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Valve Curtain Area Explained


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I wrote this up for a few other people and I didn't know if you guys would be interested in it.


Here is a picture with some equations to go along.




When you are flowing air through a valve, you can't flow air through the top of the head, it has to flow around the valve head itself. Thus the amount of air a valve can flow is proportional to the circumference of the valve, or the area that it exposes for gas exchange when it is lifted. The easiest way to increase circumference is by making more valves, rather then increasing the size as I proved mathematically in the picture. What we are discussing is valve curtain area. The curtain area is the circumference of the valve, a constant and the distance the valve is lifted or moved from the valve seat, a variable. Mathematically the valve curtain area equals the circumference of the valve times the lift. So how important is all of this? Well the larger the valve curtain area the greater the potential to fill the cylinder and the greater the potential for power. Two or more smaller valves can yield more valve curtain area than a single large valve, in a similar setup.


If I said that my four-valve top end has more valve curtain area than a similar sized two-valve cylinder setup, what would I be saying? If I stuck a single 2.08-inch diameter intake valve into a four-inch diameter cylinder. That's a pretty easy example because it's used commonly on the performance built Chevy smallblocks. Now we take our hypothetical four-inch diameter cylinder and remove the large single valve and in its place we install two smaller intake valves of 1.200-inch diameter each. We would have a valve diameter equivalent of 2.400 of an inch, more potential airflow and more potential power. When you add up the respective potential valve curtain area and the supporting port area you can quickly see that two is greater than one.


The most important thing is to match up your displacement to your valve size. Just like everything else the law of diminishing returns applies here, and it is because the gained power from oversized valves hurts a smaller engines top end power because of the linked gain of valvetrain drag due to increased size.


But when you create more valves, you are also creating many issues for yourself. For example while using more valves exponentially increases surface area on the edge of the valve where the gas exchange takes place (which is good) it also exponentially increases surface area on the inside of the valves where the drag takes place (which is really bad). Imagine if I had one really big pipe and four really little pipes, but the total volume between the two sets were the same. Since we know that the surface to volume ratio decreases with a cylinder as the size increases, that total surface area inside the bigger pipe is going to be less then the total of the four smaller pipes. The less internal surface area, the less area where the resistance and drag happens. Other problems stem from the effect created when two parallel intake valves "shroud" each other. Basically a lot less air flows through the area between the valves. Another issue is that four valve heads require that the valves be inclined closer to the cylinder.

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