Got to ask why does AM fit six cats??
If taking them out and replacing with just two gives so much more power and weight saving, but still meet UK MOT emission limits. Why wouldn't you fit two to start with. It can't cost more can it?
The V12 exhaust system splits the engine into 4 groups of 3 cylinder.
So, cylinders 1/2/3 have a catalyst in the manifold, as does 4/5/6, 7/8/9 and 10/11/12 - making 4 catalysts in total (2 per exhaust manifold for each side of the engine). Here is the manifold for cylinders 1-6, each catalyst is housed in the big can.
Each bank of cylinders then has a further, a second catalyst in each downpipe. One catalyst for cylinders 1-6 and on the other side a catalyst for cylinders 7-12 - making a grand total of 6 catalysts.
Here is a pic of one side of the engine with manifold with primary cats and downpipe with secondary catalysts;
In simplistic terms the primary catalysts are required to after-treat cold start emissions to within the legislative requirement, the secondary catalysts then provide the bulk of the after treatment when the engine / exhaust system is fully warm.
The 4.3L and pre Vantage S 4.7L V8 engines do not require primary catalysts and can cope and pass emissions standards with one big downstream catalyst. The slight technicality is that the catalyst is split internally into two catalyst within each catalyst can. So although there is only one catalyst can per bank (bank 1 cylinders 1-4 and bank 2 cylinders 5-8), and on first impression the engine appears to have a total of 2 catalysts, the V8 engine actually has a total of 4 catalysts. Here is a pic courtesy of Jessica when in her standard form;
The Vantage S had to pass more stringent emissions levels and required the primary catalyst to be moved closer to the engine (like with V12) keeping the second catalyst downstream, but Vantage S still has the same total of 4 catalysts.
The problem with catalysts is that the more there are and closer to the engine the more power they will erode.
So, to gain more power the world of aftermarket can come along, better design the exhaust system without care to cost and emissions, total focus being on performance, reduce the total number of catalysts and hence free flow catalyst is born and a bigger grin
Aftermarket can do this because all the exhaust system has to do from release to customer is pass MOT test which is quite easy even with only a low loss catalyst present. However, at point of original sale the manufacturer must prove that on aged components the emissions level passed is significantly 'cleaner' than a basic MOT test which requires more catalyst surface area, often closer to the engine.