One more update from the US.
4th and final revision of the y-pipe fabricated and installed yesterday by AutoMaster Al @ AMR!
Rasp is gone and performance almost as good if not as good as full catless.
http://legacygt.com/forums/showpost.php ... count=1626With the revisions we've gone through, our findings are similar to 'Daytona's' from the 3.0R Header thread.
viewtopic.php?f=65&t=17262&start=660#p4052431. The header runner flanges are angled a bit different depending on if the header spacers are used or not to clear the FSB.
This is useful for consideration when designing a custom y.
2. Stock cats will start to fail (its just a question of time) with the headers installed. Exhaust flow and EGTs (especially without a tune) are too high for them to take and they start to collapse, substrate separates from the housing walls. Symptoms: initially some bogging down, sluggish performance, eventually overheating front substrate surface, back-pressure spikes, random mis-fires, random knock. This can be monitored too via logging (BtSSM, FreeSSM,
3. The solution to 2 is either to go a. catless or b. replace OEM cats with larger ID metal core (higher temp tolerating) units. Catless is easier and cheaper, nets a slight performance gain, but calls for a retune (to realize it and to run safe), but results in a RASP >5K RPM that doesn't sound good at all.
Catless: Exhaust fume smell is minimal (potentially thanks to the tune). Using baffled resonators in place of the cats causes pinging within the y-pipe and doesn't eliminate the rasp, using perforated straight through resonators doesn't eliminate the rasp either but eliminates the pinging.
Catted: Using 100 cell race/metal cats only partially eliminates the rasp.
200 and 200+ cell cats eliminate (or rather muffle) the rasp enough to make it livable, streetable.
4. With the y-pipe design figured out (2.5" ID piping throughout, 2x 2.5" 200-cell metal cats), the location and way it mates to the catback is also different from car to car. All depends on what exhaust we're running (stock 3.6R CBE, modified LGT CBEs in either 2.5" ID, 3" IDs, resonated, non-resonated, flanged or not). Depending on the CBE, the inlet can be offset by as much as an 1-1.5" making coming up with a PnP y-pipe tricky.