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A stock filter has a cone inside and the material is different = restricted.
Also it’s a bit smaller than K&N –
K&N can push 3x more air = cotton filter in oil coating – so cleaner bike & more free revving yes. “650 specific” K&N is good otherwise , BMC Italy does it too (known for race filters on aprillias) – I owned some BMCs in some of my bikes , some K&Ns – they do the justice
However – an upjet is mandatory (hence carb tutorial on the forum goes in better detail on jet swapping – just search “carb” at the top of site.
Running 3x more air on the same stock jets = engine is much hotter , misfires , new piston rings soon lol.
Likewise, if you have a performance exhaust pipe –> more gases going out –> upjet is mandatory
Any air upgrades basically (intake or exhaust) = upjet
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