1. Inner Diameter
2. Removal of Pre-Cats?
3. O2 sensor location fore and after cat? (if cats are doing their job, you won't get a CEL. Some tuners also will reprogram the chip to ignore this code too - ie test-pipe option-chips)
4. Test-Pipe Option?
5. Flex Joints?
6. Cat Type
7. Cost?

So now how does each option (that I'm aware of) stack up?

Stock System
Diameter = <2.25" (I believe)
Pre-Cats = YES
O2 Sensor location fore and aft of cats = YES
Test Pipes = NO
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = Pre = Metal, Main = Ceramic 400cel
Cost = Nothing, you already have them.

<b> Piggie Pipes - OEM with gutted precats</b>
<a href="http://www.vastperformance.com/products/exhaust.php">Piggie Pipes</a>
Diameter = <2.25" (I believe)
Pre-Cats = Not anymore
O2 Sensor location fore and aft of cats = YES, often relocated 2nd O2 sensor to behind main cat
Test Pipes = NO
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = Main = Ceramic 400cel
Cost = stockers plus labor.
<img src="http://www.vastperformance.com/images/piggies_web.jpg">


APR
Diameter = 3.0"
Pre-Cats = NO
O2 Sensor location fore and aft of cats = YES
Test Pipes = YES
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = ?
Cost = $1499


AWE
Diameter = taper from 3" to 2.5"
Pre-Cats = NO
02 Sensor location fore and aft of cats = YES
Test Pipes = YES
Flex Joints = YES (ball/socket joint)
Cat Type = 400 cell per inch Ceramic Matrix
Cost = $1595


JP Engineering
Diameter = taper from 3" to 2.5"
Pre-Cats = NO
O2 Sensor location fore and aft of cats = YES
Test Pipes = NO
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = Carsound (400cpi Ceramic)
Cost = $1350
ANYONE HAVE A PIC?


Milltek
Diameter = 2.3" (60mm)
Pre-Cats = NO
O2 Sensor Location fore and aft of cats = YES
Test Pipes = NO
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = continuous stainless steel cores 200cel
Cost = $1865


MTM
Diameter = ?
Pre-Cats = NO
O2 Sensor location fore and aft of cats = YES
Test Pipes = ?
Flex Joints = YES (not sure what type)
Cat Type = ?
Cost = $6900 (part of there Stage 3 package)


RS4
Diameter = 2.3inch inner diameter
Pre-Cats = YES
O2 Sensor location fore and aft of cats = YES
Test Pipes = NO
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = Precat-precious metal, main cat=ceramic 400cel
Cost = $3000 or $1200 used.


Tantrumwerks Split Downpipes
Diameter = merge into a 2.5" pipe
Pre-Cats = NO
O2 Sensor location fore and aft of cats = YES
Test Pipes = yes
Flex Joints = YES (metal mesh/bellows-type)
Cat Type = ???
Cost = $1550 for stainless w/cats (other options available)
other note: divorced wastegate downpipes


Autospeed
Diameter = 3.0inch, tapers to 2.5 for most aftermarket cat-back exhausts
Pre-Cats = NO
O2 Sensor location fore and aft of cats = YES
Test Pipes = YES
Flex Joints = 3" Stainless Steel Braided Flex Joint.
Cat Type = 400cel ceramic cats
Cost = $1549.


GHL Downpipes
Diameter = 2.5
Pre-Cats = NO
O2 Sensor location fore and aft of cats = YES
Test Pipes = YES
Flex Joints = 3" Stainless Steel Braided Flex Joint.
Cat Type = ???
Cost = $1279

• http://forums.audiworld.com/s4/msgs/718930.phtml

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Matt's Mad Beast: Blouch K04/997 hybrids, RS4 intake&cams with port-matched heads, 60lb injectors, the rest of the supporting cast
EPL Tuning v4.0 93oct 463whp (70F) April 2007
EPL Tuning v3.0 93oct 460whp (40F) Jan 2007
EPL Tuning v2.0 100oct 480whp Jan 2007
EPL Tuning v2.1 100oct program 06-23-2006
471whp @ 7000rpm, 449 lb/ft @4023 rpm

EPL Tuning v1 93oct progam 04-05-2006
418whp @ 7150rpm, 430 lb/ft @3853 rpm

EPL Tuning v1 93oct prog w/100oct (no adapt) 5-13-2006
437whp@6714 427wtorq@3884

Custom Tuned by EPL


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