Si4 or SD4? That is the question

mark_n did give a very good technical explanation.

For what it's worth I offer a seat of the pants view.

Before ordering we did back to back test drives in a diesel manual and auto, then finally the Si4.

Having driven diesels for many years you get used to the low revving torque that is on offer from the diesel, when I drove the Si4, there was too much revving going on, sure enough the car was quick, but I have become accustomed to the diesel driving characteristics and quite like it, plus in normal driving use I don't think the SD4 is slow.

On a longer journey I'm sure the diesel presents a much more relaxed environment.

I was mindful of this discussion when I drove back from Brighton to north of Oxford this evening, mostly motorway and the Si4 at motorway cruising speeds is relaxed and very quiet, the biggest source of noise being from the road. There's no doubt though that when you're getting full power from the engine, it's quite frantic. Turbine-like the Si4 is not. That said, my journey of 160 miles at an average 63mph was effortless, aided by safe overtaking with the car in Sport and Dynamic mode. Maybe the Si4 suits the coupe better, the closest we currently have to an "Evoque Sport"?

I do think the economy benefits of diesel are overplayed however. My Si4 does 28mpg, consistently. If an SD4 did 35mpg (keeping in mind I have a very heavy right foot), the SD4 is using 25% less fuel, a bit more expensive here in the UK, call it 20% less in terms of cost. My car has done 17000 miles and I've put £4500 of fuel in it, approximately. That 20% saving would therefore translate to about £900. During that time, the car has also depreciated by at least £13000 so that if economy is the motivator, it's less than 10% of the depreciation alone, or just 5% of the total running costs.

In other countries where diesel is much cheaper than petrol, the case for diesel on cost grounds is obviously stronger.

The impact of depreciation was brought home to me recently when I considered trading my Porsche 911 for a new one. In that case, over 17000 miles and 4 years, the depreciation is more than 10 times the total cost of fuel I've put in it. While running an economical car may help your Green Credentials, the impact on your bank balance is not that great compared to the depreciation.