There's no doubt that the latest 2.2-litre HDi particulate-filter common-rail turbo diesel from PSA is some engine, but I'm coming round to thinking that the shorter-stroke two-litre still has a lot going for it. It's less expensive, it's more economical, and it's actually a lot better (in fact, the cleanest of all the C5 engine options) when it comes to CO2 emissions. And if the two-litre is slightly down on off-the-line and top-end performance, that formidable mid-range torque all but makes up for it.
In the C5 range, the SX is the top-rated two-litre HDi specification. You can't get this engine with Exclusive or Exclusive SE trim and equipment, which, on the list of saloon diesels, are available only with the 2.2. Of course, in the context of the C5, the word saloon is a little misleading. This is the five-door hatchback model, and it's called the saloon only to differentiate it from the estate.
The C5 still impresses, not just because of the much sleeker styling than Citroen has attempted in this class before, but first of all because it's so roomy. Despite the quite high seating position, there's plenty of headroom front and rear. The rear seats also have better kneeroom and (always important, this) footroom than many of the Citroen's rivals. Plenty of space in the boot, too, with its securing net and collection of side-mounted tags to keep bottles and whatever in place.
Velour upholstery is standard in the SX, and this version shares the rather suave fascia design with its sweeping, gradual curves. Without the satellite navigation - an extra-cost option here, and at £1500 for the full-colour job, by no means bargain priced - there's a smaller and neater fascia-top information screen.
Like all the C5s, the SX has multiplex wiring, which means that a service indicator, black panel lighting, Trafficmaster and a speed-sensitive sound system are all standard. So, from this model upwards, are rain-sensitive wipers and light-sensitive headlamps. And if you do the old diesel driver trick of switching on the ignition just before buckling your seatbelt (to take up the waiting time before actually starting the engine, which with modern common-rail diesels is admittedly very short) there's an instant instruction to get your seatbelt fastened. Okay, okay!
As soon as you do start the engine, there's a familiar diesel clatter, but it's heard more by people outside the car than by those inside. And it doesn't take long for the diesel noise to fade away, as the C5 accelerates away and then settles into cruising mode. These days, with most common-rail turbo diesels, there's no noise problem when they're up and running, and the C5 is very restful once under way.
The two-litre is relaxed when in long-striding motorway mode, but there's a punchy performance as you climb to the torque plateau. It gets to 188lb/ft at 1750rpm, and although that's way below the 2.2 HDi's phenomenal 237lb/ft, the smaller engine clambers onto its top level 250rpm sooner.
I never felt that it was straining, and the mid-range pull, on the level and on climbs, was invigorating. On one trip I did several miles on a hilly single-track road with passing places at each minor summit, and the C5 certainly did its stuff as it raced up the gradients so as not to "overlap" any oncoming vehicles, with the almost inevitable shuffling-about that causes.
Out on country roads, as well as on poorly maintained so-called main roads, the suspension soaked up the bumps, and yet let the car swing securely through fast corners. With its long wheelbase and wide track, the C5 sits four-square on the road.
The two-litre HDi models don't have the full auto-adaptive Hydractive 3 suspension, which is reserved for the top cars in the range. They use a simpler version, which still has self-levelling as well as that ingenious facility to lower itself on motorway runs and raise itself on really bumpy surfaces.
It's remarkable that no other mainstream and mid-range saloon and estate manufacturer has ever tried anything similar, although Citroen introduced its hydro-pneumatic suspension for production cars back in the last days of the Big Six saloon, about half a century ago.
Perhaps, like some private motorists, other companies were put off by early reliability problems, and by the fact that in the 1950s installing the system seemed to involve bending pipes any old how, at the whim of the bloke on the production line. Ask anybody who ever tried to get replacement suspension parts fitted to a DS19. But Citroen has simplified and refined the system over the half-century, and reckons that Hydractive 3 shouldn't need servicing for the first five years or 125,000 miles.
You can't do much driving in an HDi-equipped car without being impressed by the economy. I left the fuel consumption indicator running after taking the C5 over from a colleague. Neither of us did much town running, or a lot of motorway cruising, but there was plenty of quite brisk country motoring, with lots of down-the-gearbox work. Yet the screen showed almost 48mpg by the time I handed the car on.
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