Free guide · quattro GmbH & Cosworth · 2002–2004 · 4.2 V8 biturbo
Audi RS6 C5: what to check before you buy
What this car is, and the six questions that decide whether the one you are looking at is a good buy.
In this guide · 6 chapters
Who built it, and what they built
In 2002 Audi took an A6 — the ordinary one, the one your accountant had — and gave it an engine built with Cosworth.
That is not a figure of speech. The RS6's V8 was developed and manufactured by quattro GmbH together with Cosworth Technology, in Germany and in England. Cosworth is the name on the cam covers of Formula One engines that won championships — although by then the racing half belonged to Ford, and the engineering half, Cosworth Technology, belonged to Audi itself. For two years that half was building the engine for an Audi estate that looks like a taxi.
The engine
Engine code BCY: Audi's 4.2-litre 90° V8 (4,172 cc), developed from the S6 engine, with two KKK K04 turbochargers and two air-to-air intercoolers. 450 PS at 5,700 to 6,400 rpm and 560 Nm from 1,950 rpm.
And then the detail that tells you what kind of engineering this was. The S6 engine it was built from ran 11.0:1; for the RS6 the compression came down to 9.8:1 — Audi's own training manual for the car says it in one line. For a turbocharged engine that is still high.
The Plus, built in 2004, took it to 480 PS and 580 Nm with a remap. 999 of those were made.
One transmission, and it is the reason this guide exists
The ZF 5HP24A five-speed tiptronic, with lock-up in every forward gear, and a Torsen T-2 centre differential. There was no manual version of this car.
Five speeds, 560 Nm, and more than 1,800 kilograms. That gearbox is working, permanently, and the part of it that does the working — the torque converter's lock-up clutch — is the failure this car is known for. It is not the largest single bill on the car; a pair of turbochargers is more. It is the one that is most often reported, and the one buyers most often fail to notice, and those two together are what make it expensive. It gets a chapter of its own, and if you read only one chapter of this guide, read that one.
And two more things worth knowing before you look at one
This is the car Dynamic Ride Control debuted on. Hydraulic lines linking the dampers diagonally, with a central valve on each diagonal, so the car resists roll without stiff springs. Clever, and twenty years later it leaks.
The brakes are not A6 brakes. Eight-piston Brembo monoblock calipers at the front on 365 mm discs, ATE single-piston at the rear on 335 mm. Wonderful, and priced accordingly.
8,081 were built between July 2002 and September 2004.
How the figures in this guide work
How much work each job is, not what it costs. Every point in this guide carries an effort — an hour or two, half a day, a day, two or three days, or a week or more in the workshop — plus a line saying whether the money sits in the part or in getting to it. Where a documented hour count exists for this car, that is there too.
There are no prices, and that is deliberate. What a job costs depends on which country you are in and which workshop you walk into, so a figure I cannot verify is worse than no figure. If you want money, ask any workshop one question — what is your day rate? — and every effort in this guide becomes your own number, in your own currency. One question, and the answer is actually true for you.
None of this is advice about one particular car, because I have not seen the car you are looking at. Have every one of them inspected before you buy, by someone who knows this engine.
The six questions that decide whether this car is a good buy
These cars are now around twenty years old, and on this model the difference between a good buy and a very expensive one is larger than on almost anything else of its era. It comes down to six questions.
1 · The gearbox, and specifically the torque converter.
This is the one. Not because it is the largest bill — a pair of turbochargers is more — but because it is the most reported failure on the car and the one buyers walk past. It has a first symptom, and it is subtle enough that most buyers drive straight past it. There is a specific way to provoke it, and it takes two minutes.
2 · Dynamic Ride Control: original, leaking, or converted?
Three different answers and three different prices. The system leaks with age, and a good number of these cars have been converted to conventional dampers — which is a legitimate fix, not a bodge, and sometimes worth more than an original system on its last legs. What you must not do is guess which one you have from the ride alone. On this car the wrong guess is a much bigger number.
3 · What the boost is doing.
Two turbochargers, and one that is no longer making its number drives exactly like a healthy car with a split hose. You cannot tell them apart with your right foot, and the difference between the two is a set of hoses against a pair of turbochargers. How you do tell them apart is in the buying guide.
4 · The intercoolers.
Their own item, because a leaking intercooler is the smaller job, and what it can lead to is not: turbochargers. It is one of the known routes to turbocharger failure on this car, and it is preventable if somebody looks.
5 · A seal at the gearbox.
