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.

Yes and no, complex ish question to answer....

The Vantage S has the primary catalysts moved closer to the engine, so naturally you would expect a net power loss. But (catalyst inclusion aside), the exhaust manifold is 'ever so slightly' better designed than previous 4.3 and 4.7L V8 engines, meaning the manifold gains counteract the catalyst loss to deliver a power neutral condition. I suppose that the valves in the airbox is why Vanatge S is 430BHP over previous 420BHP.......

So, removal of the Vantage S manifold for a better one (better primary pipe design) and without a primary catalyst fitted will give a gain, as will removal of the secondary catalyst for a lower loss component. The performance increase this gives will be the same delta on an S compared with a previous 4.7L. However, the S will have the higher headline figure because of the valve airbox fitted as standard. This does means that if the older level 4.7L is updated with the same aftermarket exhaust manifold and catalyst system together with replication of the valve airbox system, the power output will ultimately be the same old 4.7L Vs Vantage S 4.7L.

What must be remembered here is that the base engine of Vantage S Vs prior level 4.7L engine is disappointingly, exactly the same. This fact and the exhaust system technicality between the S and prior level 4.7L engines, and S having no big power gain over previous 4.7L base engine answers the question why AM power upgrade kit is not available for 4.7L engines - it could be as it is simply a bolt-on additive like it is to 4.3L engines (valves in airbox and new ECU software, the inlet manifold is just an accessory and not needed to make more power). The outcome would be N420 or std 4.7L being N440 and greater power output than the newcomer S.

im sure 'tother BamfordMike on 'tother forum area knows the answer

I have a cunning plan forming I will discuss with him in person later in the month