I have been thinking about the design characteristics of an airbox and am hoping others may have input as to why airboxes are designed the way they are and what performance advantage one may have over another. I currently run a stock moto shifter with a Keihin PWM 38mm carb with a red Freeline airbox with two 30mm tubes. I have heard rumblings back and forth as to whether the airbox I have is better than a black RLV airbox that can be legally ran with three 29mm tubes. Pretty much everyone seems to agree that the airbox is more restrictive that the air filters that are allowed at many tracks; however, I’m not sure I buy this?
First off let me explain my knowledge (or error) of how I think the carb works. When the carb slide opens up a vacuum is created which sucks air in through it’s opening at the same time the vacuum sucks fuel up through the jets.
In terms of performance the cross sectional area of the 38mm carb is going to be less than either the red Freeline or the black RLV because of the number of tubes each one has. This leads me to think that no matter what the conditions, the carb is the restrictor when it comes to how much air can be sucked into it (imagine a straw with a small end and a large end, no matter how hard you suck from the small end their will be an ample supply from the large end). So what is the benefit of having more tubes such as is the case with the black RLV with the three tubes? If anything, I believe the air filters are potentially more restrictive because the airflow is not something that is known and as the air filter starts to get dirty the flow becomes even more restricted.
I have heard rumblings that the volume of the airbox plays a role in the performance but I simply don’t understand why because the air openings into the airbox are larger than the opening into the carb so no matter how much air the carb is gasping for, the airbox can supply?
Google cylinder wall air drag (intake tubes to airbox vs. carb venture cylinder drag). Interesting in that intake area is not all there is to air passage into an airbox.
Google intake airbox design (size and shape effect a<span style=”font-family: Calibri;”><span style=”color: #000000;”>coustic </span></span><span style=”color: #000000; font-family: Times New Roman;”>pre</span>ssure timing affects).
Karting air filters have incredibly large filtering surface area. No other asphalt-based motorsport that I’m aware of has such large filters. If maintained, are they necessary? I’d like to see real comparable data on this. I’ve tested with large and small filters on George Banke’s dynamometer (back to back in 2001 on modified CR 125) and found no difference.
Great info out there. Focus on reading technical information, not opinion, hearsay and grapevine views.
As is often the case YMMV.
It’s not just about area, it also has to do with the frequency and strength of pulsations. That’s why an airbox makes it quieter, it breaks up the main pulse wave.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience
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