I happen to know the person who imports american cats in Greece. I can ask him for techincal data for several of his cats and of various types (ceramic/metallic, street/race etc) if you want. In his brochure the cats are described with a "maximum horsepower" and number of cells (only for the race ones) indication, either 100 or 200. I suppose that the way cats behave (tube or expansion box) is closely related to their number of cells, right? He is trying to persuade me to try a wild race cat for maximum 450hp but somehow i think that this monster will be too much -or too little in terms of backpressure- for my measly 1.6 TS.
I think that your solution of two cats in the downpipe poses a difficult problem: As you probably know AR's have three lambda sensors, two after the light-off (yes, i looked the term up
) cats and one after the main cat. I was told that the ECU compares the readings from each of these sensors to deduct if there is a malfunction somewhere in the whole emission control system. If you totally remove the main cat how are you going to achieve the same difference in light-off and main lambda sensor readings? There will probably be an error code from the ECU, "Motor control system failure" i think. Of course there are ways of fooling the ECU by interposing resistors between it and the sensor but i don't like this solution and it is probably illegal as well.
On the other hand, as i may have mentioned before i have already have the light-off cats replaced with plain pipes with a big improvement in low/mid-end torque and throttle response and no error codes or emission problems as long as the main cat is in working temperature. Given this, the fact that a metallic race cat can be heated much faster than a stock ceramic one and the difficulty/cost of manufacturing a downpipe with two cats i believe that keeping the main cat instead is a reasonable and less expensive way to go. Of course you are the expert.
I plan to compare your design to the stock one of the 2.0TS exhaust system and maybe a full Supersprint exhaust system from manifold to tailpipe, if i can find its specs. I'll tell you what i find.
PS1: I hope/believe you will benefit from this quest as well, i would not feel good about you spending your time for some stranger over the net.
PS2: I think that this topic should be given a more appropriate title by the mods, how about "Modelling an exhaust system for a 1.6TS with 2.0 cams"?