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The Audi ur-quattro MB Engine

The five-cylinder inline turbo-charged MB engine fitted to the European models of the Audi ur-quattro in the 1988 and 1989 model years was stunning for its time and one of the best designs Audi ever produced. A significant number of MB-engined cars are used as daily drivers and many have reached the 200,000-mile stage. As of February 2006, my own engine has run 208,000 miles and an identical engine in my Type 44 has 309,000 on the clock. Not bad for 200bhp turbo engines that spend their lives working hard.

I have produced this page to supplement Audi's service information with data gained as a result of experience - maintaining several MB engines on a frequent basis, seeing a great many more occasionally, and stripping and rebuilding a specific high-mileage example (engine MB001293).

Some words of caution to the would-be amateur mechanic: The engine bay of an ur-quattro was not originally designed even for a turbo engine, much less an engine of the MB's complexity. It is perhaps the world's most cramped and worst organised engine bay, with hours of disassembly required to access some components. It's necessary to remove the front right headlight, for instance, to change a bulb or the air filter - the exhaust manifold nuts cannot even be inspected unless the whole of the fuel injection system and the inlet manifold are removed. There are no Haynes or Bentley manuals available, and original Audi documentation is now long out of print, error-prone and occasionally only available in German.

Audi MB engine - Theory of Operation:

The MB engine uses well-proven hardware. The I5 (inline 5) cylinder block design was around ten years old when the MB engine was first shipped - the hydraulic lifters were a novelty in the ur-quattro but had been used over three years before (in essentially the same cylinder head) to power cars as prosaic as the VW Passat (USA - Quantum). The hardware, fuel supply and ancillaries are almost identical to the MC engine used in the US Audi 5000. There essentially three modes of operation, not including OFF.


Not starting:

The MB's ECU performs a number of checks during the start sequence. Amongst these are using the flywheel pegs to make sure the engine is rotating the right way, checking the Hall sensor to distinguish the compression/power TDC from the exhaust/induction TDC, checking the coil's resistance, etc. If the ECU is not satisfied by these checks, it will store fault codes and refuse to start the engine.

Poor running:

NOTE: Some of the following tests place considerable strain on the engine. One of the potential causes of poor running is physical damage to the turbine, and stress-testing can cause further damage or even cause shrapnel to be sucked into the inlet manifold. Even though the probability of this is very small, it is recommended that the turbo-to-intercooler hose be pulled off the car and checked for metal fragments before proceeding.

Perform the output tests and pull the fault codes after a reasonably long run - at least ten miles. Address any issues found in the order that they are presented, and repeat the test run between fixes. Once the ECU is storing 4444 or no codes at all attach a boost gauge, a wastegate frequency valve light and a manual WOT override switch (if available) and repeat the test run. The boost test is performed from 35 mph in third gear without applying the brakes. If possible, the engine should be run right up to the fuel pump cut-out at redline. The boost gauge should show an initial over-boost spike (which is not measured) followed by boost pressures up to 0.69 bar relative - the wastegate frequency valve should flicker during acceleration. If this does not occur and the manual override switch is available, repeat the test run closing the WOT switch manually during acceleration. Pull the fault codes after each run.


A great many people seem to want more out of their engine than it currently delivers. There are three things to be considered before embarking on a program of modifications:

One of the major problems is the heritage of the WR engine. The earlier WR is a completely mechanical design - 'chipping' is extremely easy because the chipper doesn't need to worry about ECU control of either boost or full load enrichment. A stronger wastegate spring and a voltage divider on the ECU's pressure sensor cost only pennies and can be sold at huge profits. This technique works on the WR engine because full load enrichment is entirely mechanical, and many chippers use it on the MB engine - but the engine will receive little or no full throttle enrichment above about 4500rpm.

What this means is that the engine will run very lean (and hence hot) right at the point where it produces most power. Since cylinders 5 and 4 have a tendency to leanness in the MB in any case, this greatly increases the chance of melting a hole in one of the rear pistons.

There's another problem - controlling boost. Unlike the WR, boost control in the MB uses a servo system - the ECU is able to switch either boost or vacuum to the upper wastegate chamber to control the level of boost. The boost response of an MB engine is not linear - when full throttle is first engaged, boost is deliberately driven high to minimise turbo lag. Later, it is modified and can be significantly reduced by the ECU if it feels it sees a problem. These controls are buried deep within the actual code executed within the ECU - they are a mystery to most chippers who are not Audi specialists.

So the chippers modify the ECU in a very cheap and easy way - by putting a voltage divider circuit costing only pennies on the output of the ECU's boost sensor. This 'fools' the ECU into generating higher boost pressures until it sees the voltage from the sensor corresponding to the boost it is trying to generate. The engine, however, is seeing much higher boost.

Fooling the ECU is not a very good idea. For one thing, it may defeat the ECU's safety cut-off of the fuel pump if something like a hose failure (manifold to wastegate centre chamber, for instance) causes a sudden and massive overboost. For the second, the ECU derives ignition timing in part from boost levels (this is the only function of the WR's ECU). And for the third, it causes incorrect values to be calculated for driving the fuel frequency valve.

The first problem could be countered by adding a simple boost limiter to cut the fuel pump supply. The third can be biased to some extent using the CO adjustment in the metering head, although this causes the air mass sensor cone to be biased to the wrong point. There is no solution for the second problem other than reprogramming the timing tables in the ECU - and no UK chipper is currently doing this.


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