Why 5 valves?

[Guest mxrider426]

Does anyone know why Yamaha makes their high preformance motorcyles with 5 valves? Doesn't just as much come out of the cylinder as went in? I have never heard of any engine with more than 4.

[yamaha.dude]

The dirtbike design team at Yamaha obviously work just down the corridor from the roadbike engine guys... The five valve heads originated on the F1 superbikes... and they seemed to work well... beside, yamaha hold a patent or two on the 5 valve concept, so why use a technology that no one else can in your marketing efforts...

The inlet and exhaust ports are not the same size, and as they operate in different parts of the 4-stroke cyle, they do not need to be matched in size or flow rate...

Any internal combustion engine is basically an airpump, so the more efficient you can make it, the better off you will be... more ports and flow area mean more efficiency... Ever wonder why the Yamaha never had twin headers sprouting from the head, like a KTM? there is so little room between the valves, that the bit of metal that would separate the exhaust into two ports would be glowing red hot and probably be a failure point.

David

[Mark_UK]

its a 5 valve , becuase untill they come to there senses and bore the engine out to 750cc they cant fit 6 valves in ?

[Hick]

Originally posted by mxrider426:

Doesn't just as much come out of the cylinder as went in?

No.

Remember, E=MC^2.

[John_Lorenz]

Becouse 4 is not enough and 6 is too many I guess ?

Does it matter really as long as your winning races and the girls like it who cares

[ June 18, 2002: Message edited by: E.G.O.**** ]

[mike_dean]

You need more intake area than exhaust, because it is more difficult to suck air in than it is to blow air out. That is why intake valves are always larger than exhaust valves. Larger valves are heavier than smaller valves and engines won't rev as high with out causing damage, solution is to make 3 intake valves that have more area than 2 and are lighter, with less mass flying back and forth. You can also get better use of the cyl. head area with more and smaller valves. Mike

[ June 18, 2002: Message edited by: mike dean ]

[skoobeesnak]

Just as a side note. Ferrari, Audi and Volkswagen have all been impressed enough with the 5 valve arrangement to put in some of their production cars. I remember reading somewhere that it lets them get more valve area (on the intake side) vs. a 4 valve setup in a more compact area.

[Ed_Buratti]

Boy do I ever wish I could remember where I read this, but I remember reading that the flow area was actually the most efficient with 5 valves. To get the same flow with 4, you need bigger (heavier) valves, but if you go to 6 valves, then you lose some flow area because there are so many valves in the head that the packing efficiency of circles within a circle comes into play. It was a fantastic article, and I'll post a link if I can ever find it again.

[Hick]

Originally posted by mike dean:

You need more intake area than exhaust, because it is more difficult to suck air in than it is to blow air out.

I ain’t no engineer (nor a linguist), but I’m gonna have to disagree. The air doesn’t know, and shouldn’t care, whether it is being sucked or blown. ?

E=MC^2, so what is going out weighs less, and I assume takes up less space, than what is coming in. That probably isn’t the only thing that will affect what relative valve area sizes (exhaust vs. intake) should be (like temperature of the gas, which will be hotter on the way out), but it seems the most obvious so I’m thinking it is most important. That Einstein fella’s equation tells me that the amount of torque produced is proportional to the amount of mass consumed (mass is converted into energy when the spark lights the mixture).

Every motor I’ve ever seen had greater intake valve area than exhaust. Typical Chevy small block for example is 1.94” diameter intake valve and like 1.76” for exhaust.

[z4me]

Originally posted by Hick:

I ain’t no engineer (nor a linguist), but I’m gonna have to disagree. The air doesn’t know, and shouldn’t care, whether it is being sucked or blown. ?

E=MC^2, so what is going out weighs less, and I assume takes up less space, than what is coming in. That probably isn’t the only thing that will affect what relative valve area sizes (exhaust vs. intake) should be (like temperature of the gas, which will be hotter on the way out), but it seems the most obvious so I’m thinking it is most important.

There is a larger volume of gas flowing in the exhaust than in the intake.

The air/fuel coming in is driven purely by the mechanical suction caused by the piston lowering in the cylinder. The exhaust, on the other hand, is pressurized by both the return of the piston PLUS the expansion of the mixture during combustion (a heated gas expands.) The higher pressure "pushes" the the exhaust out through the ports more effeciently (Due to the higher pressure differential.)

It is the efficiency of the exhaust exiting that allows them to use larger intake ports than exhaust ports to ipmrove the overall flow through the engine.

