Please forgive if this has been addressed elsewhere AND for the exorbitant length. It seems important to me - BUT - in threads about K&N filters that I've seen so far, one rather important specific detail has not been adequately addressed. Namely, exactly how (and how much) these filters need to be cleaned and reoiled? I know that most come pre-oiled from K&N these days, but past experience has been that when serviced and re-oiled it is overdone...regularly. Too oily is likely as bad or worse than no oil...so what technique ensures correct oiling?
Then there's this—open filters of any kind are generally considered a bad thing on virtually all modern motorcycles—and not just for EPA noise. Ham cans pull air in sideways, in much the same way, so it's not a huge improvement per see. (There's some indication that using improper air filtration on the highly prized FCRs cuts their effectiveness considerably.)
That leaves airboxes (ala MkII-MkIII) as the best "technology" for cleanliness and performance...theoretically at least. My cursory look at the factory air box, and belief that they are a better idea, runs counter to Commando "popular wisdom". Most folks pull them off in favor of the notion they've improved performance by doing so. Does anyone have evidence/proof one way or the other? IF - the factory MkII-III airbox is restrictive, can it be modded to outflow open filters/ham cans? If not, can the MKII-III boxes be fitted to earlier Commandos? Pros? Cons?
Kevin Cameron's "take":
March 11993
Kevin Cameron
Airbox exoneration
TDC
Kevin Cameron
YEARS AGO, IT WAS CONSIDERED ESSENTIAL in any performance modification to rip off the airbox, filter(s) and intake-silencing gear, and then run either the bare carburetors or carbs adorned with “sock-type” individual filters.
There was some basis for this prejudice, in that certain models were truly strangled by their airbox systems. Yamaha’s famous RD two-stroke Twins were shining examples of what could be achieved by tossing away those plastic doghouses and rejetting to correct the mixture for all the extra air the engine was going to get.
In time, airbox removal not only achieved knee-jerk status, but indeed, open carbs came to be regarded as a manly accessory, just as well-rounded calves were viewed in the 18th century (back then, fellows with spindly shanks took to stuffing their socks to get the right effect). The definitive look in the 1980s was set by the Yoshimura GSX-R750 Superbikes, whose open Mikunis were shrouded only by sheets of insulating material, intended to keep some of the hot air out of the intakes.
Then came revelation. The production-based supersport roadracing classes arrived, and with them confusion. At first, everyone wanted to toss his airbox away in the established manner, but sophisticated competitors noticed that some horsepower went with them. In its place appeared disagreeable holes in the power curve. At first, people tried all sorts of deflectors, reasoning that air wasn’t getting to the right places. Years ago, as I had droned down the old Vineland, New Jersey, straightaway on my open-carb Honda Superhawk, I noticed that the engine revved up more if I held my knees out from the sides of the machine like a set of airscoops. But scoops and deflectors didn’t restore the missing supersports power. During that season of supersport racing when airbox removal was legal, the faster bikes all had boxes. Hmmm. Interesting.
More history. Twenty-five years ago, intake and exhaust tuning was regarded with suspicion by the official auto industry-something for hot-rodders and backyard experimenters, perhaps, but hardly a suitable subject for real engineers. Chrysler dabbled with
it (remember the giant CrossRam manifold, so big that it obscured the entire engine?) and textbooks of the period mentioned it-barely. Mercedes-Benz had taken it very seriously indeed, preparing an actively controlled, variable-length intake system for its 300 SLR racing engine back in the 1950s. But tuned-length intakes on the street? Certainly not.
Times changed when car engines had to shrink to meet emissions and fuel-economy standards. Engineered as their V-Eight forebears had been, the new four-cylinder breed would have made shrunken power, as well. To restore some of this performance loss, engineers brought their polished shoes down off their desks and began to look for ways to make small engines more fun to drive. By golly, this intake-tuning business might be just the ticket. Free horsepower!
The effect operated only across a limited range of rpm, but by choosing the right intake length, the effect (as much as a 20-percent local gain) could be put where it was most needed-for example, to fill in a hole left by a corked-up exhaust system, or as a vitamin for pallid peak power. Bike tuners, of course, had used intake length tuning routinely for years.
Now came synthesis. The sound of a tuned intake operating at its resonant frequency and full throttle is powerful-something like a big bass loudspeaker roaring away at a low, single frequency. It could give you a headache as you drone along in your tinny little car. It also exceeds noise standards set by the Federal and other governments. The noise had to go. Therefore, airbox research expanded.
A box with a hole in it, is itself a resonator (look under “Helmholtz resonator” in any acoustics book). The familiar example is to blow across the mouth of a bottle. Another example is a guitar. The vibrating mass is the quantity of air in and near the hole, and the “spring” against which it vibrates is the volume compressibility of the air inside the box. As acoustic engineers worked with various airbox combinations, trying to kill the noise, engine people were looking at the resulting power curves. What are all these new bumps? Why, isn’t this interesting? More free horsepower is coming from somewhere. Where?
What was happening was that now, added to the effect of intake-length tuning was a second effect: airbox resonant frequency tuning. As the volume of the airbox, and the length and diameter of its intake stub were varied, this resonance effect could be moved up or down the engine’s rpm scale. This was a delightful new tool for powerband shaping, giving the engineers a fresh way to fill in annoying powerband potholes that come from getting big power out of small engines. The more an engine depends upon exhaust-pipe tuning and valve overlap for its upper-end power, the more its midrange and bottom power will suffer from flat spots.
During the past five to eight years, therefore, the airbox has become an integral part of almost every street motorcycle’s powerband. It has evolved from being at first just a place for the filter and a means of silencing intake roar, into a source of modest but useful free horsepower, and of a smoother overall powerband.
At present, a reverse flow of intake technology is moving from street motorcycles into racing. Witness the giant, two-armed carbon-fiber airbox on the current NSR500 Honda. Soon, all important aspects of intake design-tuned length, cold air, ram effect, and airbox resonance-will be combined on race machines, as they have been for some time on the best street designs.
