TECH TALK
FLAT-FOUR
THE SMALLER ENGINE’S DESIGN ISN’T QUITE AS SIMPLE AS CUTTING OFF TWO OF THE B6’S CYLINDERS to tighter emissions regulations, despite the addition of more advanced Bosch L-Jetronic fuel injection. The flat-six in the 914/6 was discontinued in 1972 and replaced by a two-litre version of the Type 4 unit. This is where Porsche resumed heavy involvement with flat-four design, developing the VW lump into a twolitre engine. It used a longer-stroke crankshaft (71mm) and a 94mm bore for displacement of 1,971cc. The 914 using this engine in the USA was restricted to 94bhp, though it produced 99bhp in all other territories. The 914 was discontinued in 1976, but its two-litre flat-four still had life left in it, as demonstrated by its use in the 912 E, a US-only stop-gap model sold while the game-changing 924 was in the final stage of development.
MAKING A SPLASH
The introduction of Porsche’s transaxle family of cars brought with it a liquidcooled, inline four-cylinder engine,
signalling the end of the line for the flatfour as the world knew it. Fast-forward, however, to the reinvention of the Boxster and Cayman as the 982-generation 718 in 2016. Suddenly, the flat-four was back with a bang! Admittedly, this time around, a turbocharger and liquid-cooling were key components of the design, but this is very much a Porsche engine through and through. The B4 (as it’s called) is very closely linked to the same-age 911’s B6 flat-six and, while the smaller engine’s design isn’t quite as simple as cutting off two of the B6’s cylinders and filling in the resulting holes, the two engines share a remarkable number of components, including (but not limited to) timing chain, fuel injectors, fuel pumps, piston rings, the VarioCam Plus variable timing system, alternator and connecting rods. A major difference is the turbocharging set-up. Here, the B4 uses a single turbocharger, while the B6 makes use of two. The main reason, other than cost and packaging, is because of the four-cylinder
engine’s firing order. The B4 was initially launched in two guises: the two-litre variant used the same bore and stroke as the three-litre flat-six and produced up to 296bhp with an accompanying 280lb-ft torque, while the 2.5-litre B4 used in the 718 Cayman S and 718 Boxster S boasted a wider bore and larger valves. Additionally, the larger capacity engine was treated to a VTG (variable turbine geometry) turbocharger, which cleverly alters the shape of the path taken by the exhaust gases so that high-end boost can be maintained at high rpm whilst allowing decent response when exhaust gases are moving slowly. Peak power and torque for this engine are quoted at 345bhp and 310lb-ft respectively, later upped to 360bhp and 320lb-ft for the 718 GTS models. Given the huge round of applause Porsche has welcomed after redeveloping a naturally aspirated flat-six for the 718 twins (hello to all you Spyder and GT4 fans!), it’s unclear what the future holds for the firm’s famous flat-four configuration. Then again, with plug-in hybrid electrification on the march, we wouldn’t count against the power of four being part of Porsche’s plan for a leaner, greener future. Watch this space!
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