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The Radium King · 06-29-2017, 08:58 AM

there have been a few variations of this system on the m96/m97 engines (and the varioram system on the 993). all except the early 2.5 have had the resonance flap on the intake runners. the 987's also had a flap on the intake plenum.

a few threads on this, but the idea is 'tuning' the intake. a number of concepts at play here, tied to fluid flow properties and cylinder fill on the the suck stroke.

fluid flow - decreasing diameter of intake tract. you'll note that the intake tube gets smaller in diameter as you follow it from the airbox to the throttle body. conservation of mass says that the same amount of air that enters the pipe at the larger diameter has to pass through the smaller diameter pipe. for this to happen the air has to accelerate/compress. the faster air more efficiently fills the cylinders during intake. this produces more torque at low rpm. at high rpm the effort to accelerate the air increases and actually reduces HP, so track cars that live at high rpm typically do away with the OEM intake and just run a big pipe and throttle body to get as much air in as possible (to the detriment of low rpm torque).

the same principal applies to intake runners. if you look at the design, they start wide and narrow down as they approach the cylinder head, again accelerating the air into the cylinder. note that the 996 has longer runners than the 986, which adds to the better torque numbers the 996 makes.

now then, when the intake valve slams shut, this fast-moving air stops suddenly and sends a shockwave back upstream. IF you design the lengths of the intake runners, and the intake plenum, properly, you can make it so that this shockwave hits an open intake valve and dramatically increases the rate of cylinder fill. this is the purpose of the resonance flap - it changes the lengths of the intake runners to 'tune' the intake such that shockwaves are hitting when and where they are supposed to.

note also that these shockwaves travel back up the intake tube as well - this is the purpose of the plastic pot that sticks off the side of your intake tube - dissipate these shockwaves and make the intake quieter.

06-29-2017, 08:58 AM	#4
The Radium King Registered User Join Date: Apr 2010 Location: Canada Posts: 3,128	there have been a few variations of this system on the m96/m97 engines (and the varioram system on the 993). all except the early 2.5 have had the resonance flap on the intake runners. the 987's also had a flap on the intake plenum. a few threads on this, but the idea is 'tuning' the intake. a number of concepts at play here, tied to fluid flow properties and cylinder fill on the the suck stroke. fluid flow - decreasing diameter of intake tract. you'll note that the intake tube gets smaller in diameter as you follow it from the airbox to the throttle body. conservation of mass says that the same amount of air that enters the pipe at the larger diameter has to pass through the smaller diameter pipe. for this to happen the air has to accelerate/compress. the faster air more efficiently fills the cylinders during intake. this produces more torque at low rpm. at high rpm the effort to accelerate the air increases and actually reduces HP, so track cars that live at high rpm typically do away with the OEM intake and just run a big pipe and throttle body to get as much air in as possible (to the detriment of low rpm torque). the same principal applies to intake runners. if you look at the design, they start wide and narrow down as they approach the cylinder head, again accelerating the air into the cylinder. note that the 996 has longer runners than the 986, which adds to the better torque numbers the 996 makes. now then, when the intake valve slams shut, this fast-moving air stops suddenly and sends a shockwave back upstream. IF you design the lengths of the intake runners, and the intake plenum, properly, you can make it so that this shockwave hits an open intake valve and dramatically increases the rate of cylinder fill. this is the purpose of the resonance flap - it changes the lengths of the intake runners to 'tune' the intake such that shockwaves are hitting when and where they are supposed to. note also that these shockwaves travel back up the intake tube as well - this is the purpose of the plastic pot that sticks off the side of your intake tube - dissipate these shockwaves and make the intake quieter.