Another little bit OT question which came up to my mind during reading the part about reasonance (and is haunting me for quite a time) is: I've read a lot about reasonant airboxes and stuff on how to perform a rough calculation of it. One particular note which almost every such article contains is: if you put foam inside the reasonator volume you will effectively cancel the reasonance. OEM GK75A/B airbox is equipped with foam-type filter. I expect that this filter effectively is cancelling the reasonance. I also know that 97-on B4s do have paper filters yet these are not exchangable with GK75s.
The only paper filter I know that exists for B4 is K&N which costs about $100 per unit. And probably has to be replaced every season. The question here is how much the foam filter affects airbox ability to reasonate or vice versa how big improvement paper filter would be? If any at all. I have one piece of that K&N filter on hand but it is of unknown age and state so I am not planning to try install it into my bike.
I've seen some pretty bizarre discussions of this subject (Resonance). Like a lot of scientific issues, there's a lot of misunderstanding and crossed-up logic to be found on the internet (mostly in car and motorcycle enthusiast forums). For example, I've seen a forum discussion thread run out to 3 or 4 pages of pure confusion before it was realized that the two main people posting on the thread were talking about 2 completely different types of resonance
. One guy was referring to resonant sound waves that an airbox tends to generate as an engine climbs up through its RPM range, and the other guy was referring to the physical nature of the airflow resonance (that difficult-to-engineer, complex, compounding fluid-dynamic of airflow that occurs when the volume+length of the intake tract perfectly matches and complements the open-closed-open-closed nature of a naturally aspirated engine's valvetrain). The fact that these two items are interrelated was also adding to the confusion of an already very complex situation.
The design of the airbox (and other intake system parameters) of a naturally aspirated engine seems to exist at the border between Engineering Black Magic and Vehicle-Owner Wishful/Magical Thinking. It's a zone where strange, under-informed, mis-informed thought processes are allowed (even encouraged) to run wild.
On one hand, we all appreciate a good sounding engine (and to equate good sound with good performance) so we're inclined to install what is advertised as being a "less restrictive" air filter. This less restrictive air filter probably allows more of the engine noise to escape from the air box which sounds good to the enthusiast's ear, but does this actually help or hurt the airflow resonance? Quite often we really don't know the answer to this question, so we fall back on the old "more has to be better" argument that if the new filter "flows more air more easily" then it has to be an improvement.