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Intake Flap Mod

The stock intake manifold on the B6 S4 comes with a crazy over engineered flap mechanism that switches between short and long intake runners depending on on the engine RPMs. Below 5000 rpm the engine is using the long runners and the flap stays upwards and closed. Above 5K it drops down and lets the air through both.

Unfortunately, the flap itself is huge and even in the closed position (low rpm) it sticks some 25mm down into the throttle body. JHM was the first company to my knowledge to address the issue and they got excellent results with their ported and modded manifold. Since my car is all torn apart, i figured I might do something similar, with the focus being on low rpm efficiency... since i don't really rag on my cars anyway.

My goal here was to reduce the cross section of the flap to the bare minimum when it's closed.
Essentially all you need to get this done is:
  • Sheet of carbon fiber cut to the shape of the OEM flap
  • 4 1/4" nylon machine screws with 5mm spacers and locking nuts (Ace hardware has all of it)
  • Blue or red thread locker
All you need to remove the OEM flap and use it as a template to cut the carbon fiber. The flap mouting points actually come with convenient holes to run the bolts through, so just enlarge them a bit fit the nylon bolts through. Once that's all set, put the CF into the intake and close the flap (it helps to remove the spring that keeps it in the open position) and find the right points to drill the CF flap. I sprayed mine with red plastidip to make it seal a little better.

Finished CF Flap and OEM flap out of the way to show the mounting points



Back side of the flap


The next step is to trim the spacers to get flap sealing the whole opening in the upper manifold. Depending on how thick your CF is, you might have trim quite a bit. I ended up needing about 4mm spacers to get the flap and the plastic arms parallel with the opening surface when the valve is closed.

Once that is all done, just tighten the bolts down with couple drops of thread locker and trim the excess. You'll end up with something like this:



Finished intake with the flap open


Under 5K the airflow through the manifold is far less obstructed.

Note the black plastic arms and the flap are parallel to the opening surface when the flap is closed

Finished product with the top manifold out of the way

Same thing from the underside of the flap:


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