Before we get too excited about this, it would be as well to explain what Volkswagen is trying to do here. The Golf TSI, whose 1.4-litre engine is both supercharged and turbocharged and produces a maximum power output of nearly 170bhp, does not exist to show off how strong such a small unit can be made to be. It is a remarkable piece of work in that respect, but this is a secondary consideration.Volkswagen's real aim here is to get in on the downsizing act, which it has done before any other major manufacturer. The TSI is actually a replacement for the existing two-litre FSI direct injection petrol unit which powers the "warm hatch" models in the Golf range. And it works well, since it gives the Golf much better performance, along with more marginally improved fuel economy and CO2 emissions, than the FSI could.At 168bhp, its headline power figure is 20bhp better than that of the FSI. Even more significantly, TSI's 177lb/ft of torque from 1750-4750rpm is much higher than the FSI's peak of 148lb/ft, achieved at 3500rpm. The 0-62mph acceleration time is cut by a second to 7.9 seconds, and top speed is up by 9mph to 137mph. Combined fuel economy has gone from 35.3mpg (FSI) to 38.2mpg (TSI), while CO2 emissions have dropped from 192g/km to 175g/km.What all that means is that the TSI is substantially quicker, yet at the same time cheaper to run. It's so impressive in this respect that industry watcher EurotaxGlass's has expressed the opinion that it may inspire a rethink on the whole subject of resale values. This - it can't be emphasised too much - is the important bit. One thing that the Golf TSI is not, and was never intended to be, is a serious hot hatch.That's a difficult thing to remember, though, because when you're pushing on a voice in your head keeps shouting, "But it's only a 1.4! It can't be doing this!" The big-engine effect is quite remarkable - and it simply isn't true (as I saw suggested in a forum message written by someone who had clearly never been near the car) that a small engine will always have the characteristics of a small engine regardless of how much air you force into it. Balderdash.The whole point of the TSI is that it feels like it has a big engine precisely because of all the air-forcing. The supercharger and turbocharger allow it to take in as much air as a larger-capacity engine would, and since the power of an engine is proportional to the airflow through it the TSI really does feel as if there's a particularly strong two-litre under the bonnet.Volkswagen would not have achieved this effect simply by turbocharging the engine, or indeed by using any single-compressor system. The TSI's extra power is provided by a supercharger operating at low speeds and a turbo which chimes in further up the rev range. A 168bhp 1.4-litre turbo would suffer from a black hole of performance at low revs, and this is exactly what the supercharger is designed to fill.(This is the time to point out that Volkswagen says it isn't planning to build larger-capacity TSI engines, since there would not be the same need to create low-speed performance. A GTI TSI, for example, isn't on the cards. But a one-litre TSI - suitable for, say, the Polo - is a definite possibility.)From little over tickover speed right up to 7000rpm there's a constant surge of power, with no obvious change in the way that it is produced. But such a change does take place, since the supercharger disengages and allows the turbo to take over at roughly 3500rpm (the exact speed depending on circumstances).Since the turbo is downstream of the supercharger, it's already spinning long before it's needed, so there is no turbo lag when it comes in. There is, however, some supercharger lag, a concept I haven't come across before. As well as retiring from the scene as the revs rise, the supercharger doesn't operate in gentle driving (to improve fuel economy by reducing drag). If you suddenly floor the throttle, there is a small but distinct delay as the system realises what you want it to do.There - see how easy it is to think of this car in performance terms? If you do that too much, it's easy to become disappointed with the GT TSI, since it isn't particularly nimble through the twisty stuff. Although the GT range in general has undergone a facelift to make it appear more like the genuinely hot-hatch GTI, none of the models is anything like that car.They are pleasant enough to drive, but try to push them really hard and there is a feeling of "Oh, well, if you insist" rather than an eagerness to comply. That's fair enough in the case of the turbo diesels, but it seems odd in the context of the TSI, with its amazing numbers.Which just goes to show, once again, that you should forget about those numbers and concentrate on the fact that the TSI is really a warm hatch, albeit an impressively powerful one with unexpectedly good fuel economy.It might also be the start of something big. As well as using it in three Golf GTs (with three or five doors and manual transmission, or five doors and DSG), Volkswagen has also introduced a 138bhp version to the Golf Sport, Golf Plus Sport and Touran (all manuals to begin with, though DSG will be brought in later in 2006) though this one is only slightly more economical at 39.2mpg on the combined cycle in the case of the Golfs.The possible one-litre mentioned earlier could be used in smaller cars, both units could find their way into other Volkswagen Group products (SEAT, Skoda, maybe Audi, probably not Bentley), and of course you can bet the farm on other manufacturers looking very closely at this project, perhaps with a view to introducing something similar of their own in the near future. But if they do, they won't be able to do anything about the fact that Volkswagen got there first. Engine 1390cc, 4 cylinders Power 168bhp Fuel/CO2 38.2mpg / 175g/km Acceleration 0-62mph: 7.9 seconds Top speed 137mph Price £18,095 Details correct at publication date
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