fig1 is a left - side view of a motorcycle 1 illustrating a seat section 2 and an intake pipe 4 , and fig2 is a perspective view of the seat section 2 removed from the motorcycle 1 showing an air filter case 3 mounted underneath the seat section 2 of the motorcycle 1 . in the front section of the air filter case 3 , a conical frustum shaped air filter 10 , which is described hereinafter , is installed with its upper flat surface facing frontward . it should be noted that other shapes for the air filter can be used . in addition , the air filter may be located in positions other than below the seat . the intake pipe 4 is connected to the flat surface section . the rear - end section of the air filter case 3 has an opening ( not shown ). the airflows through this opening , and the air filter 10 removes dirt and sand from the air . the air is then sent to the carburetor ( not shown ) via the intake pipe 4 . fig3 is a cross - sectional view of the first embodiment of this invention pertaining to the air filter of the present invention . the air filter 10 is illustrated as shaped as a conical frustum . the interior section has a cavity opening 14 , which faces an upper flat surface 11 . the air filter 10 is made of a foam material such as foam urethane . in this embodiment , the air filter 10 is structured as two layers such that a foam material layer 16 covers a side surface section 13 and a bottom surface section 12 of a foam material 15 that has the center cavity section 14 . additionally , the side surface section 13 is structured as three layers , where a foam material layer 17 covers the outer surface of the foam material layer 16 . additionally , the foam material layer 15 , the foam material 16 and the foam material layer 17 are made of the same basic material and of the same thickness . however , as compared to the foam material layer 15 , the foam material 16 and the foam material 17 have a coarse foaming ratio . foaming ratio as herein defined relates to the size , or diameter , of the openings in each foam layer . fig4 shows the airflow within the air filter 10 . in this embodiment of the invention , the air filter 10 is structured in three layers at the side surface 13 and has a larger airflow resistance as compared to the bottom surface 12 section , where it is structured as two layers . as a result , the airflow “ s ” into the air filter 10 is only from the bottom surface 12 , not from the side surface 13 , thereby resulting in smooth , straight airflow and an increased volume of intake airflow . when the volume of intake airflow increases , the volume of fuel injection is also increased at the same ratio in order to maintain combustion efficiency . this combined increase in air intake volume and fuel injection volume produces a relative increase in engine output power . fig5 is a cross - sectional view of a second embodiment of the air filter of the present invention . in this embodiment , an air filter 20 has a side surface 23 in combination of a first layer 25 and a second layer 26 that covers the outer surface of the first side layer 25 , a bottom surface 22 in combination of the first layer 25 and the second layer 26 , and a center cavity opening 24 . the air filter 20 is illustrated as shaped as a conical frustum . the air filter 20 , in this embodiment , is shaped in conical frustum that is thicker at the side surface section 23 as compared to the thickness at the bottom surface 22 section . therefore , the airflow resistance at the side surface 23 is greater than it is at the bottom surface 22 . the result is a smooth , straight flow of air and a relative increase in the volume of intake airflow . additionally , the foam material layer 25 and the foam material layer 26 are made of the same basic material . however , as compared to the foam material 25 , the foam material layer 26 has a coarse foaming ratio . fig6 is a cross - sectional view of a third embodiment of the air filter of the present invention . in this embodiment , an air filter 30 has a side surface 33 in combination of a first layer 35 and a second layer 36 that covers the outer surface of the first side layer 35 , a bottom surface 32 in combination of the first layer 35 and a bottom second layer 37 , and a center cavity opening 34 . the air filter 30 is illustrated as shaped as a conical frustum . the air filter 30 has the second layer 36 , which covers the first side layer 35 , which in turn shapes the center cavity opening 34 of the conical frustum , has a finer foaming ratio than the bottom second layer 37 that covers the bottom surface 32 section of the first side layer 35 . therefore , the airflow resistance at the side surface 33 is greater than the airflow resistance at the bottom surface 32 . the result is a smooth , straight flow of air and a relative increase in the volume of intake airflow . additionally , foam material layer 35 , foam material layer 36 and foam material 37 have the same thickness . fig1 is a cross - sectional view of a fourth embodiment of the present invention . in particular , an air filter 70 has an outer layer 74 covering an inner layer 72 and a bottom flat surface 76 . the air filter 70 is illustrated as shaped as a conical frustum . a circumferential plate 78 , preferably made of a plastic material , is positioned at the outer perimeter external wall of outer layer 74 and functions to minimize the airflow entering into the air filter through outer layer 74 while maximizing the airflow through the bottom surface section 80 . fig1 is a cross - sectional view of a fifth embodiment of the present invention . in this embodiment , an air filter 40 has a side surface 41 , a bottom surface 42 including a first bottom layer 43 and a second bottom layer 44 , and a center cavity opening 45 . the air filter 40 is illustrated as shaped as a conical frustum . the side surface 41 and the bottom layer 42 are made of porous material , where the side surface 41 has a finer porous ratio than the bottom surface 42 . the second bottom layer 44 has a finer porous ratio than the first bottom layer 43 . therefore , the airflow resistance at the side surface 41 is greater than the airflow resistance at the bottom surface 42 . the result is a smooth , straight flow of air and a relative increase in the volume of intake airflow . thickness of the side surface 41 and the bottom surface 42 may be modified as necessary as long as the airflow resistance at the side surface 41 is greater than the airflow resistance at the bottom surface 42 . fig1 is a cross - sectional view of a sixth embodiment of the present invention . an air filter 60 has a side surface 61 including a first side layer 62 and a second side layer 63 , an bottom surface 64 including a first bottom layer 65 and a second bottom layer 66 , and a center cavity opening 67 . in this embodiment , the shape of the air filter 60 is reverse - conical frustum relative to the previous embodiments . the side surface 61 and the bottom layer 64 are made of porous material , where the first side layer 62 and the first bottom layer 65 have a finer porous ratio than the second side layer 63 and the second bottom layer 66 . the side surface 61 is thicker than the bottom surface 64 , and each layer can be designed so that the airflow resistance at the side surface 61 is greater than the airflow resistance at the bottom surface 64 . the result is a smooth , straight flow of air and a relative increase in the volume of intake airflow . thickness of the side surface 61 and the bottom surface 64 may be modified as necessary as long as the airflow resistance at the side surface 61 is greater than the airflow resistance at the bottom surface 64 . even though the number of layers , the thickness and the foaming ratio of the foam materials are varied independently in order to modify the airflow resistance in the embodiments noted hereinabove , in actual implementation any combination of these factors can be utilized . in the embodiments described above , the airflow resistance through the side surface is increased relative to the bottom surface . however , it is also acceptable to increase the airflow resistance through the bottom surface as compared to the side surface . the point to note is that to create a smooth , straight flow of air within the air filter , different thicknesses at the bottom and side surfaces are provided . while the invention has been described with reference to its preferred embodiments , it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention . in addition , many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings .