@SkylineUSA:
I really like this discussion as well, So keep up the good work.
@RazorgtR:
I as fas as i know the bigger the plenum the better. The bigger the plenum the better turbulence is turned into quite air (colisions (crashes) with other aotoms/moleculs). Also for the engine it looks like it is sucking air from a room where the pressure is pressure at sealevel. It can suck air without any obstruction( airfilter, badly designed box to plenum pipes)Therefor the plenum must contain air for more then one cilinder to keep the pressure the same as the surroudings. Like a Common rail diesel.
Runners.
by opening the the inlet valve a pressure wave (negative -) is travelling into the cilinder. by closing the cilinder a pressure wave (positive +) is bouncing upon the valve. This +wave is going back to the plenum where it returns as a -wave back to the valve. It bounces back on the closed valve and it travels back to the plenum still as a -wave. By entering the plenum the sign (-/+) switches (here from - to +) and begins he travel to the inlet valve as a positive wave.
Now the engineer who works on the intake has to get the +wave at the inlet valve as it is 1 mm open.
In case of my car I have to get 9 meters (29.52747 feet)of runner (one runner 4 in total) under the hood to get more torque on the by my desired(there is something wrong on this word) rpm (2500).
The pulses (positive/negative wave) travel at 343 meter/s (0.2131303 mile per second) at 25C (77 Fahrenheit) and every 20 C that the air is hotter the speed raises 11m/s so lets take an air temperature of 65 C the speed of sound is 365 m/s at this temp. (0.2268005 mile per second). 7000 rpm is 116 rps. Every 720 degrees a combustion. So every 720 degrees the intake valve opens once. So from those 116 rps the inlet opens 58 times (please people correct me if I'm wrong. It is late for me). I pick a cam that is used for an NA Renault Alpine (I got the data on my desktop... everybody is lazy sometimes
). Every work process takes 1/116 seconds (0.008620). From every work process the inlet is 255 degress open. so the inlet is closed for 720-255 = 465 degrees. That is 0.005567 seconds. So a +wave(back) a -wave (to) a -wave (back) and a +wave(to the valve) has to travel in 0.005567 second. The speed of sound is 365 meter per second, so the inlet has to be 365*0.005567 = 2 meters (6.56166 feet)(that is a lot...) long to have advantage of the pressure waves in the inlet. This advantage is used for this rpm but it also works when the inlet is already open. (not when closed of course) So it works for something more then one rpm. This creates a mountain landscape in a graph.
(This is all just an example, I took the time for this but it still can contain mistakes)
It also works on the 3500 rpm. but the pressure waves have to travel 4 times (up and down). But the effect is less cause of efficiency.
This is all for NA engines. I still don't know how it works for turboed engines. I think it works the same because of the natural effect. But if someone thinks different please let it know.
No offence SkylineUSA but shorter runners work only for high reving motors. Like F1. The best would be a engine without any kind of runners...
I hope this clears something up. This however is not the only way to desing runner length. There are two other ways to come to a runner length. The art is to combine all of them.
Most of todays intakes are designed around the exhaust. The exhaust works on a similar way. But there are also some other things that work on a exhausts. Cause of the change between negative effect and positive, the intake is designed to compensate for the negative effects of the exhaust.
Derby