If you look at the factory exhaust manifold – common to all V12s – you can see how short the exhaust gas runner is for cylinders 3, 4, 9 & 10 and how close it is to the entry cone of the primary catalyst.
Occasionally during the life of the V12 engine, usually due to a defective or aged ignition coil, there can be a slight misfire on one of the cylinders. As the V12 is so smooth anyway – 11 cylinders will carry and mask the misfire – a driver probably won’t even notice. However, the misfire transfers unburnt fuel for each cycle (when there was not complete combustion in the cylinder) directly onto the face of the primary catalyst. The primary catalyst is usually running at 480C (900F) so of course, the fuel automatically ignites upon contact with the catalyst.
In the Aston Martin V12s, the primary catalyst is made of a ceramic material. The uncontrolled ignition of the fuel upon the face of the primary catalyst starts to nibble away and erode the ceramic material of the catalyst brick. The dust and debris caused by this erosion action congregate in the catalyst cone collection area which, as you can see from looking at the OEM exhaust manifold, is very close to the exhaust outlet port for cylinders 3,4,9 & 10.
So how does this debris impact the engine? To pass emissions tests it is common industry practise for inert gas (NO2) recycling to be part of a modern engine’s design. In Aston Martin’s case internal gas recycling is used. A large degree of overlap is set between the exhaust and the inlet cam so the engine will for a very brief time suck back up the inert gas from the exhaust into the cylinder. This inert gas in the case of this problem can also contain the catalyst debris from the misfire.
Some of these ceramic particles will simply be mashed onto the piston but some will get mashed down the side of the piston ring and scuff the bore, the piston and the piston ring. This is when the real trouble starts.
As the oil lubricates away into the sump it will contain what is now effectively ceramic grinding paste. The oil with this paste now spreads around the engine and can fail the crank and cam journals and bearings causing irreversible damage to this componentry and necessitating an engine rebuild.
This is why we at Bamford Rose strongly recommends primary catalyst removal and ECU recalibration as insurance against this problem as the V12 cars age (see Phase 1 and Phase 2 V12 engine upgrades for further details).