Why does the exhaust growl more aggressively for the first few minutes after starting

Originally Posted by Elan

Let me translate:

"My good friend had a single tip exhaust system installed. Gosh darn when he engages the ignition, it makes me go wow. He had the engine tuned up and it was faster than other peoples cars. He told me that after the tune allowed his car to produce more horsepower, he was able to claim victory in a race with an elderly gentleman the previous day".

Either that or after the tune, he mauled an old man.