Can someone here tell me exactly what the split air pipe that goes between the UIM and the catalytic converter does? Also, if it is removed, should the hole in the manifold be blocked?

Thanks!

- k -

 

if remove don`t work your VDI, this backpressure what generate at 4,000 RPM open yours 5th and 6th port

 

On S4, you need the split are pipe so the exhaust gas pressure opens up your ports.

On an s5, the airpump creates pressure to push the actuators and open the 5/6th ports.

If removed, I would plug that hole on the manifold. Bad things can happen if you leave it open.

Other reasons for the pipe, provides fresh air to cats during idle to pass emissions and not burn up your cat. (I believe under 3k rpm)

 

It wasn't posted, but I know from other threads that this is an S4 TII. The air delivered through the split air pipe is very minimal. The split air solenoid is only activated when in 5th gear, and the port inside the ACV is only 1/16" in diameter (the split air solenoid opens an additional 3/16" port). It's really small. Air from the pump is delivered primarily to the exhaust ports, and both paths are only open under 3500 RPM. Above that point, the relief valve opens inside the ACV and dumps air into the silencer tube. If you remove it, you can put a small block-off plate over it to keep dirt out. There's no need to seal it with a gasket; just something to cover it.

 

It reduces NOx emissions under highway cruising conditions.



Now technically that is describing the FD but I suspect the FC is similar. The main cat operates in two stages. When port air is supplied (airpump air goes into the exhaust ports), the cat functions only to elminate HC and CO (oxidation process) in both stages. When split air is supplied directly to the cat, in the first stage three ("ternary" process) emissions are catalyzed: HC, CO, and NOx. In the second stage the oxidation process (HC and CO) occurs only.