ufcnancy
Powered by
  • WheelsWheels
  • MOTORMOTOR
  • 4X4 Australia4X4 Australia
  • Street MachineStreet Machine
  • Trade Unique CarsTrade Unique Cars

Inline 6 versus V6 – why are straight-sixes making a comeback?

By Tony O’Kane, 18 Feb 2019 Car Advice

Inline 6 versus V6 – why are straight-sixes making a comeback?

You may have heard that a bunch of carmakers are returning to six-cylinder engines arranged in-line, rather than in a V-shaped configuration. Interesting, but why should you care?

Jaguar Land Rover recently announced that they would once again be putting inline six-cylinder engines into its cars and SUVs, phasing out its venerable range of V6 petrol motors in the process. But why is that important? With both the old V6 and the new inline six displacing an identical 3.0 litres, will you even notice the change from behind the wheel?

It’s a question that also applies to those following developments at Mercedes-Benz, which has also made a similar switch from V6 powerplants to inline sixes. BMW, meanwhile, never moved away from the inline six format. So, why the resurgence of interest in a type of engine that many thought was dead?

Read next: Australia's top 10 engines

Well, though the cylinder count remains the same, the switch from arranging them in two banks (as in a V6) into just one brings some surprising differences. Here are the ones that will matter most to you, the driver.

REFINEMENT

Yep, an inline six is actually more refined than a V6 with the same displacement. In fact, improvements in refinement were one of the main reasons why Jaguar Land Rover decided to switch back to inline sixes (an engine configuration the company had abandoned decades ago in favour of V6s).

In an inline six, each cylinder that’s undergoing a combustion stroke is balanced out by another cylinder that’s undergoing an induction stroke, and with these ‘paired’ cylinders often being located symmetrically around the centrepoint of the crankshaft, there’s very little vibration generated by an inline six-cylinder engine as a result.

V6s, by contrast, do not enjoy the same harmonic advantage.

Read next: Engine oil development explained: Geek Speak

PERFORMANCE

There are certain other advantages to the inline-six configuration – advantages that focus on making power through more intelligent packaging. With turbocharged six-cylinder engines effectively taking the place of bigger V8s in many modern cars, the simpler in-line layout provides more space to put performance-enhancing devices such as turbochargers, superchargers, and their associated plumbing.

A V6 engine, meanwhile, must either find room in the valley between each cylinder head (such as in turbocharged V6-powered Audis) or in the limited space on either side of the engine (Like Nissan’s GT-R) which makes for a cramped and complex turbocharger installation. Shoehorning in other power-adders like electrically-assisted turbos and/or superchargers would be extremely challenging on a V6.

And with performance cars increasingly making use of electrically-boosted turbos and superchargers – often both in the same installation in a lag-reducing sequential arrangement – having a maximum of space to place these things means greater performance potential. It’s somewhat ironic, given one of the big reasons for the adoption of V6 engines decades ago was their compactness and ease of packaging – but that was back in the day when turbocharging wasn’t as commonplace as it is now.

Read next: Mercedes-Benz ditches V6 engines

SOUND

This benefit can vary depending on how an individual car is engineered, but generally speaking inline sixes tend to generate more pleasing exhaust notes than their V6 counterparts. Why? Because having all six exhaust ports on the same side of the engine means they can be merged together in a way that neatly separates the exhaust ‘pulses’ from each cylinder, something that’s harder to do on a V6 (but not impossible). Result: sonic bliss for straight-six performance cars.

COST AND COMPLEXITY

Here’s the REAL reason why inline sixes are making a comeback. It’s now more cost-effective for carmakers to simply set some core dimensions for their inline engines and add or subtract cylinders as necessary – an engineering technique known as ‘modularity’. BMW has been doing it for years now – its inline six, inline four and inline three engines all share the same critical bore-spacing (the distance between each cylinder) and cylinder displacement measurements as each other, the main difference being how many cylinders are cast into their engine block.

That’s not something that can be easily done with a V6 format. Mercedes-Benz tried to do so by making its first production V6 a shortened version of its existing V8 engine architecture, but it introduced compromises in design (namely the use of a 90-degree angle between each bank of cylinders, rather than the 60-degree angle that’s more common for V6s) that gave the six-cylinder motor poor refinement.

And that saves money by allowing the same production line to process different engines of different sizes. What’s that mean for you? Simply put, manufacturers can use the money saved in engine design and production on other things, like in-car technology, higher-quality materials, or simply keeping the price tag as low as possible.

NO SAFETY COMPROMISE

Safety concerns were a primary reason why V6s took over from inline sixes, with their shorter length allowing bigger crumple zones and minimising the chance that the engine might enter the cabin in a massive frontal collision. It was a primary reason for safety-conscious Mercedes-Benz, so why is the company moving back to inline-sixes for its large cars?

Technological advances mean that the engine’s “accessories” – the power steering pump, air conditioning compressor and alternator – no longer have to be mounted off the front of the engine, a position that added overall length to the engine. Now able to be electrically-driven, they can be positioned elsewhere in the engine bay and thus bring the dimensions of a straight six down to a level where crash safety isn’t compromised.

The height of inline engines was another safety issue, this time for pedestrian protection. Again, technology comes to the rescue here, with pop-up hinges now able to physically lift the bonnet to give unlucky pedestrians more clearance from the hard metal of a cylinder head.

CAVEATS?

So with that many advantages in the inline-sixes favour, is the V6 on borrowed time? Not really, because of one simple fact – inline sixes are extremely difficult to package in any car that isn’t longitudinally-engined, where the engine points the same way as the car’s direction of travel. Large transverse-engined vehicles (where the engine is aligned sideways) like the Toyota Kluger require six-cylinder power in a compact package, so for those vehicles a V6 is still the best choice.

But for rear-wheel drive (or all-wheel drive) performance cars and large luxury sedans, it looks like the inline six is very much back in fashion.