Why 5 Valves?

23 replies to this topic
  • Thumper33

Posted September 22, 2001 - 07:55 AM


I had someone ask me why 5 valves is better than 4. I answered by saying that it gives you more room for the gasses to get in (or out) of the combustion chamber. I didn't know if the 5th valve is intake or exhaust, but he had a good point. What's the point of having more intake than exhaust valves or vise/versa. Seems like one would be a bottle neck and would make the other extra valve not worth it.

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  • Hick

Posted September 22, 2001 - 08:19 AM


E=MC^2 ??

What goes out ain't as big as it was when it came in. I'd bet most engines have quite a bit more intake valve area than exhaust, regardless of the total # of valves. The Yamaha Genesis design is no different, it has 3 intake valves and 2 exhaust.

As for 5 being better than 4, if you have ever looked at the combustion chamber on this motor you can see that it would be very difficult to fit as much total valve area in that limited space with fewer but larger valves.

That is why this motor revs so well, they did a good job of creating a lot of valve area so it could flow 11,500 rpms worth of air. Nobody has been able (or willing) to match that yet in a dirt bike. As good as the new CRF may be, it is a 4 valve motor and it wouldn’t surprise me if the YZF still makes more HP per liter.

[This message has been edited by Hick (edited September 22, 2001).]

  • trailguy

Posted September 23, 2001 - 04:29 AM


Try to think of it this way. Air going in is being forced in by atmospheric pressure, due to the piston going down and creating vaccum if you will. Air going out is going to be pumped out by the piston coming up.Duh. The intake side is larger to give a better airflow into the combustion chamber, therefore, better power. That is why a blower or turbo increase power so much. They increase the flow of the air/fuel mixture into the engine. Hope this helps.

  • DOC

Posted September 23, 2001 - 11:27 AM


Here is my totally uneducated answer. Obviously a five valve head will flow better than a four valve head, but why are there more intake valves?
The way i see it is, what comes out isn't as big as what comes in, like Hick said. Once the air fuel mixture is drawn into the combustion chamber, it is compressed and ignited. This releases engergy in the air fuel mix and creates force on the piston, heat, light and sound. Once the combustion process has completed, you are left with what you started with, only without the energy that it once had. So, the air fuel mix, minus the energy equals waste (exhaust gas) and therefore there must be less than what was originally drawn into the motor. Guess that also explains why the intake side of the motor (carburettor) is so much larger than the exhaust side.

  • DOC

Posted September 23, 2001 - 11:38 AM


just on the issue of forced induction, i would like to add a bit onto trailguys comment.
A naturally aspirated engine can only draw in a certain amount of air from the atmosphere. This means, it is only limited to a certain amount of oxygen.
A forced induction will physically draw air from the atmosphere and ram it into the motor. If it rams air into the motor, then it will also ram more oxygen into there too. By giving this air more fuel, you now have a air fuel mix that is rich in fuel and oxygen. Bigger bang. Turn up the boost, and you increase the power again for the same reason.
This is a general description because the more boost a turbo charger makes, the more heat is generated. This heats the air and makes it less dense (less dense means less oxygen, thats why hot air rises), but if we cool the air after it leaves the turbo with an intercooler, we now have dense, compressed air rich in oxygen and ready for the engine.

  • Taffy

Posted September 23, 2001 - 12:32 PM


can you tell me more about this hot air please?


  • DOC

Posted September 23, 2001 - 11:30 PM


I really hope someone else can add to this answer, as i don't know all about this particular phenomenon.
Lets say a T3 turbo on a 6 cylinder car is running 6 pounds boost. The air leaving the turbo is about 50 degrees C. Turn the boost up to 15 pounds, and expect to see the air temperature leaving the turbo reach around 80 to 90 degrees C.
There are a few reasons for this, but the main reason i believe, is that there is more exhaust gas passing through the exhaust turbine on the turbo, instead of passing through the wastegate (as it does on lower boost). This creates a build up of exhaust gas pressure around this turbine, and therefore more heat is being transmitted to the body of the turbo itself. The air leaving the turbo and heading towards the engine gets heated by the body of the turbo charger. Its not uncommon to see a turbo glowing red when being run on big boost for long periods.
Now this is just a thought, and i guess it could be proven by comparing small turbos with larger ones. A T4 turbo (bigger) won't heat the flowing air quite as bad as a smaller turbo on big boost. Firstly, most large turbos run external wastegates which doesn't cause turbulence in the exhaust housing (area where exhaust turbine lives) when the wastgate is open. Internal wastegates operate next to the exhaust housing, so when they are open (to control boost) they interfere with the exhaust gas leaving the exhaust turbine.
Secondly, because larger turbos are physically bigger than their smaller companions, they naturally have larger exhaust turbines and housings, so more waste gas can flow out better.

