Six-cylinder engines are generally available in two basic configurations – the inline-6 and V6. Subaru and Porsche notably offer a third, the flat-6, but we’re leaving that discussion for another day.
Most petrolheads ought to be relatively familiar with the general pros and cons of the inline-6 vis-à-vis its V-shaped counterpart.
The inline-6 layout has the longer history and is generally regarded to offer superior refinement thanks to it being a naturally-balanced configuration. Its biggest problem, however, is packaging – the inline-6’s length complicates installation and particularly difficult to accommodate in a transverse engine setup. Didn’t stop Volvo though, the current S60 and XC60 was once offered with a transverse-mounted 3.0-litre inline-6.
The V6 solves the inline-6’s bulk problem by arranging the six cylinders in a more compact footprint. Early attempts at V6 engines involved lobbing two cylinders off existing V8s, enabling both engines to be assembled from the same line, helping manufacturers reduce overall cost.
The issue with the V6, however, is that it is effectively two three-cylinder engines. Each of the two cylinder banks produce vibrations that cannot be easily cancelled out, meaning that V6 engines inevitably require balancer shafts to achieve acceptable smoothness. Also, a V6 engine requires two valvetrain sets – one for each cylinder bank – adding to the engine’s overall complexity.
A short video produced by the folks at Car Throttle summarizes several key differences between the inline-6 and V6 engine. Some of these differences are common knowledge, others less so.
The video points out that the inline-6’s natural mechanical simplicity over the V6 makes the former easier to maintain with leads and ancillaries all easy to access. A lesser-known disadvantage of the inline-6 is that its lengthened block and camshafts are more susceptible to flexing in operation compared to more compact engine designs. An inline-6 also raises a car’s centre of gravity, not the kind of trait you’d want for cars with ambitions of sportiness.
For a second opinion on the debate, we found a six-minute video produced by US-based Mercedes specialist Kent Bergsma, who gives a mechanic’s viewpoint comparing the M103 inline-6 from the W124 E-Class against the M112 V6 in its W210 successor.
After a somewhat lengthy intro, Bergsma highlights a couple of maintenance issues with the older M103 engine – its accessories, particularly the water pump and belt tensioners, are inconveniently located and difficult to access for replacement. The M103 is built on a cast iron block mated to an aluminium head and magnesium front cover. These dissimilar metals expand and contract at different rates, regularly resulting in head gasket leaks.
On the all-aluminium M112 engine, Bergsma praises its mechanic-friendly configuration, noting that spark plugs aside, all other components are easier to access than in the M103. He also opines that the M112 is a better engine to drive than the M103 – highlighting its greater fuel efficiency, smoothness, and more driveable torque curve.
Packaging advantages along with increasing competence in vibration engineering have seen the V6 become the dominant configuration among six-cylinder engines. BMW is the one manufacturer that is noteworthy for refusing to jump on the V6 bandwagon despite reportedly building a number of experimental prototypes over the years.
A lesser known devotee of the inline-6 is actually Ford, which offers inline-6 petrol engines in its soon-to-be-defunct Aussie-only Falcon and Territory models. Mercedes-Benz is resurrecting its own inline-6 engine as well, its upcoming family of modular engines set to offer both petrol and diesel powerplants in the said configuration.