Sticking to dogma can be a dangerous thing. It can lead us to places we'd rather not go and turn reasonable people into hidebound bores. It can also compromise great cars, ignoring sensible engineering solutions because 'that's the way things are done around here.'

I can think of many examples straight off the bat and BMW's perfect 50:50 weight distribution would probably be one of them. Such weight distribution is fine for their performance M-cars but do you really need perfect balance when you're lugging loads in an estate or cruising in a convertible? Evidently not, as the latest 3 Series drop top features a centre of gravity not measured to millimetric precision in the middle of the car.

Engineering any folding hard top means putting a good deal of metalwork and motors in the rear of the car. In order to get the perfect weight balance, BMW would have needed to undertake radical and costly design changes such as relocating the fuel tank. Of course, you could probably achieve this end a lot cheaper by keeping a bag of cement in the passenger footwell but I couldn't really see Munich green lighting a solution like that. Besides, who really cares if the 3 Series Convertible does have a rearward weight bias? Porsche 911 owners rarely complain.

BMW has been selling 3 Series Convertibles for many years, first through Baur and then manufacturing them in-house, and they've refined the art of building a desirable drop-top very well, the latest Z4 and 6 Series convertible models featuring triple-layered fabric roofs that are about as good as this sort of hood gets. With the 3 Series, BMW felt the time was ripe to follow a divergent path and get with the technology that's driving huge customer demand - folding hard top roofs. Like all the most modern versions, the BMW system is a three rather than a two-piece system.

"The goalposts in this sector of the market look to have subtly shifted"

The advantage of this is that you don't need a distended bottom on the car to accommodate the larger sections of a two-piece roof. The downside of stacking three pieces atop each other instead of two in the boot is that when they're folded, there's not a great deal of luggage space left. With the roof in place, you'll get 350 litres of load space. Fold it down and 140 litres is instantly pared from that figure. That's less than all this car's key rivals - the Mercedes CLK Cabriolet, the Audi A4 Convertible and the Saab 9-3 Convertible. Never mind. From this point the story improves.

The operation of the roof takes a so-so 22 seconds (compare that to 12 seconds for a Mazda MX-5 Roadster Coupe) but there are some real practicality benefits offered. Because of the larger glass rear screen and side windows, visibility has improved by 38 per cent when compared to its predecessor. Not only does this mean that rear seat occupants will feel less claustrophobic, it's also an important safety feature, with cars and bikes less able to sit in your blind spot on motorways. The designers have also factored in some key practicality benefits too. The rear seat backrests can be folded flat to provide additional luggage space if the 210 litres in the boot isn't cutting it. As an option, customers can also specify a through loading facility for bigger items that can still be used with full folding hard top functionality.

The depth of engineering is impressive. Take this issue that many convertible owners will identify with: It's a beautiful summer's day, you arrive back in the office after lunch, park the car in your reserved space, pop in for a couple of hours work and then get into the car to drive home and promptly cremate yourself on upholstery that's hot enough to fry an egg. I once seared the imprint of a Lotus Elise gear knob into my palm on just such an occasion, having to withstand weeks of ribbing from friends that I was the Nazi agent from Raiders of The Lost Ark. With this 3 Series, BMW has pioneered Sun-Reflective Technology for its leather upholstery. Specialised pigments are embedded into the hide that reflect infra-red radiation. This means that areas that are exposed to sunlight for a prolonged period remain up to 20 degrees cooler than similarly coloured but untreated leather would. Just make sure you remember the factor twenty.

The line-up starts at £30,915 with the 320i SE Convertible. At the top sits the flagship variant and the version that's got everybody in the know excited, the 306bhp 335i Convertible that costs £40,670 in sporty M Sport guise. This is a revolutionary engine insofar as it marks BMW's first turbocharged petrol unit ever offered in an open top car.

Accelerating from zero to 60mph in just 5.8 seconds, this is a car that's almost as fast as the old-shape M3 yet which is far less temperamental. As is the norm with high-end BMW models, the top speed is electronically limited to 155mph. Ignore the badge on the back: this is no 3.5-litre unit. Instead it's a twin-turbocharged 3.0-litre powerplant, each blower supplying compressed air to three cylinders. Using smaller turbos helps to overcome the inertia of one big unit and this lack of turbo lag in turn helps this forced induction engine to replicate the progressive power output of a big, normally-aspirated powerplant.

In order to get this sort of power from a normally-aspirated engine, you either need to throw a lot of money at it (like the old M3's unit), or you'd need a bigger capacity with more cylinders and, correspondingly, more weight. Turbocharging, no matter how retro it seems, was the most sensible solution. Piezo fuel injectors help the 335i Convertible to a very creditable 29.7mpg combined fuel economy figure. To put this into perspective, an Audi A4 3.2 with its trumpeted FSI engine manages 2.2mpg less and generates 50bhp less. No wonder Munich is pleased with the results. Mind you, even the entry-level 2.5-litre petrol engine is no sluggard, punting the Convertible to 60mph in 7.6 seconds and on to 154mph. Fuel economy is rated at 37.2mpg, helped by BMW's patented Valvetronic technology and the EfficientDynamics package. This engine also uses the lightweight magnesium-aluminium composite cylinder block that debuted on the 630i coupe.

BMW customers have a lot to get excited about in the 3-Series Convertible range and dealers are expecting buyers to start flipping their lids in large numbers.

TOP 5 3 SERIES DEALS

The results below show the top 3 SERIES deals on buyacar

	BMW 3 Series 335i M Sport 2dr Convertible
	ETR	Mthly	Saving £749	Price £40,121
	BMW 3 Series 320i SE 2dr Step Auto Convertible
	ETR	Mthly	Saving £423	Price £32,087
	BMW 3 Series 330d M Sport 2dr Step Auto Convertible
	ETR	Mthly	Saving £780	Price £40,570
	BMW 3 Series 325i SE 2dr Step Auto Convertible
	ETR	Mthly	Saving £523	Price £34,542
	BMW 3 Series 320i SE 2dr Convertible
	ETR	Mthly	Saving £363	Price £30,657