I've been looking to do this too for a long time. Once I've got my car perfect in every other way, a custom exhaust is on the list.
I'd be looking for an exhaust that makes no noticeable difference under light throttle, so as not to spoil the tranquillity and refinement at cruise, but one that will let the roar and rasp of the V8 escape when driven enthusiastically. I'd also like the exhaust to release a few more horses, if possible, by improving exhaust gas flow, for which reason I'm considering a custom exhaust that has twin pipes, all the way back, to keep gasses from the two banks separate.
I've spoken to a number of friends and acquaintances in the trade, and one company that has been recommended a few times is
Pipe Werx who, I'm told, are particularity knowledgeable in all things VAG. Might be worth giving them a call if you're not based too far away.
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Mark
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2002 FE S8 Ebony Black Pearl
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Cars Owned:
The Tesla Era: 2020 Model S Performance Ludicrous+ (present)
(Black, with all black premium interior and carbon fibre décor, 21" sonic carbon twin turbine wheels and FSD capability)
The Audi Era: '97 A8 4.2 (Ming Blue) --> '96 A8 4.2 QS (Dark Green) --> '02 FE S8 (present)
The Citroen Era: '84 BX 1.6 RS --> '89 BX 1.9 DTR Turbo --> '94 XM 2.0L Turbo --> '96 XM 2.0L Turbo Exclusive --> '00 Xantia Activa 2.0L Turbo
The Banger Era: '76 1.2L Lada VAZ-2101 (Ruski Fiat 124) --> '80 1.7L Morris Ital HL, finished in Ermine White and Rust