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[Engine & Performance] SOHC and DOHC revisited:

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  • [Engine & Performance] SOHC and DOHC revisited:

    Kita bila cakap pasal overhead cam je mesti org kata Double semestinya lagi baik dari Single 100% tanpa mengambil kira apa yg berlaku disebalik valve, tarpet, cam, timing belt/chain bla bla bla...

    nah pasal SOHC vs DOHC - setiap satu ada kelebihan

    SOHC vs DOHC Valvetrains

    posted on 22/6/2005 by Paul Tan in Cars, Howtos

    Plenty of people have been talking about how the Savvy only has an SOHC engine so it is inferior, and old technology. So I decided to post this entry so that people will be better informed on what SOHC or DOHC really is.

    SOHC and DOHC are basically two different ways to configure the valvetrain. In the whole valve train setup there are camshafts, lifters, rockers, valves and springs. It is best that the valvetrain is light. Valves open at intervals of 25 times a second at 3000rpm. If the valvetrain is heavy, when the camshaft lifts it the valvetrain will just keep going up until the valve spring catches it. This is called valve float. You can overcome this with stronger (which means heavier) springs, but this extra weight increases the power requirements to open the valve.

    The cams are driven by the crankshaft, using either a belt or chain called a timing belt or timing chain. If the timing belt/chain snaps and the camshaft stops spinning, the piston coming back up the combustion chamber might hit the open valves. This is very costly. Thatís why you should always change your timing belt/chain at the interval specified by your car manufacturer.

    SOHC refers to Single Overhead Camshaft. In the days before DOHC, it was known as OHC, with no need to differentiate between a single or double camshaft. In SOHC, the camshaft is situated in the cylinder head, above the valves. The valves are opened and closed either directly with a shim between the cam lobe and the valve stem, or via a rocker arm. SOHC engine valve configurations typically have 2 or 3 valves per cylinder. It is also possible to have 4 valves per cylinder using SOHC but this translates into a complicated combination of rocker arms and cam lobe shapes. An example would be the 4G92P in my car which is a SOHC but has 4 valves per cylinder.

    DOHC refers to Double Overhead Camshaft. This arrangement uses two camshafts in each cylinder head. Two cams per cylinder head means that a DOHC V engine has 4 camshafts because it has 2 banks of cylinder heads. This allows the manufacturer to easily implement a 4 valve per cylinder setup. Most of the time it also allows the engine to rev higher. It also allows better placement of the valves in an optimized setup that gives you maximum performance. But the disadvantage of such a setup is more weight, more cost and more complexity. It takes more stuff to drive two camshafts. The main reason to use DOHC is to drive more valves per cylinder. If a SOHC setup can allow 4 valves per cylinder, having a DOHC engine will not bring that much benefits over SOHC and the additional weight becomes a burden instead. DOHC engines also allows the spark plug to be placed right in the middle of the combustion chamber. This promotes efficient combustion. With SOHC, the camshaft is usually in the middle of the head because it has to drive both the intake and exhaust valves, robbing the sparkplug of itís optimal location.

    In the end, a SOHC 16 valve engine would have better torque on the low end where the DOHC valvetrainís weight results in lower torque. But at high engine speeds, the 16 valve DOHC engineís peak torque and horsepower would be greater. Thatís the trade-off. With the amount of valves being equal, SOHC has better low-end torque because the valvetrain package is lighter while DOHC has better top-end power.

    Other benefits of DOHC would be making it easier to implement variable valve timing technologies (which I will cover in another blog post) and also you can tweak it better with adjustable cam pulleys. If you were to put high-profile cams in your DOHC engine, the cam lobe profile can also be more optimized than a SOHC engine because you can play around with the lobe shape easier with separate camshafts for the intake and exhaust valves.

    Why more valves per cylinder? Why not just make 1 huge intake valve and 1 huge exhaust valve? Bigger valves weigh more than the smaller one, so controlling the extra weight as it gets flung open and close becomes difficult. The spring has to be stiffer. A stiffer spring means more energy has to be spent overcoming the valve pressure. This partially oversets the gains which a bigger valve has to offer. Another problem with a single big valve is at lower RPMs the intake velocity will be lower. Iím sure you guys know thisÖ the same amount of air going through a big pipe will have lower pressure than the same air going through a smaller opening. Think of how you can control the water pressure of your garden hose by adjusting the opening size with your finger. Because of the velocity drop, low RPM torque and driveability will suffer. Although two smaller valves weigh the same as 1 big valve, and with the extra rocker arms and springs they can actually end up weighing more, this is offset by less mass to be overcome when opening and closing the valves.

    So, are more valves per cylinder really that beneficial? Hereís comparing a Nissan VG30E to a VG30DE. Both are 3 liter electronic fuel injection engines, but one is a SOHC with 2 valves per cylinder and the other is DOHC with 4 valves per cylinder.




    We can see that torque at low RPMs are about the same for both engines, but at higher speeds the 2-valve per cylinder engine has reached itís peak and has to switch to the next gear while the twincam 4 valve per cylinder engine continues making more power at the top-end of the powerband. The powerband is also longer.

    So to sum it all up, SOHC has better low-end power, DOHC has better high-end power and overall maximum power. 4 valves per cylinder is much better than 2 valves per cylinder and it doesnít matter whether 4-valves is achieved via SOHC or DOHC.
    sumber paultan blog : http://paultan.org/archives/2005/06/...c-valvetrains/

  • #2
    Re: SOHC and DOHC revisited:

    orang secara automatik anggap yg DOHC superior sebab kalau DOHC konpem lagi banyak valve... dan secara am more valves = more power...

    tapi pada pendapat aku lah... beza DOHC ngan SOHC ni cuma ketara kat trek. Kalau tak main racing (atau rempit), rasanya kalau orang tak perasan pun...