A driveline seal at the gearbox weeps, and it does not leave the evidence you would expect, which is why it gets missed. It is not a large bill. It is just one nobody looks for.
6 · What has already been paid for — and here the usual advice is inverted.
On this car, the specialists say it plainly: take the high-mileage example with a documented, properly maintained gearbox over the low-mileage one without that history. On most cars paperwork beats mileage. On this one it is not close.
What the honest answer to all six looks like
On a car with history, where the known items have demonstrably been done, an owner gets through a fraction of the list in the first two years, and none of the heavy jobs — not because things break, but because things arrive that were always going to arrive.
The buying guide takes each of these six apart, adds the things that are simply twenty years old, and gives every one of them how much work it is, what tells you about it at a viewing, and whether it has to be done now or can wait years. It ends with a card you print and take with you, in the order you do it, including the two moments at a viewing that come only once.
Which years, which specification
The RS6 C5 was built from July 2002 to September 2004 with one engine and one transmission: the BCY 4.2 twin-turbo V8, 450 PS, and the ZF 5HP24A five-speed tiptronic, driving all four wheels.
There was one transmission, and it is an automatic. That is your fastest filter on an advert.
Saloon and Avant. Both were sold and both drive the same. The Avant is the one people picture, and it is the one with the extra corrosion check in this guide.
The RS6 Plus, 2004, 999 cars — and it was built as an Avant only. 480 PS and 580 Nm from a remap, sold in the last six months of production. So a "Plus saloon" is not a car that exists: if you are offered one, you are being offered a standard saloon with a description attached. And if a seller says Plus about an Avant, ask for evidence rather than for a description — the difference is a real premium, and the standard car is not worse to own.
And the confusion to clear up, because it costs money. An RS6 is not an S6 with a chip. The S6 C5 has the naturally aspirated 4.2; the RS6 has the Cosworth-built twin-turbo BCY, different brakes, different suspension and different bodywork. There are also S6 cars wearing RS6 parts, and on a car at this price checking is not suspicion, it is a standard check: the engine code BCY on the sticker on the toothed-belt guard, the charge-air pipework to two intercoolers, eight-piston Brembo calipers behind the front wheels.
What you must establish per car:
Whether the DRC is still DRC. Original system, or converted to conventional dampers. Both are acceptable answers. Not knowing is not.
Whether it has been remapped. On a twin-turbo V8 already making 450 PS with a five-speed automatic behind it, more torque is not free — it is spent on the gearbox. Ask, and ask for the map to be shown.
Beyond that, look for a car nobody has modified. Factory wheels, standard ride height, the original exhaust. On this model an original car is more often a sign of maintenance than of restraint.
What it is worth
This is the hardest thing to establish on your own and it does more than anything else to decide whether you buy well. I am not going to give you a price list — it varies by country and by month, and an out-of-date number is worse than no number. I am going to give you the method.
On this car, compare on the gearbox first and everything else second. That sounds reductive and it is the single most useful sentence in this chapter. A documented gearbox history — fluid, and any converter work — moves the value of one of these more than mileage, colour or body style.
There are three kinds of car for sale, and each one deserves a different offer.
The documented car. Full history, invoices tied to readings, and an owner who knows when what was done. Asking price at the top of the market, usually justified.
The tidy car with no paperwork. Drives well, looks well, and the owner says everything has been done. The hardest of the three, and on this model the most dangerous, because the two things you most need to know — the converter and the DRC — are exactly the two that leave no trace outside a folder.
The project. Known work, price adjusted to match. Be careful here in a way you would not be with a cheaper car: a project RS6 with a tired gearbox and leaking DRC is not a project, it is two four-figure bills with a car attached.
How to set the three against each other. Take the asking price, work through both lists, and add up what is coming in the first two years. Call that the entry price. Then compare entry prices rather than asking prices. On this model the gap between the cheapest and the best of these is frequently larger than the price of the cheapest one.
Where to look: your country's big classified sites, the RS specialists, and the auction sites. Follow the market for a month before you go and see anything. You do not need to know what one is worth; you need to know what is happening — which cars sell quickly, which sit for months, and at what price.
What it is actually like to own one
The thing owners say first is that it does not feel like 450 PS until it does. It is quiet, heavy, softly sprung by modern standards, and completely undramatic at any speed you would use in traffic. Then you flatten the throttle and 560 Nm arrives from under two thousand revs, all four wheels take it, and the horizon changes shape. There is no wheelspin, no fuss, and no noise to speak of. It just goes.
