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I was thinking, a 2v and 4v are even numbers so it can be 1,2 valves for intake and exhaust. I was thinking how do 3,5 valve engine even work? Wouldnt it be more intake than exhaust? or vice versa?
 

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MM's Ginger
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lol i was wondering about that too
 

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It utilizes 2 33.8 mm intake valves and 1 37.5 mm exhaust valve along with 2 cam phasers to advance or retard the cam timing as needed. The intake runner design is pretty straight forward so it makes good power especially with a few mods. If the engineers could have designed the head the 2 exhaust valves it would have been the ultimate.
 

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the idea is to get more air into the engine, so 2 intake valves do that well. the reason there is only one exhaust valve is because the exhaust is getting pushed out by piston force so you only need one... Now these are only theorys until proven and anytime you can lighten the valve train by using 2 smaller parts rather than one big part to do the same job you can affectively make more higher rpm power with less stress on the parts because they are smaller and lighter..
 

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~C~, The Other White Meat!
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why not just make it a 4v?
 

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Priest of the Car Gods
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well, even when you have just 2 valves, the size of the intake and exhaust valves is never the same, with the intake always being larger than the exhaust.
 

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Priest of the Car Gods
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why not just make it a 4v?
Most 4v setups are DOHC. The 3v setup is still SOHC, which means less moving parts and better packaging - look at the size and weight of 4v heads vs 3v heads. Less parts also means cheaper; cheaper to make, cheaper to repair, and cheaper to modify.
 

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Most 4v setups are DOHC. The 3v setup is still SOHC, which means less moving parts and better packaging - look at the size and weight of 4v heads vs 3v heads. Less parts also means cheaper; cheaper to make, cheaper to repair, and cheaper to modify.
i like your words
 

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Total number of valves really does not make a difference. What matters is the total combined cross sectional area for the intake valves and combined cross sectional area of the exhaust valves. For a given cylinder diameter, you will be limited by the number of holes or how big of holes you can put in the head. Too many holes and/or too large of holes will weaken the head which will result in a lower limit on peak cylinder pressures. Lower cylinder pressures means less horsepower. Therefore, the head is optimized taking into account peak cylinder pressures and flow in and out of the cylinder (volumetric efficiency).

The reason the combined cross section of intake valves are always larger than the exhaust valve cross section is based on the compressibility of the working fluid (i.e. air and fuel). Let us look into an example where we have the same size intake and exhaust valve. The intake charge is “cold” when compared to the exhaust gas, therefore it is denser than the hot exhaust gas. Cold gas will choke, or reach the a maximum flow limit through a given hole size before a hot gas flowing though the same size hole. In other words, you can push more of a less dense fluid though a hole than a fluid with higher density through the same hole.

Let us remember that the same amount of mass or “stuff” entering the cylinder will be exiting the cylinder. We want to maximize the amount of new charge entering the cylinder so we make the intake valve area large, or in our case have two intake valves. Now that we know that the exhaust gas is less dense, you can use a smaller size valve (or one valve in our case), to get that same amount of charge out of the cylinder.
 

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Admanistrator
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It's like double penetration.
 

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Total number of valves really does not make a difference. What matters is the total combined cross sectional area for the intake valves and combined cross sectional area of the exhaust valves. For a given cylinder diameter, you will be limited by the number of holes or how big of holes you can put in the head. Too many holes and/or too large of holes will weaken the head which will result in a lower limit on peak cylinder pressures. Lower cylinder pressures means less horsepower. Therefore, the head is optimized taking into account peak cylinder pressures and flow in and out of the cylinder (volumetric efficiency).

The reason the combined cross section of intake valves are always larger than the exhaust valve cross section is based on the compressibility of the working fluid (i.e. air and fuel). Let us look into an example where we have the same size intake and exhaust valve. The intake charge is “cold” when compared to the exhaust gas, therefore it is denser than the hot exhaust gas. Cold gas will choke, or reach the a maximum flow limit through a given hole size before a hot gas flowing though the same size hole. In other words, you can push more of a less dense fluid though a hole than a fluid with higher density through the same hole.

Let us remember that the same amount of mass or “stuff” entering the cylinder will be exiting the cylinder. We want to maximize the amount of new charge entering the cylinder so we make the intake valve area large, or in our case have two intake valves. Now that we know that the exhaust gas is less dense, you can use a smaller size valve (or one valve in our case), to get that same amount of charge out of the cylinder.
Great explanation of why intake valves are bigger/ can be bigger than exhaust valves!!!

Not so great an explanation of how 3v's work... But damn good explanation none the less!

And from a newb! **** hot man.

The way the 3V works with a single cam is the way they have the rockers positioned. They are set up like this ^v^. The contact area of the 3 rockers are almost lined up which lets them all be activated by one cam. If they were to have 4 valves they wouldn't be able to accomplish this in the limited lateral space available. DOHC 4V's are set up like this

^^
v v


Another benefit with 2 valves over 1 larger valve with the same combined area is that the 2 valves will be much stronger than the 1 large one. They can only increase the stem size by so much before they run into limiting flow. With a small stem and a large valve there is a whole lot of metal that is unsupported.

To add to the reason intake valves need to/ can be larger than exhaust valves is vacuum is less effective than compression. It is much easier to create flow with pressure than it is with a vacuum. Additionally the exhaust gasses are expanding at a very rapid rate when they exit the cylinder which also aids in the evacuation.
 
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