Good afternoon team, I have a 2020 p400 here, can anyone tell me what is going on with the exhaust at startup and why it is so loud? The exhaust is real loud for about 8 seconds and then the defender purrs. It reminds me of draft induction fan, the kind you hear that starts up before your HVAC runs a heat cycle.

 

That is your catalytic converters warming up upon a cold start.

 

Pretty normal these days...one neighbor's X5M has a really loud exhaust at start-up, only topped by a Macan Turbo exhaust a few houses away.

 

Ok, I learned something today -- thanks.

In a nutshell, the engine at start-up is ejecting very fuel rich, hot, dense exhaust out at high rpms to quickly activate the converter.

Similar to applying a propane torch flame to the inside of the converter.

Interesting...

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Methods for Fast Catalytic System Warm-Up During Vehicle Cold Starts 720481

During vehicle cold start, emissions, mass flow rates, and catalytic converter space velocities vary by orders of magnitude. Therefore, catalytic exhaust control systems must be designed to operate at high efficiency almost from the moment of engine start-up. Catalysts must reach their operating temperature as quickly as possible. Therefore, the utility of different methods for improving the warm-up characteristics of catalytic systems is illustrated.
A very elegant method to speed the warm-up is the use of the engine itself as a “preheater” for the catalytic converters. High exhaust gas enthalpy to raise exhaust system mass up to its operating temperature is obtained by the use of extreme spark retard, stochiometric mixtures, and fully opened throttle. Intensive studies to investigate the effects of concurrent changes of spark timing and air/fuel mixtures on exhaust gas temperature, enthalpy, NOx and HC emissions are discussed.

 

Thank you all for the info on this, now I get it. I can imagine what that BMW must sound like.