Mercedes-Benz M176 / M177 / M178

AMG's 4.0-litre V8, and the engine behind every modern 63-badged Mercedes. Its defining feature is the "hot-V" layout — both turbochargers sit inside the vee rather than outside the banks, which shortens the exhaust path to the turbines and makes the engine far more compact than the arrangement it replaced.

That packaging is also the diagnostic reality. Everything in the vee runs extremely hot and is extremely hard to reach, so heat-cycled wiring, coolant hoses and sensors in that space fail earlier than their equivalents on a conventional V8, and the labour to reach any of them is substantial. Assume access, not diagnosis, will dominate the estimate.

The M178 is the same engine in dry-sump form for the AMG GT. Both are direct-injected and carry the usual intake-valve deposits at mileage; both are also frequently modified, so establishing whether the car is running standard software and standard boost is a reasonable first question on any driveability or knock complaint.

The family has three members and the number says which. The M176 is the standard-range engine — the S 560 and GLS 580 — the M177 is the AMG version, and the M178 is the mid-engined GT's. They share the block and the hot-vee layout; output, cooling and the oil system are not the same, so a specification quoted for one describes none of the others.