Six-cylinder engines have featured in some heroic cars over the years, with stuff like the E-Type Jag, Toyota Supra and BMW M3 paving the way for the straight-six and the Honda NSX, R35 GTR and Lancia Stratos cementing the V6 into the automotive psyche. Sadly, the glory days of the straight six seem all but over, with turbocharged V6s still featuring as downsizing alternatives to V8s in today’s performance cars. So what are the advantages and shortcomings of each setup and why does the V6 now dominate?
Firstly, like any in-line engine, straight sixes are a nice, simple design. With no cylinder offset, manufacturing costs are low and there is no need for separate heads or valvetrains like in a V-configuration. Instead of using double the amount of smaller camshafts, a DOHC I6 can simply use two longer camshafts to open and close its valves.
The simplicity of these engines is emphasised also when it comes to working on them, with the straight configuration allowing great access to all the spark plugs, leads and ancillaries for general maintanence, making the I6 an amateur mechanic’s friend.
The biggest advantage however comes through engine balancing. Due to the normal firing order of a straight-six, the pistons move in-tandem with their mirror image on the other side of the engine block. So pistons 1 and 6 reciprocate, followed by 2 and 5 and finishing with 3 and 4. As pistons 1 and 6 reach top dead Centre, the other four pistons are evenly spaced at 120 degrees and 240 degrees respectively around the engine cycle, meaning that the reciprocating forces balance each other out. This makes a smooth-revving engine for which units like the S50 and RB26 have become famous for.
Unfortunately, there are numerous reasons why the straight six is all but dead. Packaging has always been an issue, as the extra two cylinders when compared to an I4 means that mounting the engine in a transverse configuration is very difficult. If mounted sideways, there usually isn’t enough room for the size of transmission and drivetrain needed to use an I6 in a front-wheel drive setup. And with manufacturers looking to create powertrains that can be shared across as many chassis as possible, the elongated I6 just doesn’t quite cut it.
The long engine and its components also lack rigidity when compared to a more compact engine setup. The long camshafts and crankshafts naturally try to very slightly flex during rotation, along with the engine block lacking the stiffness of a V6 equivalent. The dimensions of the I6 also don’t help it in terms of the car’s centre of gravity, with rotating and static mass sitting slightly higher in the engine bay than other more compact engine options.
Found normally in 60 or 90-degree configurations, V6s can still be found in numerous performance cars, with the art of turbocharging creating upwards of 500 horsepower from the likes of the MY17 GTR and the latest tech-fest NSX. V6s have also been used in much more docile platforms like my own Mondeo ST200, so one of the largest plus points is the engine’s versatility.
Due to its much more stocky, compact nature, it is capable of being shoehorned into numerous engine bays within a manufacturer’s fleet, thus cutting huge costs from having to perform R&D on other engine options.
The precious space offered up by the neat package therefore opens the door for forced induction, leaving room for turbochargers to nestle deep within the engine bay. Front-wheel drive setups can also utilise a V6 as a powertrain, which can lead to some truly epic performance bargains like the MG ZS180 which used a Rover KV6 and the Mazda MX-6 which squeezed in a 2.5-litre V6 in its second generation. So the V6 allowed car companies to easily produce a performance variant of their usual boring four-cylinder cars without having to drastically change chassis dimensions or engine bay organisation.
Although it may feature the same number of cylinders as its in-line brother, the V6 does not have the same intrinsic balance. Effectively made up of two in-line three-cylinders stuck together, the V-style engine needs balancing shafts that use specially placed weights to counteract the unwanted inertia created by the reciprocating engine. Without these balancing shafts, large vibrations would travel through the crankshaft and offset the efficiency of the reciprocation.
The engine balance is worsened as displacement increases (longer piston stroke) and an increase in bore size (increasing the mass of the piston). The counterweights needed therefore add complexity to the engine’s design and manufacture, increasing overall costs. Naturally a DOHC V6 must have four camshafts and potentially 24 valves in total, so the complexity of the additional valvetrain components needed to fill each cylinder head further increases the complexity of this engine setup which can make working on V6s an intimidating prospect for a less-experienced petrolhead.
Although many car guys have been mourning the lack of modern straight six engines, it seems that times may be about to change. In recent weeks, Mercedes has come forward with a new in-line six that will use a 48V battery to power the ancillary components and help the powertrain. And even with this possible rebirth of the I6, remember that BMW specifically made its name with four-cylinder engines through the original M3 and 2002.
In the I6’s absence, the V6 has taken over and will probably be around for some time to come. But with the calibre of cars utilising the V-format currently, it’s hard to complain with the performance that they can offer with few drawbacks.
Which engine format do you prefer? Do you want to see the straight-six returned to the engine bays of the current crop of performance cars? Comment below with your thoughts!