i have tried 2 times to get a primary fuel rail for a s4. i thought that i bought the right one the second time (over the internet). but instead i got a s5 with a built in dampener. i thought that i read some where that they are interchangable. but it would be nice to get some confirmation .. thanks

 

I see no reason why the primary would work, as the engine block is pretty much identical. The secondary rail may be a different story becuase the manifold is different than the S4.

 

The primary rails are interchangable, however you will not be able to repalce the PD on a S5 rail, like youc an when using a S4 rail.

You will have to replace the entire rail, instead of just the PD when using a S5 rail/PD combo.

 

You can put an primary S5 fuel rail on an S4 engine, but you can't put an primary S4 rail on an S5 engine. The intake manifold is the determining factor here... The VDI chamber on the S5 intake hits the S4 fuel rail.

 

The S5 rail is alot better. the PD is built in.. Hardly ever leaks like the S4 one.

 

That would be radically incorrect.

#1 because of the design you must replace the whole rail if the PD leaks. This means you spend $50-$100 more (depending on where you get your parts) when replaceing the PD (which you should every 100-120K miles or 10 years).

#2 I personally have seen more S5 PDs fail and leak than S4 versions. And in fact have seen more FD PDs leak (which use the same diaphram design as the S5 PDs) than any other PD out there.

#3 the S5 rail/pd redesign over the S4 design was simply a cost cutting measure for production rather than a better design.