Forum Chat #57 – 4.3 V8 Vantage exhaust manifold on a 4.7?

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Welcome to BamfordRose and another forum chat. This week I’ve seen a forum chat talking about fitting 4.3 exhaust manifolds to a 4.7 V8 Vantage to eliminate the primary catalyst that the 4.7 exhaust manifold has where the previous 4.3 V8 doesn’t. It just has a downstream secondary catalyst.

So, if we look at the post, it says, “Anybody interested in these 4.3 exhaust manifolds, ideal for fitting to 4.7 to replace the restrictive catalyst manifolds”. So, now we zoom in and just show the picture of the manifolds. Obviously, this is the left side and right side. And where you can see the red circle which is highlighting the exhaust gas recirculation pipe, that’s about the location on the 4.7 manifolds where the catalyst sits. You can see the green line that I’ve drawn in there shows that the primary pipe length is actually quite long. And if we remember that when we look at the 4.7 exhaust manifolds, which I show here in their entirety.  So, now we’ve got a shorter primary pipe length leading into the collector cone and the catalyst.

In this next picture, I’ve drawn the primary pipes in red, the collector area in blue, and the catalyst is there with a green squiggle on it. So, when the factory went from the EU-4 exhaust manifold from 4.3 to the EU-5 exhaust manifold for 4.7, they achieved that emissions level change but the power output of the engine remained the same at 420 bhp. Now obviously, the inclusion of a catalyst in the location where I’ve marked it with green pen, you would think would be restrictive and it is. But the factory maintained 420 bhp by losing power from the addition of that catalyst, but gaining it back because the primary pipe lengths are now tuned a bit better, in the red area, leading to the blue area of the collector cone. So, this results in a net neutral power situation. The primary pipe length and collector design is aiding exhaust gas scavenging. And although, there is a catalyst present, the diameter of it is quite big.

So, like this post would lead you to believe and you’d naturally assume the statement to be correct. To get rid of your restrictive catalyst on a 4.7, you can fit these 4.3 exhaust manifolds and they will mate up to the downstream catalyst of the 4.7 exactly the same as your 4.7 manifolds would do. So, it will fit perfectly fine. But because the 4.7 manifolds have that better primary pipe and collector and gas flow arrangement, to take the 4.3 manifolds and then fit them to your 4.7, although you are removing a primary catalyst from the system, you’re not going to get any power increase.

So, instead of fitting a 4.3 exhaust manifold to a 4.7 thinking that it’s going to get a power increase, actually the correct thing to do if you were after a bit of a budget performance increase would be to remove your existing 4.7 exhaust manifold with the catalyst in the manifold and actually remove that primary catalyst from the manifold.

What we do here is put that exhaust manifold on a jig and at that seam line, you can see where I’ve colored the blue line, showing the collector can going into the catalyst; we cut along that weld seam line, then we get easy access to the primary catalyst which we can knock out and remove, then fit the exhaust top section back onto the jig and re-weld that top seam line back up. So, basically the manifold looks totally standard from the outside.

So, you’ll get a performance increase by removing the catalyst. That will need an ECU remap to stop error lights coming on the dashboard and now you can reoptimize spark and fueling for best power. And you can finish it off with a high flow 200 cell secondary catalyst. That’s a nice neat way of getting a budget performance increase instead of going for exhaust manifolds and catalysts.

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