Audi 2.5 TFSI (five-cylinder)

Audi's turbocharged inline-five — the engine of the RS3 and TT RS, and a deliberate revival of the five-cylinder layout the company built its rally reputation on. It has won International Engine of the Year in its class repeatedly.

The layout is the point and the diagnostic frame. A five-cylinder's firing order gives it an odd-cylinder-count vibration signature and a distinctive sound, and its misfire arithmetic differs from a four or a six — which matters when reading a misfire counter or interpreting a rough-running complaint by ear. The later aluminium-block version dropped substantial weight over the original iron-block engine, and the two are quite different to work on.

In service the themes are those of a very highly boosted direct-injection engine: intake-valve carbon, high-pressure fuel-pump wear, and heat management that leaves little margin. These cars are also modified as a matter of course, so establishing the software and hardware state is the first question on any knock, boost or misfire code, ahead of any component test.