Lexus has always described the IS as a sports saloon, but if you take terms like that seriously the car you should really be interested in is not the IS 250 or its diesel relative, the IS 200d, but the IS F. Its name doesn't appear to mean much until you discover that the F stands for Fuji - no, not the mountain, but the Fuji Speedway race circuit where the car was partly developed.
While its outer shape is clearly based on that of the rest of the IS range, there are many detail changes, including more purposeful-looking bumpers, larger air intakes, wider wheelarches and quad exhaust tailpipes. The more observant among you will realise that it's also longer both at the back (mainly for aesthetic reasons) and at the front, which is where the main action takes place.
The front is longer to make room for a much larger engine than can be found in any other IS. It's a naturally-aspirated 417bhp five-litre V8, and while Audi is providing more power from a smaller-capacity engine of similar layout these days, the Lexus unit is still very impressive. It's whisperingly quiet when you're not asking it to do much (few manufacturers can beat Lexus for noise suppression) but it becomes considerably more vocal when a second intake passage opens up at 3600rpm, and by the time you've reached the 6600rpm at which it develops its peak power it is fairly screaming. You could just about get your money's worth from the F simply by accelerating it to the max in the first two gears every couple of days and revelling in the sound.
Speaking of gears, there are no fewer than eight of them, contained in an automatic transmission first seen in the Lexus LS 460 luxury saloon. You can leave the F to pick its own gears, of course, but the sportier option is to select M mode and choose them yourself using the paddle shifts mounted on the steering wheel. Since the torque convertor operates on first gear only in M mode, and second to eighth are locked-in, the effect is similar to that of a twin-clutch semi-automatic (even though that is a quite different system).
Changes are very quick at 0.1 seconds on the way up and double that on the way down, the latter including the time required to blip the throttle so that the engine is turning at the correct speed for the next gear, which is not only the best way of doing things but also makes the F sound even better.
With the possible exception of the looks, the engine is probably the best feature of the car. To cope with its power, which allows a 0-62mph time of 4.8 seconds and a top speed which is as modest as 168mph only because an electronic limiter makes it so, Lexus and Brembo have developed a braking system with 360mm discs and six calipers at the front and 345mm discs and two calipers at the rear (if you didn't follow much of that, let me assure you that the brakes work very well indeed).
Suitably grippy tyres are mounted on lightweight 19" BBS alloy wheels, and for the 2010 model year Lexus has introduced a real, proper, actual limited slip differential - not one of those electronic equivalents - which improves traction and is said to have improved the F's lap times around Fuji Speedway by two seconds.
There's also something called Vehicle Dynamics Integrated Management (VDIM). This looks after the ABS, traction control and vehicle stability control, as well as automatically shifting the gearbox through its Sport, Normal and Snow modes as it sees fit. Lexus says that VDIM has been created "to achieve the right blend of handling and control", but systems like this are only ever as good as the suspension settings allow them to be, and I'm afraid this is where the F begins to fall short of the ideal.
Lexus has always been at its best when designing luxury cars. The LS is a wonderful piece of work, and I would have one if I could afford it. By contrast, no IS has ever struck me as being anything like as good as Lexus has said it is, largely because they have all felt as is they required another six months' chassis development. The F feels much better than any other IS in relatively gentle driving, but since a regular IS going absolutely flat-out is operating at nowhere near the full potential of the F this should not come as much of a surprise.
I suspect that all those laps at Fuji have something to do with it. I have never been to the Fuji Speedway, but I think I'm safe in assuming that it has a very smooth surface and well-designed corners, thereby distinguishing it from almost any interesting road in the UK. Over here, the IS F struggles as you start to push it hard. Like many road cars set up for track use (or ones developed on the superb tarmac considered normal in Germany) it is underdamped at the front, which not only harms the ride quality but makes the driver uncertain about whether the amount of steering lock applied at the entry to a corner was correct, or too much or too little.
The next issue is that, despite the limited slip differential and all those electronic controls, the back end can easily be made to struggle when the throttle is applied. Of course, this is the sort of thing you should always be careful about when 417bhp is available, though I can't help thinking that a car costing over £57,000 should be able to do this sort of thing better. Similarly high-performance Audis don't suffer in the same way, but of course they're all four-wheel drive - the more telling comparison is that Jaguar was able to feed the same amount of power through the rear wheels with greater control nearly ten years ago.
Combine the uncertain front end with a rear which would be very wayward if the traction control didn't beat it into subsmission, and you end up with a car which is nothing like as precise or enjoyable to drive as you feel it should be. The best way to use the IS F, if you live in Britain and don't have access to a race circuit, is to enjoy its ability when it's well below its limit and don't risk disappointment by pushing it as hard as it will go.
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