http://www.lilevo.com/mirage/Cyclone...vr4cyclone.htm
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One reason nobody has much to say about them is they are usually set up improperly and then removed. There*IS* a reason Mitsubishi engineered these expensive manifolds and put them on their top-of-the-line vehicles, and didn't waste them on the inferior American-market products!
The idea behind the Cyclone (variable-length intake manifold) is to physically split the air passage up into a set of short runners and long runners to modify the low-end torque curve via changing Volumetric Efficiency AND set up a Helmholtz Resonance. Air flows more efficiently over a LONG path at LOWER velocity, and more efficiently over a SHORT path at HIGHER velocity.
The long runners are always open, and provide a narrow path for the air at low RPM where the engine doesn't consume enough air to need both runners (which add up to ALMOST the same capacity as a 1G manifold, there is only a few CFM difference between them). Torque, spool and throttle response are increased slightly.
The valves for the short runners open at 4100 RPM around the same time the turbo fully spools on a stock Evo3 RS. Here, the engine can consume much more air, and the short runners open to put the manifold back to "1G-size".
Determining the best time to open the secondary runners is critical in making the manifold do what it's supposed to on your setup, and depends mainly on volumetric efficiency - turbo size, dimensions of your intake pipes and intercooler, cylinder head porting, etc. Open the short runners too soon and spoil the Helmholtz effect with turbulence; too late and you choke the engine. If you run a monster turbo, you've already forfeited 80% of your power band anyway, so get a SMIM instead
WTF is Helmholtz resonance? It's the whistle effect you get from blowing on an empty bottle - air pressure waves compressing and expanding like a bouncing spring. The Cyclone manifold also generates this resonance. It actually smooths the flow of air, and creates a mild supercharging effect. Air that doesn't make it into it's cylinder on the current stroke is pushed back a little by the intake valves snapping shut, and sends a pressure pulse back through towards the manifold plenum. At low RPM, the pulse pressure is stored in the blocked short runners and is pushed back into the long runner when a set of intake valves open, which forces a little extra air into the cylinder. The higher the RPM, the faster the pulse frequency gets - until the long runner is too far for the pulse to travel before the next valve open event. Opening the secondary runners provides a shorter path for the pulses to travel, allowing them to survive and keep up the supercharging effect.
This is why the Cyclone manifold makes kind of a "growl" noise. You can also hear this growl on the Ford Taurus SHO engine. The SHO has a Yamaha 6-cylinder with a cylinder head MUCH like their motorcycle heads, and a dual-runner intake manifold like the DSM Cyclone. And, believe it or not, this is why VTEC engines "growl" too. Poppin' Tec dawg!
NOTES:
Mazda has produced engines with a combination of variable valve timing, variable length intake manifold, and variable backpressure exhaust systems. All are just ways to dynamically tweak VE to widen the power band.
The SHO engine can also benefit from a variable backpressure exhaust, just like other Yamaha engines do with their EXUP valve.
The Cyclone butterfly valves don't open all the way! This is intentional by the engineers who designed it - with the Helmholtz resonance in mind. Since the short runners don't account for half of the high RPM airflow (longer runners' = greater volume), they don't need to be fully open.
Mitsubishi prototyped, if not manufactured Cyclone manifolds for the 6GXX engines too.
Here's a good article on how to make a Cyclone work with a DSM:
http://www.dsmtuners.com/forums/arti...-manifold.html <<--The dyno plot at the bottom of the page is confusing as hell, I don't think it's right so ignore it.