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Old 15th July 2010, 12:55   #1
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Default 8 Valve SOHC vs 16 Valve SOHC

I have seen some manufacturers giving the number of valves in comparison charts in their adds.

Both being SOHC, I really doubt the need for more valves per cylinder.
I have been told that, in a SOHC configuration, more valves per cylinder = more NVH & wear tear.

Expecting your technical comments, the PROS and CONS of 8v vs 16v SOHC.

Thanks
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Old 15th July 2010, 14:47   #2
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Extract from SOHC vs DOHC Valvetrains: A Comparison


Quote:
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. (Note: Iíve added this graph here to show you the difference between 2-valve and 4-valves, not SOHC vs DOHC. Couldnít find any graphs for SOHC vs DOHC. Sorry!)
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.
I hope this clarifies your doubt
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Old 8th February 2016, 17:54   #3
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Default Re: 8 Valve SOHC vs 16 Valve SOHC

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Originally Posted by jaguar.runs View Post
Extract from SOHC vs DOHC Valvetrains: A Comparison
I hope this clarifies your doubt
Pretty clear. Some queries though...
  1. Why are some manufactures in India like Toyota (Etios), Nissan (Micra) and the more current Ford (Aspire) using the 8-valve technology? Is that from a cost-cutting perspective OR is it that they have these engines that are good and do not want to invest in upgrading to 16-valve technology?

  2. Is this an easier way to achieve better mileage?

  3. Is this difference of valves applicable to petrol cars too?
Thanks,
C_
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Old 10th February 2016, 08:47   #4
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Default Re: 8 Valve SOHC vs 16 Valve SOHC

1. Accountants rule all sectors of the automobile industry. Instead of spending millions of dollars on investing in new technology, cash strapped companies go on with old engine designs.

2. Mileage is a VERY subjective term at the best of times. Mileage, as far as valves are concerned refers to pumping losses when the piston goes down the cylinder at the intake stroke. The resistance the incoming air faces when travelling to the cylinder, is proportional to the power output at a particular rpm. At low rpm's the difference between DOHC & SOHC aren't too much. It's at higher rpms that the difference is noticeable. Less pumping losses at higher rpm's means more fuel efficiency.

3. Yes, all four stroke engines benefit.
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Old 10th February 2016, 12:02   #5
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Default Re: 8 Valve SOHC vs 16 Valve SOHC

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Originally Posted by Coolman View Post
Pretty clear. Some queries though...
  1. Why are some manufactures in India like Toyota (Etios), Nissan (Micra) and the more current Ford (Aspire) using the 8-valve technology? Is that from a cost-cutting perspective OR is it that they have these engines that are good and do not want to invest in upgrading to 16-valve technology?

  2. Is this an easier way to achieve better mileage?

  3. Is this difference of valves applicable to petrol cars too?
Thanks,
C_
Hello Coolman,

1. Definitely, cost is a major factor. Another factor is the requirement. I believe that you are referring to the diesel engines for the three models you have mentioned, which do not always require 4 valves per cylinder as multi valve technology mostly benefits engines which usually reaches a higher RPM for maximum power output.

2. More valves means better flow during intake and exhaust cycle, resulting in better combustion and thereby increasing the efficiency (better power and fuel efficiency).

3. Multi valve engines are more popular in petrol engines due to the higher RPM required for the maximum power output. Newer generation diesel engines are also switching to multi valve technology for improved efficiency and better emissions.

I hope this clarifies your doubt.

@1self: Thanks for pitching in!
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Old 12th February 2016, 22:06   #6
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Default Re: 8 Valve SOHC vs 16 Valve SOHC

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Originally Posted by 1self View Post
The resistance the incoming air faces when travelling to the cylinder, is proportional to the power output at a particular rpm.
Don't follow. Explanation would be helpful.


Quote:
At low rpm's the difference between DOHC & SOHC aren't too much. It's at higher rpms that the difference is noticeable.
Cams or valves?

Regards
Sutripta
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Old 12th February 2016, 22:23   #7
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Default Re: 8 Valve SOHC vs 16 Valve SOHC

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Originally Posted by shineshine View Post
I have seen some manufacturers giving the number of valves in comparison charts in their adds.

Both being SOHC, I really doubt the need for more valves per cylinder.
I have been told that, in a SOHC configuration, more valves per cylinder = more NVH & wear tear.

Expecting your technical comments, the PROS and CONS of 8v vs 16v SOHC.

Thanks
SOHC stands for single overhead cam while DOHC stands for double overhead cam. In both the cases, the cam shaft is in the cylinder head. In SOHC, the cam shaft will have both the intake and exhaust lobes that will be acted upon by rocker arm while the opposite ends of the rocker arm acts upon the valves, intake and exhaust.
In DOHC, we have 2 cam shafts, one for the intake valve and the other for the exhaust valve.
Generally it is desirable to keep the valve train inertia as low as possible. In DOHC since we have 2 cam shafts (more mass), valve train friction will be more here. If more engine power is desired, then DOHC will be preferred to SOHC.
Now a days manufacturers talk of 16V ie 4 valve / cylinder for a 4 cylinder engine. This implies 2 intake valves and 2 exhaust valves for a cylinder. Do note intake valves are usually bigger in size compared to exhaust valves. Bigger the size of intake valves, better will be the engine breathing resulting in better volumetric efficiency.
The spark plug will be centrally mounted. Going for more number of valves does not necessarily mean more mileage. Providing more valves complicates the packaging.
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