New Zealand Datsun Club

Bartman wrote:long story short, the airflow inside a manifold doesn't run like a river, it fluctuates in pulses. If a pulse runs down the runner and the valve is shut, it bounces back, and then rebounds to the valve. When the pulse hits the valve while it is wide open, the intake is at max effeciencey. For each time the pulse rebounds before hitting the valve while open, the strength of the pulse weakens. Hence 1st, 2nd, 3rd harmonic etc. The longer the runnner, the lower in the rpm range this sweet spot will be acheived. And also the fewer times it will bounce before it is reached. the hemholz equation relates to the speed of sound in a vacuum, which is how fast these pulses are bouncing in the manifold. (Bare in mind that the speed of sound is something like 330 m/s)
During R&D jaguar were able to acheive volumetric effeciency exceeding 100% (thats like having boost!) at one given specific rpm by using runers that were extreemely long. (Possiblely even too long to sit inside the engine bay of a car?)

Now consider having a purpose built race engine where, Your exhaust header primary length, intake runner length, camshaft profile (and timing) and cylinder head flow numbers, all hit their sweet spot at the same rpm. The engine would have an insane torque peak!
Now imagine all the R&D and time/money required to build an engine that had all these things factored in. OUCH!

The Japanese Market Cefiro has a Variable Intake called the MEVI.

It's not a true Dual-Plane intake (like the Taurus SHO, or the 3.8 SPI from the later model Windstar/Mustang) which has two distinct air paths from the air filter to each cylinder (one long, for low revs - one short for high revs). Instead, it's a Dual-Plenum design that has a second plenum directly above the lower intake, providing a shorter path for the Dynamic Supercharging effect (described below). The second plenum is 'activated' by opening a set of butterfly valves. These valves are opened by a vacuum actuator at the appropriate engine speed.

Dynamic Supercharging is the concept that when the intake valve closes it causes a pressure wave to travel from the outside of the intake valve back towards the air filter. When the pressure wave reaches the plenum (the common part of the intake track between the throttle body and the individual runners) it bounces back towards the intake valve. If the intake valve opens just as the pressure waves gets back, then more air is forced into the cylinders and it increases the volumetric efficiency. There is a specific engine speed at which this Dynamic Supercharging effect happens, and the length of the runners changes this engine speed, because of the time it takes the wave to travel.

Intake spacers change this effect to a lower engine speed (because the wave takes longer) giving the engine more bottom-end grunt (marginally, and at the cost of mid-range grunt)

The dual-plenum design gives the engine TWO speeds at which dynamic supercharging happens - at high speed the upper plenum valves open and the shorter path to the upper plenum gives the dynamic superchargin effect at higher engine speeds, increasing breathing and prolonging the powerband.

That's it, in a nutshell