  • lewichris

Posted September 24, 2001 - 05:18 AM


Originally posted by DOC:
Here is my totally uneducated answer. Obviously a five valve head will flow better than a four valve head, but why are there more intake valves?
The way i see it is, what comes out isn't as big as what comes in, like Hick said. Once the air fuel mixture is drawn into the combustion chamber, it is compressed and ignited. This releases engergy in the air fuel mix and creates force on the piston, heat, light and sound. Once the combustion process has completed, you are left with what you started with, only without the energy that it once had. So, the air fuel mix, minus the energy equals waste (exhaust gas) and therefore there must be less than what was originally drawn into the motor. Guess that also explains why the intake side of the motor (carburettor) is so much larger than the exhaust side.

There are two intake and three Exhaust. just to let you know. The intake valves are bigger then the Exhaust. go to yamaha website and get into the YS page and download the screensaver. It has a cool pic of the head with the valves showing.

If life is not a blur. You aren't going fast enough

[This message has been edited by lewichris (edited September 24, 2001).]

  • DOC

Posted September 24, 2001 - 10:59 AM


You had me worried for a minute! there are three inlet and two exhaust, look in your manual.


Posted September 24, 2001 - 01:05 PM


when a gas is compressed it appears to heat up and when it expands it appears to cool down. the reason for this is that there is a certain amount of heat energy in the gas and when you compress the gas you also compress the energy, you have more heat in a smaller area which makes that area hotter

the reason for five valves has 2 primary reasons.

small light valves will be able to rev higher. hp comes from displacement or rpm yamaha chose rpm

5 small valves have more surface area (flow) than bigger valves would. (this also aids the search for rpm)

as far as only 2 exhaust valves a certain amount of restriction in the exhaust is wanted in all but very hard core racing engines. the more you open up the exhaust tract the hotter your engine gets and the narrower the power band becomes. fuel consumption also increases and becomes less efficient

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Posted September 24, 2001 - 01:13 PM


i almost forgot what goes out is larger that what goes in. think for a second the explosion that forces the piston down is the rapid expansion caused by the burning of the fuel you are trading fuel and oxygen for expansion.

  • Brian_in_Long_Beach

Posted September 24, 2001 - 02:32 PM


Be nice Taffy.... You crack me up


  • darbsitton

Posted September 24, 2001 - 04:08 PM


There are 3 intake and 2 exhaust valves. This info is given on page 3-21 of a 2001 WR426 manual. You can also verify by removing the cam cover and counting the cam lobes next to the carburator/exhaust. Hey, while you are at it why not check the valve clearances :)

If you want to learn more about how an engine works, try out the following link.

Trailguy and JoshuaZ2 have answers that are most consistent with what I know. The only thing that I would add is that the reason gas heats up when you compress it is that by compressing gas you actually add energy to it. Added energy = added heat in this case. When gas burns more gas is created, and heat is generated. This causes the pressure in the combustion chamber to increase which will push the piston down, make the rear wheel go round and put a smile on your face :D

  • DOC

Posted September 24, 2001 - 09:55 PM


I don't understand how you can finnish with more than you started with. Where does the extra gas come from. I understand that when the fuel is burnt it expands and pushes the piston down, but from here im am lost.
I don't know much about physics, but isn't there something called entrophy? Take a air fuel mixture and compress it. This has potential energy (potential to do work). Ignite the air fuel mix and the energy is released into the forms of heat, sound, light and pressure. Once the available fuel and oxygen have been used up, the energy has now been converted from a high grade of potential energy, to a lower grades of energy (heat, light, sound and pressure).
Im not trying to be smart, just wondering how this works. Please share your knowlage on this topic.
Also, im not sure if there are only 2 exhaust valves because a certain amount of restriction is needed. Wouldn't a header pipe and muffler provide this needed resriction? I know removing your head pipe and muffler will dramatically reduce power, but this is due to a lack of resonance rather than a lack of restriction (back pressure).

  • MN_Kevin

Posted September 24, 2001 - 10:22 PM


Entropy: a measure of the unavailable energy in a closed thermodynamic system that is also usually considered to be a measure of the system's disorder and that is a property of the system's state and is related to it in such a manner that a reversible change in heat in the system produces a change in the measure which varies directly with the heat change and inversely with the absolute temperature at which the change takes place.

