coolant temp rises enough and the valves get hot enough to encourage better atomization of the fuel. Secondly typically we increase the RPM of the engine idle during cold start because higher revs means more airflow velocity to try and get that fuel into the combustion chamber where it belongs. And last, the engine has some variable components like camshaft timing or profiles (some have air tumblers or intake plenum components that can shift airflow) when combined with different ignition advance settings, help cold start. The combination of higher revs, increased fuel, different cold start cam/tumbler/ignition advance, and exhaust temps all play into it.