I wonder why they think 900 headers work better on anything but a 583 Supersport as all Monsters already use a 40mm system and I thought the 620 Sport also used 40mm heades

Sorry for joining in late -Spaghetti headers always seemed to be rated well on the 900's but I've hardly heard of them being used on anything smaller?

I've also heard some claims that cutting the deflector plate out from within the collector 'box' helps midrange??

My 583 system had standard diameter headers 35/36mm IIRC.