E=MC^2 is a pretty broad equation to try to apply to this situation. Really the combustion is an oxidation reaction where air and fuel are the reactants and the spark is a catylist.

[Hick]

Originally posted by z4me:

E=MC^2 is a pretty broad equation to try to apply to this situation. Really the combustion is an oxidation reaction where air and fuel are the reactants and the spark is a catalyst.

Okey dokey.

But isn’t energy one of the products of the oxidation reaction?

[z4me]

Yes, but the conversion is not purely mass to energy as Einsteins equation was intended. In other words, after the reaction you still have plenty of mass. Carbon dioxide, carbon monoxide, N2O all are present after combustion (among other molecules) - equations for exothermic reactions are more appropriate.

[Hick]

Thanks for the chemistry lesson.

Or was that physics? I flunked physics twice, so you think I'd know...

[Guest mxrider426]

Ya, Im gonna go with ego's answer.

[big_G]

Cool, its thermodynamic geek wars!!!

You all have forgotten your basic chemistry I see... you lose both energy and heat in the combustion and, academically speaking, those two "items" don't leave thru the exhaust.

Further, even if the cross sectional flow area was equal with 3 intake and 2 exhaust, or 2 intake and 2 exhaust or even 1 intake and 2 exhaust, is pointless as an argument since equal area in and out does not necessarily make equal flow, or more power or better power or whatever. The same goes for reciprocating mass and the heavy/lightweight valve arguments. Just because there are differences does not make one better. However, for a fast revving engine style, lighter is usually always "better". If you don't think so, go slap an 8 oz flywheel on your bike and see what a half pound does to your powerband.

If you really want to get into this stuff, go lookup the Otto or Carnot cycles, bring some coffee, and get comfy with terms like adiabatic expansion and isobaric intake. Engine power is to this day one of the biggest sciences that still requires a bit of art. Here, I'll start you off for free...

See how a thumper really works... Otto cycle

Or the superior 2-stroke thermodynamics... Carnot cycle

You'll notice valve area missing on both graphs for a reason... ?

[z4me]

Hey - the geek cap is yours as far as I am concerned. I just knew we weren't splitting atoms here.

[MotoGreg]

I read a tech article on it from a Yamaha engineer years ago in road racing magazine. Yamaha only uses 5 valves on high performance bikes with large cc's per cylinder. The R1 (250cc per cyl) uses 5 valves but the R6 (150cc per cyl) uses 4 valves. On road racing bikes, the tuners who specialize in these motors always use a bigger than stock center intake valve and leave the other two intake valves stock size.

[Mark_UK]

ok in my vast engineering experience,,,(none)

the definitive reason yamaha has 5 valve,,,is they like what it makes the back wheel do ?

Just as a side note. Ferrari, Audi and Volkswagen have all been impressed enough with the 5 valve arrangement to put in some of their production cars. I remember reading somewhere that it lets them get more valve area (on the intake side) vs. a 4 valve setup in a more compact area.

i knew there was a reason i loved my Audi,,,,, hmmmmmm anyone know of a manufacturer who makes griper seat, tripple clamps, flywheel weights for Audi's?

[Mikeanic]

Any engine relies on atmospheric pressure to fill the cylinder . It is indeed a pump . Larger valve area is needed on the intake side to allow more air in . That is why at high altitude your engines power suffers , less atmospheric pressure. Superchargers & turbo's effectively increase that pressure . Seven psi boost would be like adding 1/2 an atmosphere . Three small intake valves is a way to increase valve area and keep them light to enhance high rpm performance . The exhaust has the benefit of expanding combustion gasses so smaller valves are needed . Of coarse there are many other variables that effect how an engine flows , but we could discuss this for hours . In general I think that's it .

[Boit]

In layman's terms, and not to get into much of an engineering or thermodynamic discussion, it might help to point out that the intake charge is relatively dense. Once combustion takes place, there is a need to exhaust this expansive gas quickly in order to get the next intake charge into the combustion chamber.

Actually, there is a computer software program that will pretty much dial in the engine's output once variables such as valve surface area, bore, stroke, compression ratio, fuel characteristics, are inputted. Honda experimented with oval bored cylinders a few years ago. The uniqueness to this design was that the valve area could be maximized. The machining costs were too high for them to go into production. I don't remember who tried it, but there was a triangular shaped valve cylinder head tested some time ago. Interesting stuff. With the steady advancements in CNC machining, expect some fantastic acheivements in 4-stroke dirt bikes in the near future.