the one on rx7.com doesn't have an airtube that i see to actuate the ports, and it says it only replaces the main cat. Would this http://cgi.ebay.com/ebaymotors/86-92...spagenameZWDVW

with this work ? http://cgi.ebay.com/ebaymotors/Mazda...spagenameZWDVW

 

The S4 6PI doesn't work off exhaust velocity as such. They work off pressure in the exhaust pipe partly caused by velocity and partly caused by the restriction caused by the pipework system. The faster the exhaust, the higher the pressure will be. The more restrictive the path is, the higher the pressure will be.

"Backpressure" is a misleading term, but it's easily measured by putting a pressure gauge in the exhaust system. The more gas being pushed down the system (i.e. the higher the load), the higher the measured pressure will be. Pressure will be at it's highest at the very beginning of the system and zero at the very end.

 

Will there be enough exhaust pressure/velocity/whatever to actuate the 5/6th ports if I remove the cats and run a straight pipe (same diameter as stock)?

 

If you're so worried about those ports opening for you, just save the extra 100 or 150 dollars it is going to cost you to order the parts from RB and get it right.

 

Right on, that's my plan now. No more cheap ways out i guess. Anyone that has a RB downpipe for a s4 n/a, let me know.

 

Probably, but that doesn't mean the system will be working right.

What people don't seem to get is that there is a specific point where the auxiliary ports need to open for get the best performance, and any reduction in exhaust backpressure will result in them opening later than ideal. As I said above, the pressure in the exhaust system rises with exhaust flow (i.e. a combination of revs and throttle opening), so reducing the exhaust's restriction will mean the pressure required to open the ports will occur at higher revs. This will mean a drop in mid-range performance (where you spend most of your time) because the ports are closed when they should be open.

IMO any changes to the exhaust should be done in conjunction with a change from exhaust backpressure actuation to an airpump/solenoid/rpm-switch set-up mimicking the S5's.