![]() The engine using this system, running at 8500 rpm, takes a very considerable 46 crank degrees before any signal from the cylinder can reach the runner end (assuming no movement of the air in the runner). In an intake runner at room temperature the sonic speed is about 1,100 feet per second (340 m/s) and traverses a 12-inch (300 mm) port/runner in 0.9 milliseconds. The wave/flow activity in a real engine is vastly more complex than this but the principle is the same.Īt first glance this wave travel might seem to be blindingly fast and not very significant but a few calculations show the opposite is true. It only occurs at 1/2 stroke (90 degrees) with a connecting rod of infinite length. For normal automotive design this point is almost always between 69 and 79 degrees ATDC, with higher rod ratios favoring the later position. This point occurs at different points depending on the length of the connecting rod and the throw of the crank, and varies with the connecting rod ratio (rod/stroke). That is, any time a change occurs in the cylinder – whether positive or negative – such as when the piston reaches maximum speed. This is why port/runner volumes are so important the volumes of successive parts of the port/runner control the flow during all transition periods. The speed that the signal can travel is the speed of sound within the runner. The principle is the same as in the water hammer effect so well known to plumbers. It is this phenomenon that enables the so-called “ram tuning” to occur, and it is what is being “tuned” by tuned intake and exhaust systems. This very considerable pressure rise can be seen on the graph below, it rises far above atmospheric pressure. The runner entrance continues to flow at full speed, forcing the pressure to rise until the signal reaches the entrance. The closing valve causes a buildup of pressure that travels up the runner as a positive wave. Not shown in this animation.)Ĭonversely, the closing of the valve does not immediately stop flow at the runner entrance, which continues completely unaffected until the signal that the valve closed reaches it. (Once the low-pressure wave reaches the open end of the runner it reverses sign, the onrushing air forces a high pressure wave down the runner. Up until that point all that can happen is the higher pressure gas filling the volume of the runner decompresses or expands into the low-pressure region advancing up the runner. It is only then that the entire runner can begin to flow. The air at the runner entrance does not move until the wave reaches all the way to the end. All the air on the upstream side of the moving disturbance boundary is completely isolated and unaffected by what happens on the downstream side. When the valve opens, the air doesn’t flow in, it decompresses into the low-pressure region below it. Note the green "valve" opening and closing. This highly simplified animation shows how air flows as waves in an intake system. For major modifications the ports must be welded up or similarly built up to add material where none existed. When a modification is decided upon through testing with an air flow bench, the original port wall material can be reshaped by hand with die grinders or by numerically controlled milling machines. In that context, air can be thought of as thick, sticky, elastic, gooey and heavy (see viscosity), and head porting helps to alleviate this. However, an engine running at high speed experiences a totally different substance. Ports can be modified for maximum power, minimum fuel consumption, or a combination of the two, and the power delivery characteristics can be changed to suit a particular application.ĭaily human experience with air gives the impression that air is light and nearly non-existent as we move slowly through it. Cylinder heads, as manufactured, are usually suboptimal for racing applications due to being designed for maximum durability. ( Learn how and when to remove this template message)Ĭylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve their air flow. Please help improve this article either by rewriting the how-to content or by moving it to Wikiversity, Wikibooks or Wikivoyage. The purpose of Wikipedia is to present facts, not to train. This article contains instructions, advice, or how-to content.
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