► The beating heart of the RS3
► Why it’s great
► And why its days are numbered
Audi was the first car brand to make a hero of the five-cylinder engine, promoting the layout on the basis of providing the economy of an inline-four with the refinement of a straight-six in its ’80s ads. But is that true?
Most friction in an engine comes from the pistons reciprocating in the bores, so reducing the number of these (or at least their total circumference) will help with fuel economy. It won’t easily get you to four-cylinder levels, though, so Audi was indulging in some advertisement latitude there.
As for the other claim, an inline-six has what we call perfect balance but a five only has perfect force balance – it has unbalanced couples of both primary and secondary orders. The primary can be mitigated by a balance shaft and for various reasons the secondary is usually ignored but, again, the five cannot touch a six for smoothness. For the second time, the sales team was not being entirely accurate…
However, there was a very pragmatic reason why Audi adopted the arrangement. When you’re mounting your engines longitudinally, in front of the transmission, engine length was very important and a six often won’t fit: five cylinders it was, then. And that gave rise to the inline-five becoming an Audi thing, an almost-USP. And when Audi put a turbo version in the Quattro, legends were born.
I say ‘almost’ a USP because the layout isn’t actually unique to Audi. Among others, Volvo, Ford and Fiat have all used it. Between them, they took advantage of its packaging benefits – it’s much easier to fit one transversely in an engine bay than a straight-six. Widespread adoption of balance shafts essentially made all the layouts equal in terms of vibration, giving less concern in this area.
But, as turbocharging became more common, other issues manifested for the five. They perform really well as naturally-aspirated engines, because you can adopt long primary exhaust headers which allow great exhaust tuning and therefore avoid pulse interference wrecking the scavenging. A turbocharged inline-five engine, with its 144° firing intervals, a simple, compact exhaust manifold, and high back pressure due to catalysts and silencers, is plagued by pulse interference.
This occurs when the valves are still open towards the end of the exhaust stroke and a slug of burned gases gets rammed back into the cylinder – because when the exhaust valves of the next cylinder in the firing order open, a high-pressure wave travels back into the manifold. This creates high levels of trapped residuals in the combustion chamber, and then you end up with all sorts of problems with knock.
With turbocharged inline-fours and -sixes you can fairly straightforwardly ‘pulse-divide’ the exhaust, but you cannot with the five. Audi tried to address the issue with different exhaust runner orientations and lengths but the fact is that you cannot solve this challenge without long primary pipes – impossible on turbocharged road engines, for all sorts of emissions-related reasons. So, a roadgoing turbocharged inline-five is hobbled with respect to high specific output and thus, when downsizing began in earnest, the layout was essentially ignored. Audi keeps it alive – for now at least – for emotive reasons of brand heritage. Get an RS3 while you can!