i guess it might actually depend on the motor, i then would have to assume that the bigger the motor the more air that needs to be pulled in. My buddy has some race cars, dragters and things like that and he has told me that they never really like K&N because they had to clean the motor more than they should. They never were disappointed with any power decreases, cause in fact im sure it gave them more power. But the principle of it is that the bigger the motor the more air being sucked in. On a motorcycle, or ATV's, or dirtbikes, all of the smaller motor vehicles probably dont have big enough air intake abilities to suck in alot of dirt. The dirt comes with the air, thats common sense, but if something doesnt pull in as much air then obviously there is going to be less dirt. That might be why people with smaller motor vehicles dont notice a difference with K&N. All in all we may never know how these filters affects our motors, but i guess it all comes down to personal preference, and whether or not you actually believe its good or bad for your motor.