Sekedar Sharing: "The Port and Polishing Myth" + Apa itu Three Angle Valve Job
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  1. #1
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    Sekedar Sharing: "The Port and Polishing Myth" + Apa itu Three Angle Valve Job

    It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed units where the actual size of the ports has become a restriction. Often the size of the port is reduced to increase power. A mirror finish of the port does not provide the increase that intuition would suggest. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow bench testing shows that the difference between a mirror finished intake port and a rough textured port is typically less than 1%. The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust ports may be smooth finished because of the dry gas flow and in the interest of minimizing exhaust by-product build-up. A 300 - 400 Grit finish followed by a light buff is generally accepted to be representative of a near optimal finish for exhaust gas ports.
    The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal wall and the air, the air speed is ZERO (see boundary layer and laminar flow). This is due to the wetting action of the air and indeed all fluids. The first layer of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past which develops a velocity profile (or gradient) across the duct. In order for surface roughness to impact flow appreciably, the high spots must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this.
    dapet dari internet..tolong ditranslate sendiri aja bagi yang ga ngerti..

    bagaimana tanggapannya dari para tuner?? apa bener polishing malah bisa menurunkan performa? ditunggu sharing nya..

  2. Delapan Satu
  3. #2
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    Often the size of the port is reduced to increase power. A mirror finish of the port does not provide the increase that intuition would suggest. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do.
    nah loh..gimana nih

    jadi abis porting ga perlu dipolishing dong sebenarnya? saking kinclong nya, malah bias merusak flow udara..gimana nih??

  4. #3
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    setau gw, makin di polish, range tenaga geser ke rpm atas.. knp? karena kalo tetep rough, aliran fluida rendah udah turbulen, bagus untuk ruang bakar, tapi kalo udah mulus kinclong, aliran turbulen baru terjadi ketika putaran mesin tinggi.. tapi ga tau juga deh.. hehehe..

  5. #4
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    disini malah dibilangnya dengan adanya gritty surface (gak di polish) itu, membantu meng increase flow dengan menciptakan bantalan udara di sekitar permukaannya, sehingga flownya makin lancar.

    logikanya seperti dimples di bola golf. dengan adanya dimples, ada bantalan udara yang ngebantu si bola golf meluncur lebih jauh..

    tapi ga tau juga sih.

    hehehe...

  6. #5
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    mampir...

    gimana nih para suhu2 jago mesin..?

    mohon dibantu...

  7. #6
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    yang gw tau yang ga boleh di polish itu mobil yang masih karbu
    klo injeksi sebaiknya di polish n di buat selicin mungkin permukaan port-nya

  8. #7
    Kapten
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    yang bagus yang udara nya jadi muter kan,, turbulen ya namanya?
    Black Espero '97

  9. #8
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    Smile

    mirror finish diperlukan pada combustion chamber, supaya tidak mudah terjadi timbunan kerak disana.. kl di exhaust port cukup polish aja, supaya tidak ada turbulensi yang bisa menghambat alur kluar dari gas buang.. emang ga perlu jg sampe jadi kayak kaca licinnya kl disini..
    kl di intake port sebaiknya dibiarkan kasar, supaya tetap ada turbulensi yang bisa ngebantu menghomogenkan air-fuel mixture..
    besar ukuran in-ex port disesuaikan dengan kapasitas dan kompresi dari mesin..

    jangan sampe salah, kl salah yang ada jadi boyo mobil bukannya kenceng.. makanya pastikan PnP dilakukan oleh orang yang bener2 berkompeten supaya hasilnya juga tidak mengecewakan..

    kl perumpamaan dengan bola golf kayaknya kurang tepat kl diterapkan pada in-ex port..
    pada bola golf, pimples mengakibatkan turbulensi shg timbul daerah bertekanan rendah pada permukaan bola tersebut.sehingga mengurang gesekan udara pada permukaan bola, jadi berkurang juga tahanan udara terhadap bola tersebut..
    kl pada in-ex port, daerah yang bertekanan rendah yang terjadi pada permukaan dalam dari port malah mengakibatkan pengecilan jalur lewat udara dr port tsb.. sehingga malah menjadi hambatan dalam port tsb..
    air speednya jadi lebih kenceng tapi air volumenya jadi berkurang..

    air speed berfungsi untuk meng-homogenkan air-fuel mixture, sedangkan air volume menentukan banyaknya air-fuel mixture yang masuk ke dalam combustion chamber.. dua2nya berhubungan dan saling mendukung..
    semakin besar ukuran jalur lewat udara, air speednya makin kecil dan air volumenya makin besar dan sebaliknya.. kuncinya gimana bisa menemukan kombinasi yang paling pas..



    yang gw tulis diatas tu pemahaman yang gw dapet selama ini, semuanya IMHO lho.. kalo ada kesalahan mohon dikoreksi..

    thx..

  10. #9
    Quote Originally Posted by gedeman View Post
    nah loh..gimana nih

    jadi abis porting ga perlu dipolishing dong sebenarnya? saking kinclong nya, malah bias merusak flow udara..gimana nih??
    Halo,

    bagus nih thread...kita coba sharing ya
    Pengalaman dari kita kerja dengan flowbench ..beda antara diporting saja dan dipolish sampai licin seperti kaca, hanya beda maks 1% di flow karakternya..:o.Malah kalau kita sudah porting optimal dan terukur dengan baik..sebaiknya jangan coba2 di rubah apapun ..karena kuatir bukan memaksimalkan, tapi malah menaikkan resiko menggangu port desainnya, yang ujung ujungnya nya menggangu velocity - cfm - dll.

    terima kasih

  11. #10
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    @ gardenspeed..

    Iya om, dari beberapa sumber yang saya baca, mereka bilang dalam pekerjaan porting itu yg paling penting 99% adalah ukuran..sisanya polish hanya menyumbang 1% kepada hasil akhirnya.

    nah berarti selama ini PNP banyak yang salah kaprah dong dengan mempolish selicin mungkin.

 

 
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