As any engine tuner will tell you, the most basic part of the whole enterprise is to push more air in at one end and give it an easy route back to the outside world at the other. All motorsport engines work on this basis (or are not permitted to if the regulations have been designed to limit power outputs) and the same principle is used by boy racers, who have for many decades fitted free-flowing air filters and exhausts to their cars in order to make them faster at reasonable cost, and incidentally - if not in some cases primarily - to make them sound better.Since the invention of electronic control units, another fairly inexpensive way of gaining power has been to modify these from the standard manufacturer settings, which are usually intended to give a compromise between performance and fuel economy. This modification and the special air filter and comprehensively non-standard exhaust system which cover the points made in the first paragraph make up the Mountune performance package with which you can boost the performance of any current 1.6-litre petrol Ford Fiesta.It costs £1299, including VAT but not the cost of fitting - that takes around four hours and can be done at any of the Mountune Performance Ford dealerships which can be found all over the British mainland from St Ives to Fort William, with two more in Northern Ireland. You're probably going to use more fuel once the conversion has taken place, but insurance remains in Group 6 (and since this is a Ford-backed venture which does not affect the car's warranty you won't be heavily penalised as you would if you had bought all the parts from an accessory shop and fitted them yourself) and VED payments remain the same because the car is not sold new in this form and therefore officially retains the standard 139g/km CO2 rating.Mountune provides other packages for the Fiesta too. One is purely cosmetic, consisting of side graphics and new door mirrors and costing £149, but you can also have lightweight wheels for £529 and Dunlop Sportmaxx tyres for £120.The car I tested didn't have any of that, though, and in fact apart from the tuning bits it was a completely standard Ford Fiesta with regular tyres, brakes and suspension. That may sound slightly alarming given that the power output has increased by nearly 17% to 138bhp (or, to use the measurement quoted in the car's title, 140PS), and the 0-62mph times has fallen by two seconds to 7.9 seconds, but as I suggested in a previous road test the brilliantly set-up 1.6 Fiesta already feels like a hot hatch in unmodified form and it laps up the extra performance with no problem whatever.With enough power to make life interesting but not enough to upset the chassis, the Mountune Fiesta is an absolute hoot to drive. Ten years ago it would have seemed like a miracle, and even now it's good enough to gladden the heart of any boy racer on a budget.But it's not all about handling or acceleration. Those things are important, but so is the way the car sounds while they are happening, and the Mountune Fiesta sounds fantastic. The noise levels are never painfully high, but even on a motorway cruise there's a sort of racey rumble which won't let you escape the fact that interesting things are going under the bonnet - and if you're the kind of person this car is aimed at, why would you want to escape it?It's even better when you start pushing on, because there's quite a bit of induction roar to make the car sound more butch than perhaps it really is. If you're in the right sort of mood you'll find yourself blipping the throttle on downchanges, not because you need to, but just because it sounds so good. And girls think that's cool, don't they? Of course they do. Best of all, there's even some occasional popping and banging if you lift off the throttle. Score!You may think all this is trivial, and of course you're right, but you're also not the kind of person who could be persuaded to buy the Mountune package in the first place. Personally I loved it, partly because it was all such good fun and partly because it reminded me of what it was like to be 19.At one point during this test I thought of a single sentence which summed up the car, and I stopped to write it down: "This car is more than the fun of its parts." Corny, of course, but I stand by it.