The Alpina B3 S Biturbo is the marque's take on the 335i. The renowned BMW fettlers have tuned the B3 S’s 3.0-litre, twin-turbocharged six-cylinder engine to produce 395bhp, almost 90bhp more than the current 335i, and 398lb ft of torque, a rise of more than 100lb ft. That means that even in convertible form, as driven here and which is some 200kg heavier than the saloon, the B3 S will hit 62mph in 4.9sec and go on to an unrestricted 185mph. 

However, comparisons will doubtless be drawn with the M3 as often as with the 335i, and it’s worth noting that while the B3 S is two cylinders and 25bhp down on the V8-engined BMW, it delivers a remarkable 103lb ft more torque – and it isn’t restricted by a speed limiter. 

Chassis-wise the B3 is way softer than the M3’s, and that’s an entirely deliberate decision on Alpina’s behalf. Apart from Alpina’s own, lighter 19-inch wheels and bespoke Michelin Pilot Sport tyres (235/35 at the front, 265/30 at the rear), the B3 is virtually identical to a 335i Sport in set-up, and is none the worse for that. Notably, the lack of runflats do give the B3 S’s ride a suppleness and civility that’s pretty much unmatched by any 3-series, without any major sacrifice in body control.

The only obvious omission is the absence of any form of limited slip diff from the B3’s spec sheet. The B3’s open item makes for poorer traction on less than perfect surfaces, and rules out the 200-yard power slides you will be able to pull off in the M3 with it’s awesome M-diff. Which, you could argue, is a pity, but it’s perhaps also a good indicator of the greater maturity of the B3’s typical driver.

On the road the B3 feels monumentally quick, even down the shortest straights. And the auto gearbox works perfectly in conjunction with the engine, blipping downshifts smoothly and with just the right number of revs when you shift manually, and picking off gears with similar speed and dexterity on the way up.

If you want to go slowly in it, you can, and the engine and automatic gearbox combo works beautifully in laid-back mode. But the moment you want to summon end-of-the-universe acceleration, all you do is flatten the throttle and go, cheeks all-of-aflap like a proper time traveller. 

Criticisms? At nine tenths and above the B3 can feel a tiny bit soft, and vertical surface undulations can cause it to float a little. But the number of occasions on which you are likely to drive your B3 hard enough on a UK road to uncover this minor flaw would be limited in the extreme, and you’d probably lose your licence long before losing your patience with the car’s suspension. We’d also like the option of a manual, and we wish the steering was a little lighter at low speeds for urban use. 

And in drop-top form as tested it’s not a light car – but then neither is a regular 3-series convertible. 

We’d be very tempted by a B3 S Biturbo, although we’d probably look at the saloon, coupé or estate before we’d buy the heavier drop-top.

If you really must have the ‘ultimate’ 3-series then you’ll probably still buy an M3. But anyone who hankers after the old six-cylinder E46 version’s six-cylinder rasp – and there are plenty of you out there, we know – should give the B3 S a long, hard look.

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