    Comment


    • #3
      Re: SOHC and DOHC revisited:

      Originally posted by FunctionX
      tapi pada pendapat aku lah... beza DOHC ngan SOHC ni cuma ketara kat trek. Kalau tak main racing (atau rempit), rasanya kalau orang tak perasan pun...
      mengikut penerangan paul tan, akoo rasa tak juga. DOHC di luar trek boleh ada teknologi VVT yg boleh membantu utk menambah power dan optimize fuel consumption

      Comment


      • #4
        Re: SOHC and DOHC revisited:

        orait...mmg betul...kurang vavle kurang beban...
        tp kena ingat benda nie bagus utk tork shj...top aku rase menjerit vavle tu...

        PROS

        koo bayangkan camnie jerk ek...jumlah minyak masuk same cuma bebanan kerja berlainan...

        SOHC lg berat...sbb byk minyak masuk tp...valve kena bekerja keras, kena byk kali buka...RPM tinggi n so on...

        DOHC ringan nak nye lg byk valve...valve rilek jooo...buka skit jerk, sbb ade valve lain leh bantu...RPM mase kat higway, rilek jo...cruising...

        CONS

        SOHC valve besar, korek lg...mungkin pickup utk beberape ratus eter jee laaa leh pegang...pastu tak penin pale pomen nak grand valve..

        Comment


        • #5
          Re: SOHC and DOHC revisited:

          ader lagik nozie...

          secara logiknyer... kalo material yg buat head SOHC ngan DOHC same... campur ngan kompleksiti komponen DOHC i.e. 2 cam, valve lagik banyak... logiknyer DOHC lagik berat darik SOHC ....

          Comment


          • #6
            Re: SOHC and DOHC revisited:

            Mercedes SLR McLaren masih guna V8 SOHC

            DOHC
            Nih sesuai untuk orang banyak wang, sekali tukar hot cam, tukar 2 batang terus ( twin cam ).

            SOHC
            Nih sesuai untuk orang yg budget dan suka buat research sebab connecting parts yg kurang, so senang nak paham konfigurasi engine.

            Kereta yang guna 20Valve 4 cylinder ( 5valve per cylinder )
            Toyota AE111 20Valve
            Volvo 850 20valve
            Audi A4 20Valve

            Comment


            • #7
              Re: SOHC and DOHC revisited:

              Originally posted by badakcipan
              ader lagik nozie...

              secara logiknyer... kalo material yg buat head SOHC ngan DOHC same... campur ngan kompleksiti komponen DOHC i.e. 2 cam, valve lagik banyak... logiknyer DOHC lagik berat darik SOHC ....
              tp dia dibantu dgn dua pulley...tu pasal DOHC byk dipasang pada CC lebih 1.6...kaloo 1.5 mungkin berat skit n tak larat dia nk bawa...

              Comment


              • #8
                Re: SOHC and DOHC revisited:

                In the end, a SOHC 16 valve engine would have better torque on the low end where the DOHC valvetrainís weight results in lower torque. But at high engine speeds, the 16 valve DOHC engineís peak torque and horsepower would be greater. Thatís the trade-off. With the amount of valves being equal, SOHC has better low-end torque because the valvetrain package is lighter while DOHC has better top-end power.
                Better low end torque ni bukan bagus ke utk jalan yg banyak kornar2?

                Comment


                • #9
                  Re: SOHC and DOHC revisited:

                  yup...
                  tu pasal...V6 turbo...tak pick-up...tp top power...tp tu boleh bikin lg

                  Comment


                  • #10
                    Re: SOHC and DOHC revisited:

                    Originally posted by gogsard
                    mengikut penerangan paul tan, akoo rasa tak juga. DOHC di luar trek boleh ada teknologi VVT yg boleh membantu utk menambah power dan optimize fuel consumption
                    Keyword "ketara".

                    Comment


                    • #11
                      Re: SOHC and DOHC revisited:

                      Paul Tan nih penulis upahan ker? Hehehe. :P

                      Setau aku yg tak berapa tahu nih... DOHC ni lebih baik dari SOHC. Tu jerk... detail nya baru aku tahu hari nih. Tapi explanation dr Chinozie tu bagus. Aku rasa ader raleven nya.

                      Comment


                      • #12
                        Re: SOHC and DOHC revisited:

                        bagi aku plk DOCH lebih bagus dari SOCH.sbb sperti di atas.dia ringan.beban enjin kurang sikit apa bila kite tekan minyak.

                        Comment


                        • #13
                          Re: SOHC and DOHC revisited:

                          nak paham lg jom kita baca ni: http://auto.howstuffworks.com/camshaft.htm ada gamba dgn animasi lg...

                          wassalam.

                          Comment


                          • #14
                            Re: SOHC and DOHC revisited:

                            Originally posted by FunctionX
                            Keyword "ketara".
                            ?? what im saying that .. akoo tak setuju jika perbezaan "ketara" hanya ditrek. perbezaan "ketara" juga boleh nampak pada normal driving.

                            Comment


                            • #15
                              Re: SOHC and DOHC revisited:

                              yg korang susah sgt nak pikir wat pee...tah tah paul tan...tan tan sri...mendaaa aaa...ntah org upahan proton...mane laa tau...dia nk upkan savvy...internet ini kena berhati2 tak semua fakta tu betul...kena pandai laaa pikir...jgn percaya pada satu sumber...cari sumber yg lain gak...baru leh clarify...

                              Comment

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