Quote:
Originally Posted by V1H
Hi Tony, thanks for your helpful explanations. 
I still have some questions.
I notice a distinctive design difference between your 2.5" and 3" TDX.
The 3" seems to use 3" TP in place of the downpipes, with the catalysers (100cell) now moved downstream into the lead pipes. On the 2.5" the cats still sit in the downpipes (factory positions), the lead pipes are thus straight (or resonated).
I wonder why the cats moved into the lead pipes. Is it just more temperature-friendly? And since the 3" uses testpipes only, does the O2 bung extension on the TPs still prevent CEL?
I'd be all up for going 3", not 2.5" if the only difference is better Turbo spool.
Any sound differences considering both non-resonated?
Most importantly I still wonder whether the 100cell cats pass M.O.T. here in the U.K.
Problem is I don't know how modern cars are emission tested (I've had an OBD1 Prelude before) and my Z is still too new for M.O.T 
Do they actually sniff the emissions on the tailpipe (then the 100cells would surely fail?) or does the ECU merely give a pass/fail in which case one could trick the ECU into satisfaction? |
Don't mention it!
There is no difference in design between the 2.5" and 3" versions. The 2.5" version can also have a full down pipe starting at the exhaust side of the turbo. What we do is start the down pipe in 3" and than smoothly taper it down to 2.5". You can run the 100 cell cats in either system. They are moved down further in the exhaust stream to reduce the amount of heat that they will see. They will hold up much better in that location and being that they are 100 cell, they will do very little to hinder performance. Mainly they quiet down the exhaust note and help reduce the raw fuel smell coming from the tail pipes.
It is a safe bet to say that these cats will not pas your M.O.T. inspection considering that they are a race cat.
The "J" bend anti foulers are in place to trick out the ECU and keep the CEL off.
In the end, the 3" will be the way to go if you want to maximize turbo spool and reduce restriction. Achieving that zero psi back pressure on the exhaust side is imperative to maximizing performance.
Think that this covers it for now.
Thanks, Tony