The chain driven V8 is the B6 S4 is pretty good. 54mm (IIRC) shorter than the belt driven version but otherwise very similar. The chain is at the back of the engine, the reason for this is that due to being supported by the gearbox the crankshaft vibration is less pronounced than it is at the front of the crank where it has no support. This meant that the chain could be single width rather tha a twin duplex chain as the length was paramount to fir that, lovely, warbly V8 in the nose of the A4 chassis - that's why the A8 4.2 D3 of simalr vintage still had belt drive.
The issues arise if, as suggested, the chain or tensioners have problems. The chain tensioner on the right hand bank near the top tends to be a weak point in my experience but still Ive not seen many actual failures, usually just a brief rattle at start up as the oil feed non retrun valve fails in them. So long as the cam timing and variation is fully operational they are OK. If any cam is out of time it can mean the chain has stretch, but again, this seems rare.
If however, the chain syste, does need work it obviously requires engine removal to rectify as its at the back of the engine so not an easy job just with gearbox removed!
The AHC 340bhp 32v and AVP etc 360bhp 40v are all great engines. Only ever seen block wear if cambelt has failed casuing impact damage and ring damage. The cam link chain timing variators on the 40v do have issues, either with the oil strainers blocking causisng poor operation or the plastic guide guard can wear and come clean of inside the head. Either of these faults can be picked up upon inspection though and pretty easy to remedy. Besdies that, oil coolers are obviously an issue and Air Mass Meters are dear but great engines all in all.
The FSi V8's are wonderful but bring a whole load more issues! The increased fuel pressure (130BAR versus 3BAR!!) means they employ twin, cam driven mechanical fuel pumps. These can have internal seal issues, allowing petrol to enter the heads and contaminate the oil, and fuel pressure suffers causing high correction factors and missfires under load. They also require more interms of wiring and RS4 V8's use 2 engine ECU's but still, if you're happy to do the work, revving a V8 to 8500rpm is worth it!!!
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