I have always misunderstood the dynamics behind the opening of the ports. Why do they only open under load? They use positive pressure being fed in from the cat so why would they not open when revving the engine? Could someone explain it? BTW S4

 

It takes more air induction to move a car 40 mph at 3.5K than it does to rev an engine with no load to 3.5K, to put it simply. More air induction means more air exhausted...Accelerating the mass of the car makes all the difference.

 

i thought s5s only open under load and s4s open by rpm

 

Not OEM cars...Mine open relative to rpm 'cause I've got the air pump doing the work that the stock exhaust used to do...On S4's, once you go open exhaust, there's no way to generate the kinds of back pressure required to open the ports, so you have to "create" alternative means...

 

Wayne's first post is right...you make more exhaust gasses while under load, those creating more backpressure. Same reason a turbo spools much harder and makes more boost under load.

-Joe

 

Theoretically you can still use backpressure on the S4 with a free flowing exhaust - the Racing Beat presilencer has a "ram-air" tube that feeds the split air pipe with backpressure.

-=Russ=-

 

they both open by load, but the s5's are electronically opened under load. the s4's are dependant on the backpressure.