The vehicle passed its emission test procedure as delivered. Making changes to that, if done by a shop, is a severe fine. It's a little messier if the owner does it. By taking longer to warm up, the vehicle will also use more fuel. Catalytic converters require some excess fuel to bring them up to temp to work properly. By opening up the flap early, it will be using more fuel for longer to achieve that. All this for a bit of sound? Personally, I like it quiet. Now, if I floor it, that's another thing, but just sitting there, quiet is better.
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