355 CATs - can someone explain the set up to me?

Nope. To get your MOT this is the hurdle to get over plus the actual emissions test. As noted your primary catalytic convertors must be in place AND they have to pass the emissions test. A check engine light on flunks the Emissions test.

Do you know if the car will pass the Exhaust and the Emissions test now? Most of these cars catalytics are well passed their point of effectiveness. New replacements from Ferrari are about $5,000 USD.......each.

2.5 Exhaust system
The exhaust system will be inspected to check that:

• it’s secure and complete
• a catalyst isn’t missing where one was fitted as standard
• it’s without serious leaks and not too noisy

 

The F355 has two different types catalysts. The first ones are the primary cats. Those are the ones you remove to install test pipes. It is my understanding they are a ceramic based catalyst which can break up in the exhaust. That is why I removed mine.
When the bypass valve opens in the higher RPM range, the exhaust is routed through the louder, second stage. That Y pipe contains a catalyst which is, I think, made out of stainless steel. Not many people replace those as they don't go bad.

As INRange has said, removing the primary cats WILL cause you to fail emissions. The great news is that if you get the 2.7 there is no post cat O2 sensor to give you a check engine light. You can just install the test pipes and away you go. So if you decide to do this (and you should because the 2.7 is THE BEST one to get) make sure you pass emissions first before deleting the cats!
I have been told the car makes a little more power with the cats on. But I prefer the sound with the cats out. An added benefit as I am pretty sure it makes the car shoot flames.

I hope you get the car. Enjoy!

 

The exhaust will always flow through the main cats. The exhaust valve opens and allows exhaust to flow additionally through the secondary cat pipe.
2.7 (1995 only) does not have post cat O2 sensors and can have a cat delete without a CEL. I have never tested for horsepower but it sounds like 50 extra so I say its worth it. (I know it is not any more HP)
The O2 extenders with cat material in them rarely prevent a CEL in my experience and I have used them on a number of cars.
It is my opinion that if you need to pass emission testing, keep the cats and add headers and the muffler of your choice. I do think the aftermarket headers make them sound better.
Again, this is just my thoughts. Your thoughts may be different, some assembly required, results may vary, batteries not included, not actual size.

 

With the cat delete pipes, you are essentially always in by-pass mode (there is nothing in the cat) so the sound is pretty loud. There is not going to be much exhaust flowing through the Y pipe since there is no air resistance in the main cats (there is nothing there).

 

OP you are saying valves. The OEM system has one valve, and the preferred Capristo 355 exhaust also has one valve...using either the OEM valve or their replacement version. I would not recommend using their muffler with two valves. It does not have the same crisp F1 sound that their single valve systems have. Even with oem exhaust manifolds and catalysts, the Capristo will still sound glorious, loud and amazing. It did on my 355. Removing catalysts simply increases the volume even more. Go with catalysts initially...you can always remove them later if desired.

 

Lots of info on all this in the archives, only point I'll add is that at least in my experience there was very little noise difference between cats and bypass pipes. I changed nothing other than the cats, and on my stock muffler car, I'd say it has a 10% louder exhaust note now, if that. Barely noticeable.

 

Sorry, these were the primary cats. Not the Y-pipe.

 

Mark, most people don't change the y-pipe. They don't go bad.

This guy clearly changed out his cats for test pipes. Maybe this will help you:

 

May have to do with the stock exhaust. I had stock catalysts with a Capristo exhaust...very loud. My friend had cat delete pipes with his Capristo and it was literally ear splitting when he would pass me. I actually thought it was too much of a good thing.