If you look at the Mollier diagram, it displays the graph of temperature versus entropy. We used it all the time in our thermodynamics class while in Nuclear Power school in the U.S. Navy. It helps explain the steam cycle of electrical power plants and ship's steam driven proplusion plants.

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  • DOC

Posted September 25, 2001 - 01:06 AM


Good, a nuclear physicist. Kevin, can you please answer my question. Its good that you can quote from text books, but a person who has a true understanding should be able to answer in laymans terms. Can you help?

  • MN_Kevin

Posted September 25, 2001 - 02:18 AM


I only know about non turbocharged nuclear reactor plants! :D (unless you are referring to super heated steam systems that use moisture separator re-heaters to re-heat the exhausted steam from the high pressure turbine prior to entering the low pressure turbine)

Now what was the original question?

I was wondering if 3 smaller valves were better than two bigger valves due to size and throttling characteristics of the valves themselves. The more valves, the more controllable the power due to the more linear flow characteritics of the passing fuel...[???]

The expelled gasses will have a larger volume due to expansion. I think you have mentioned a law of thermodynamics that energy can neither be created nor destroyed, only altered in form. The chemical energy of the gas transformed to heat energy, mechanical energy and noise (energy?). None the less, you still have expanded gasses. Can you can use a P1V1=P2V2 relationship where P is the pressure and v is the volume; 1 being initial and 2 being final. There is a third variable with this calculation which I cannot recall off the top of my head (I am working the mid or "graveyard" shift at work. I am lucky if I can remember to wipe my a** after taking a dump, let alone recall computations I learned 15 years ago! :) )

How about this old navy saying, "Works fine, lasts a long time". Enough said! :D

[This message has been edited by NH Kevin (edited September 25, 2001).]

  • tbronco

Posted September 25, 2001 - 03:27 AM


OK, Here's my take on this.

There ARE 3 intake valves, 2 exhaust.
Even though the intake valves are smaller, in total they provide more area to flow than the 2 larger exhaust valves.

Why?, The volume of gas leaving is greater isn't it?

Yes, but that doesn't matter, the gasses have completely different prorerties after combustion.

Let's assume (for my sake) that the chemical comp. of the gasses stays the same (it doesn't). Also, whan I say gas I am refering to air, both on intake and exhaust.

We know that the flow is restricted by the area of the valve opening. If you force more gas through the valves in the same amount of time, the air must speed up. Simple enough.

There is a limit to how fast that air will go. When it reaches the speed of sound it forms a shock wave or, just a "shock" across the valve opening. This "chokes" the flow. It will not speed up any more.

Here's where the answer is found.

The speed of sound is dependant on the temperature. It is proportional to the sq. root of temp. That means that if the (absolute)temp goes up 4x (say 60F to about 1500F), the speed of sound doubles.

Soooo, you have nice, cool intake flow with a much slower choked velocity, and as a concequence, less flow. And a very hot exhaust flow that can move much faster through it's smaller opening.

Add to this the pressure differences between the cylinder and manifold for each intake and exhaust and you will see that you need more opening (area) on the intake side.

98 WR400 Dualsport!

  • tbronco

Posted September 25, 2001 - 03:32 AM


There's your fix of HOT AIR Taffy :)

  • darbsitton

Posted September 25, 2001 - 07:03 AM


Gasoline is a big molecule. When it reacts with oxygen it creates several smaller molecules. I don't know the formula for a gasoline molecule, but here is how propane burns:

C3H8 + 5 02 = 3 CO2 + 4 H20 + heat

On the left side of the equation you will notice there are 5 molecules, while on the right side there are 7 molecules.

The formula P1V1=P2V2 is only true if you do not add heat and do not change the amount of gas molecules in the system. Neither of these assumptions is true for burning gasoline.

I believe that more smaller valves serves 2 purposes.
1) Smaller valves can be made to move faster.
2) Using a larger number of smaller valves you can get more air flow since they will have a greater combined area. (Imagine if you only had 2 valves. They would only cover about half of your head. With 5 valves you can probably cover 2/3 or more of your head with valves.)

The intake area needs to be larger because, as trailguy mentioned, you only have atmospheric pressure pushing the intake gas in. Turbochargers and superchargers are used to increase this pressure and push even more gas into the combustion chamber. They are basically just big fans which help blow the gas into the combustion chamber.

The exhaust gas has the force of the piston pushing it out. This allows a greater volume of gas to pass out through the smaller exhaust valve area with decent efficiency.
Hope this helps. I am rapidly reaching the end of my knowledge on the subject.



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