number of valves

[240racer]

Most of us on this forum will agree that the 4-valve head on the DOHC flows better then the 3-valve head on the SOHC. Also, I think all of us would agree that both of them flow better then a 2-valve head with pushrods and both valves on the same side. What I want to know is, will more valves flow better? Would it benefit an engine to have 6 valves for instance?

[Jeff240sx]

I don't have any numbers to back this up, but some common sense.
A 2 valve head doesn't utilize the total surface of the combustion chamber. A 3 valve head utilizes more of the combustion chamber, and therefore allows more intake air in (2 valves, 1 exhaust?).
A 4 valve chamber uses nearly all of the combustion chamber area, and flows well. Using 6 valves I don't think you would gain power, because the valves would have to be smaller. If the head deviated from the standard round-valve, perhaps you could make more use of the available surface area of the combustion chamber.
-Jeff

[orange-grey]

Yeah, in theory. Thats why Ferrari and VW use 5-valve heads. Problems crop up with valvetrian friction and mass at high rpm though, from what I hear. The fact that cars like the S2000 still use 4 valves should tell you that proper design is more more important than gimmicks.

[mrdirty]

I once asked a professor I had who was a designer for Mercedes this; his answer:

-More valves mean that each one can be smaller
-Smaller valves don't have to be extended as far to get proper flow therefore smaller movements of the valvetrain and potentially higher rpm.

[d240t2]

That is where you get into the law of diminishing returns...as you put more and more valves on there...you can potentially have more valve opening. If you can control the valves independently, then you can only utilize some for low RPM to improve air velocity and get a mixing/swirling effect in the combustion chamber.

If it gives you a hint, no manufacturers that I know of use more than 5 valves/cylinder...most of the really good engines are all 4 valves. If done right...you can get more out of 5 valves...but that doesn't mean that a crappy 5valve head will be better than a good 4 valve head.

Dennis

[mrdirty]

Originally posted by d240t2
That is where you get into the law of diminishing returns...

I think it has more to do w/ running out of room in the head and a compromise between higher flow cross-section and higher turbulence...