The B6 exhaust manifold looks very similar to the S4 manifold. And the inside on the S4 has a 3 way merge collector like the inside of the 3.0 collector that you posted above. And beem posted that the outlet on the S4 is just a tad smaller then 2.5" (from his memory, he said he wasn’t positive about it). But I’d be willing to bet he meant tad larger, well if your measurements are accurate, b/c 65mm converts to 2.55" so I’d imagine the S4 would match that same size based on similar design.
All that said, the S4 has found that the stock manifold isn’t a problem until about 350whp, after that it starts to become one of the larger restrictions. So if headers was a route someone wanted to take, they should be the extreme long tube equal length headers like JHM built for the S4 so that the torque could be maximized (same thing the 3.0 could use, a big torque bump). However, I’d be willing to bet, just like on the B8 S4 & B7 RS4, those supersprint headers do almost nothing, well at least for their cost. The design looks nice, but it just isn’t a proper approach to enhance the weak parts of the motor, low end grunt.
Here are some S4 comparison pics:
http://i278.photobucket.com/albums/kk89/mbgt72/My%20Audi%20S4/MCW%20supercharger%20Build/IMG_0717-1.jpg
http://i456.photobucket.com/albums/qq289/joeycuccaro/S4%20Engine%20Removal/P1000635.jpg
http://i456.photobucket.com/albums/qq289/joeycuccaro/S4%20Engine%20Removal/P1000637.jpg
http://jhmotorsports.com/products/pictures/Headers/JHM_B6-B7_S4_Longtube_Headers.jpg
All that said, I certainly agree that a set of custom made DP’s would really go a long way for this car. The big factors to remember if someone does go that route is:
1)Mandrel bent 2.5" from start to finish.
2)Welds REALLY matter. Slip fits, or sloppy welds just don’t cut it.
3)HFC’s can easily be added if needed, just do it much closer to the catback then near the exhaust collector. There’s plenty of room back there, so it shouldn’t be an issue.
-Just to touch on the welds, CountVoughn explained this to me about exhaust work. You need to think about each pulse from the motor as a heart beat. Every pulse the gas is trying to push outward on the pipe, so it’s crucial that the interior sections remain the same diameter as well as being very smooth. If the diameter starts changing or the welds are poor, turbulence develops and the pulse starts to break down.
The ability for the exhaust gas to scavenge (the act of the lead pulse creating a vacuum and pulling the next pulse resulting in increased speed of flow, very important on a NA motor) is strongest when it acts as a uniform singular pulse. Exhaust is all about speed, not diameter, thus when you can speed up the pulse by making sure the restrictions are minimized and using quality piping and the correct diameter, the motor is able to work at a much higher efficiency and create significantly more power with out really “doing” anything to the motor itself. This is how the JHM LTH work, even without a tune update the power is significantly increased, then with proper tuning, the motor can really take advantage of the new flow characteristics.
There’s lots lots more that goes into it, just trying to help establish a good base for understanding. This is old news to a lot of people I’m sure, but there’s no reason not to post up quality information to help inform anyone unsure or put a end to misinformation. And I’m just trying to paraphrase information given to me by CountVough, I feel he’s a pretty trustworthy informer haha.
Edit: looks like ya beat me to the post Count, glad to hear from ya and hopefully I didn’t butcher the info you’ve passed on to me.
