I got my shorty pair from Armour Motor Products in the UK.
Although they are marked as road legal, they are certainly much louder. Which I like. They only have a very simple perforated spoon baffle at the front, which provides enough back pressure.
I would not modify them to 'improve the air flow' as without the muffler/baffle you get a nice big hole in the mid range that would break the heart of any rider. Don't ask me how I know : (
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