Tässä kun on ollut noista runnereiden pituuksista, niin sekoitataan vielä lisää. Tässä kirjasta Practical Engine Airflow: Performance Theory and Applications: Wave Tuning The practice of improving VE by increasing the pressure in the intake port just before the point of IVC is referred to as wave (or ram) tuning. It is a method that seeks to take advantage of finite amplitude waves, or pressure pulses, in the intake and exhaust system to add energy to ramming the incoming charge or extracting outgoing exhaust gases. This section elaborates on the use and effects of wave tuning in the intake system. Here is the Runner Length Formula: L = [(k × c) ÷ N] + 3 Where: L = length of the inlet path in inches k = mathematical constant (Chrysler chose 72,000) c = 1,100 fps N = engine speed (rpm) + 3 = recommendation to encourage experimentation to determine the actual “weet spot” You can find simplified optimum runner length constants (k) by dividing the selected constant by the RPM: 2nd pulse = 108,000 ÷ RPM 3rd pulse = 97,000 ÷ RPM 4th pulse = 74,000 ÷ RPM 5th pulse = 54,000 ÷ RPM Chrysler Ramcharger Research Sharp-eyed readers note differences in the equations presented below and those above. They are not erroneous. The ones offered below are taken from work performed by Chrysler Ramcharger engineers more than 60 years ago. They are still correct in that they represent a range of possibilities. The Chrysler recommendations fall almost directly between the third- and fourth-wave equations provided in modern engine simulations. Hence they split recommendations that encompass a broader range based on 60 additional years of testing and evaluation. In the absence of very tightly controlled dyno testing, these calculations are meant to get you in the ballpark, particularly if you are already stuck with fixed-dimension hardware. The calculations may suggest that your combination has more to offer with modified dimensions. You can model it in a simulation program (which is also a generalization) to see if it predicts a favorable outcome, or you may choose to bite the bullet and reconfigure your dimensions with new components and then test. It’s important that you gain a better understanding of your engine’s potential. Given enough data, any engine’s performance can be reduced to the mathematic principles that define it. Discovering those pesky variables is why you calculate, why you model, and most important, why you test. Harmonic Cycle Although much is made of the activity in the combustion chamber, an extraordinary world of tuning potential exists in the captured column of air between the back of the intake valve and the opening of the intake flow path, either at the plenum entrance to the intake runner or the opening of an individual runner ram tube. A similar environment also exists between the exhaust valve and the opening of individual header primaries to the collector. On either side, a harmonic cycle defines this unique phenomenon. Here, I examine the intake side. A harmonic cycle occurs in sets of four events, and each initiates a pressure and velocity change within the intake runners at supersonic speed. Each event starts at one end or the other of the flow path between the valve and the plenum or ram tube opening. The first event occurs when the relatively fast moving air/fuel charge slams into the back of the closed intake valve causing a pressure spike relevant to the air density of the charge, its velocity, and the local sonic speed depending on temperature. As more charge slams into the stalled mass, it creates a secondary pressure spike that separates and reflects back up the runner at sonic speed. The piled-up air/fuel charge remains in place, but the pressure pulse reverses and moves through the charge mixture as if it didn’t exist. When the leading edge of the pulse reaches the plenum, the larger area causes local air molecules to disperse away from the opening. A phase change occurs, and negative pulse is reflected back down the runner at sonic velocity (with the air in the runner at zero pressure and negative flow). When the pulse reaches the valve again, the air experiences a negative pressure equal in magnitude to the initial positive pressure. This causes another reversal and another pulse to travel back up the runner to the plenum. It arrives at negative pressure, and plenum air begins to flow into the runner again, but there is minimal pressure with positive velocity. This constitutes one harmonic cycle. When the pulse reaches the valve another cycle initiates immediately. Research by the Chrysler Ramchargers racing team in the 1950s determined that the best time to open the intake valve was after the third harmonic cycle, which means that the pulse traversal between the valve and the plenum has occurred 12 times. When timed properly the pressure peak arrives at just the right moment to add energy to sweep more charge into the cylinder than typically occurs. These pulses reflecting back and forth in the system, and their arrival at the valve, can be calculated and controlled by the length of the inlet passage. Additional research determined that the first reflection is very strong and arrives too quickly to do much good. Chrysler engineers in the 1950s initially felt that the second reflection called for excessively long runners and too large of an air/fuel mass to accelerate successfully. Fuel-injected engines that add fuel near the valve don’t suffer that problem, so the second reflection is often applied to modern sprint engines and fuel-injected high-torque OEM engines. The second and third reflections typically generate the most torque. The third and fourth reflections are generally used to apply useful intake tuning to race and performance engines. The third wave is most useful on Pro Stock and Comp Eliminator drag engines to produce maximum horsepower. In most applications the fourth reflected wave is the most effective with single-plane intakes, delivering less peak torque, but still good horsepower. And, it is typically useful for packaging induction systems under stock hoodlines. The Chrysler Equation Calculated values for tuned lengths vary depending on the length of the flow path and the RPM for which you’re tuning. The Chrysler equation has proven to be very effective and is still in use today: N × L = 84,000 Where: N = the desired tuning RPM L = the tuned length of the flow path 84,000 = mathematical constant To achieve ram tuning at any given engine speed, simply divide the constant by the desired engine speed. For example, adjusting the formula to tune for 6,500 rpm, you get 12.92 inches (84,000 ÷ 6,500). This means that you should make your total flow path from plenum to valve as close to 12.9 inches as possible. In many cases, you’re likely to be working with fixed-length ports and runners, and you must adjust the formula to find the RPM at which your engine tunes with existing hardware. To calculate RPM, bolt the intake to the head with the appropriate gasket and measure the flow path centerline. You can use a piece of welding rod or a length of string. As an example, for a flow path centerline measurement of 10.4 inches you get 8,077 rpm (84,000 ÷ 10.4). Some anomalies have emerged in subsequent testing. In a 1999 Hot Rod magazine test conducted by Steve Magnante and me with on-site support by one of the original Ramcharger engineers, Bill Shope, Chrysler’s initial equation was more aligned with short-duration camshafts. After a few thoughtful moments with a calculator, Shope offered that longer camshafts with faster ramps tune slightly different. He suggested revising the equation to compensate for longer-duration cams: N × L = 80,300 In this case the ideal tuned length is 12.35 inches (80,300 ÷ 6,500). It is reduced by approximately 0.6 inch, which follows logically given the increased overlap periods and longer seat-to-seat times.