I've seen all kinds of racing, from 70cc tuned mopeds to superbike road racing. I think when it became most apparent for me that K&N and other low layer count cotton gauze filters like Sprint filters wreck engines was when I raced the Aprilia SXV550 supermoto bike. Amazing bike, crappy head design and that thing blew head gaskets constantly leading to all kinds of issues. What really struck me as odd was when re-doing the cylinder head gasket once, 500km after having totally redone the engine after a crank case seal leak and I'd put in new liners and pistons and the cylinder walls looked like they came out of a clapped out MX bike with no filter. It should not have looked like this.
In Tallinn, my friend has the most advanced dyno place for bikes and also builds some of northern Europes most high end race bikes and many bikes have been run through there to test the differences of stock, sprint, BMC and other filters. The stock filters run maybe 1-3hp behind the cotton gauze filters on the top end when the engine is brand new. But after a season, the bikes are on par with each other.
The cotton gauze filter engines all display the same symptoms on visual inspection which is cylinder walls looking much more scuffed up than bikes running stock filters. The issue seems related to the size of the debris that can enter the combustion chamber and all those bikes also have sand in the airbox.
Our theory is that the scratching induced on the cylinder walls by the debris becomes a passageway for gasses to escape during compression, combustion and exhaust stroke which robs the engine of power over time.
Foam filters in our experience is a better alternative than cotton gauze filters however the very best solution for an engine you want a lot of power out of while keeping longeivity is to stick with stock filters and do some kind of airbox modification.
That's also why I liked the MR5 airbox mod although my plan is to probably 3D print a miniature stock airbox which fits some OEM stock filter which essentially just increases the airbox volume and filter surface area allowed the engine to breathe better.
... Winter project