That combination — a car nobody looks at, doing something absurd, in near silence — is the whole reason to own one. It is also why they were bought by people who did not want to be noticed, which is why a good one often has a real service history behind it.
Then it goes back to being an A6. Four doors or a tailgate, a boot you can put a family's luggage in, and a ride that eats motorway. The Avant will take a dog and a bicycle. This is a genuinely useful car, which is not something you can say about most things with these numbers.
What it costs you in attention. More than a modern car, and concentrated in two places. The gearbox wants its fluid and it wants you to notice its first symptom early. The DRC wants looking at, and one day it wants a decision. Everything else is ordinary running.
And it drinks fuel. Four point two litres, two turbochargers, four driven wheels and nearly two tonnes. Be honest with yourself about that before you work out what it costs to run.
What it gives back. A car with a Cosworth-built engine that has stopped losing value the way a modern one does, and that will be rarer in ten years than it is now. That is not an investment and you should not call it one. It is an engine Cosworth built for an estate car, and nothing about that sentence is going to happen again.
The buying guide — €24
Everything above is what the car is. This is what you check.
Six faults that belong to this engine, taken apart one by one, with how much work each repair is and how to tell which of them you are looking at. Twelve things that are simply twenty years old, every one with how much work it is, what tells you about it at a viewing, and whether it has to be done now, within a year or two, or not at all. Six places to check for corrosion on this bodyshell, including one on the Avant that nobody looks at. The gearbox, the torque converter inside it, and the two-minute test that tells you whether this car carries a four-figure bill. The viewing in twelve steps, including the two moments that come only once. The sentences you say to the seller, and how to build an offer out of what you found.
And a card you print and take with you — fifty-one lines, in the order you do them. That comes as a separate file, because nobody wants to print a whole guide to take a few pages to a car park.
This is what one of the twelve looks like
4 · Front discs and pads: eight-piston Brembo, 365 mm
Half a day · expensive part, easy access This is not an A6 brake service. Monoblock Brembo calipers on 365 mm discs, made for one model over two years, and priced the way scarce performance parts are priced. Good alternatives to the original discs exist and it is still a real number. How you notice it: a lip around the edge of the disc, scoring, and thickness measured on the lift rather than guessed through the wheel. Also look at the caliper faces for weeping and at the pistons for corrosion — seized pistons on a caliper of this value are a repair, not a replacement.
Expect it within a year or two even if it just about passes now. And budget it properly: this is the item buyers of these cars most often underestimate.
And this is how the card reads
- 1 · The cold start · do nothing else
- 2 · The engine bay, cold
- 3 · The tops of the front dampers · the quickest DRC check, in two minutes
- 4 · Round the outside
- 5 · The boot
- 6 · Inside, ignition on
What €24 buys, put next to what it saves
A torque converter is two or three days in the workshop. A pair of turbochargers is two or three days and the most expensive parts on the car, and a leaking intercooler is one of the things that takes them there. The guide is €24. It will not stop any of that from being needed — but it does tell you which of them you are looking at before you agree a price, and that is the whole of the difference between paying for a car and paying for someone else's postponed maintenance.
Why there are no prices in it
Because a price we cannot verify is worse than no price, and because a price would be wrong for most of the people reading this anyway. Workshop rates across Europe differ widely, and so do parts prices. A quote from a specialist in one country tells you nothing in another.
What does not change is how much work a job is — that follows from what has to come off the car. So every point in the guide says whether it is an hour or two, half a day, a day, two or three days, or a week or more in the workshop, with a labour figure where a source gives one. And it says whether the money sits in the part or in getting to it, which is what lets you tell a fair quote from a bad one.
If you want a figure, ask any workshop one question — what is your hourly rate? — and every effort level in the guide becomes your own number, in your own currency. One question, and the answer is actually true for you.
How this guide was put together
Because you should know before you pay for it.
- Every fault and symptom comes from a source the guide names, the manufacturer's own documents first. The effort levels are our assessment, unless a source gives the hours.
- Where the usual buying advice is wrong for this car, it says so: on an RS6 you take the documented high-mileage example over the undocumented low-mileage one.
- The effort of every job is counted item by item and checked against the list before every edition.
- It says as plainly what you can leave alone as what you have to do. Four of the twelve age points start from leave it.
Leave your email address at the checkout and both files arrive straight away: one guide and one card, as two pdfs. No subscription, no account.
The full guide · €24
22-page guide, A4 and phone · 4-page inspection card · instant PDF · VAT included
Have every car inspected before you buy by someone who knows this engine. This is general information about the model and not advice about a specific car.