‘ The worst B7 RS4 we had was around about 80hp down over stock. That’ s almost a 20 % reduction in power due to carbon buildup’
Audi revised the V8 for its later RS4, which helped reduce carbon buildup, but the BNS in the B7 is still a delightful and impressive engine
The engine is turned to close the next set of valves so another inlet can be cleaned. This process is repeated until all eight inlets and the back of all 16 inlet valves no longer have any black carbon buildup on them and they look factory fresh. Steven is keen to emphasise how particular they are at Regal during the process,‘ We always clean up afterwards to get rid of any residue before we put it back together’.
Regal likes to know just how beneficial its decoking is, too.‘ We do pre- and post-dyno power sweep tests to verify the airflow improvement. We have found a direct correlation between how much power an engine is down and how severe the buildup is.
‘ The worst B7 RS4 we had was around about 80hp down over stock. When you consider it has 420hp-ish as standard, that’ s almost a 20 % reduction in power due to carbon buildup. Whereas the B8s we see have more like 10 to 15hp drops. It’ s a really worthwhile procedure.’
‘ The worst B7 RS4 we had was around about 80hp down over stock. That’ s almost a 20 % reduction in power due to carbon buildup’
It’ s far better than actually using crushed walnut shells, which was the traditional method.‘ There’ s a separate product that is supplied, it’ s a solution they give us to dissolve the granules should some of them end up not where you quite want them to be. If there’ s any residue left behind, we can clean that up too.’
How long is the fix likely to last?
Considering that this is a clean, and that no parts are modified or improved, you have to expect that carbon will continue to collect on the inlet valves and intake side of the head. When it might need cleaning again, though, varies hugely.‘ We’ ve had cars with 100,000 miles where the buildup hasn’ t been as bad as ones with fewer miles. So there’ s not a point where we can say,“ At this mileage, at this age, it will be dreadful”. Every car is different; they’ ve all had different lives and uses.’
As you might expect, looking after an engine with frequent servicing and using high-quality products keeps that carbon buildup to a minimum.‘ The cleaner you can keep it, the better. Regular oil changes help to minimise the amount of oil particulate matter in the inlet tract. Good quality fuel is one of the better things, too.’
Spring 2026 The Engine Rebuilder 57