Master of the Class: BMW’s 3 Series Convertible

une 5 (Bloomberg) — It’s a brilliant spring day, and I’m slipstreaming BMW’s latest 3 Series up a squiggly road into the low mountains, the automotive equivalent of a salmon charging upstream. I love the outdoors, and it feels like a homecoming: After all those winter months, the sun is finally kissing my face.

Oh blessed thing, the invention of the convertible.

Less than 20 minutes later, though, Northeast weather reasserts itself: rain. I yank the 335i to the shoulder, engage the roof and, 22 Mississippis later, a metal carapace has settled over my head. Then I’m back on the asphalt, seeing how the performance tires mate up to wet road under heavy acceleration (courtesy of that lovely new 300-horsepower, twin- turbo engine).

Even with the vagaries of spring weather, life’s pretty good.

When it comes to $40,000-to-$50,000 performance coupes and convertibles, the BMW 3 Series is the benchmark. The problem with an enduring benchmark, of course, is the bench itself can be surprisingly high if one falls off of it.

So when Bimmer refreshes the line, owners worry. After all, at times the company can bewilder even its most serious fans (remember, cough, iDrive?). And this time around, the company made a big change in its best-selling ragtop version — they got rid of the rag. Like the Volvo C70, it’s now a hardtop.

When the roof is up, you might not realize it’s a convertible at all. The attendants at my parking garage, who know their cars and relish seeing what I’ve got coming in week after week, didn’t.

Muscular Design

I haven’t loved all of BMW’s designs over the Chris Bangle era, yet both the roof-up convertible and the regular coupe are lovely. Muscular yet taut, like well-conditioned athletes. The best angle is from the front: Hung from the end of the angled hood, the diminutive double-kidney grill looks like the openings of a double-barrel shotgun.

You can’t really tell the difference between the convertible and the standard coupe until you notice the lines that break the skin in the rear three-quarters — all those places that hinge when the roof folds into the trunk lid. (Yes, a large portion of the trunk space is lost to the stowed top.)

When the hardtop does its three-layer, mechanical jig into the back, you know it’s not a bad compromise. Transformed, the car is just as handsome, and the design ensures wind in your hair without bugs in your teeth. It’s quiet enough, too, for normal conversation (top up, expect signature BMW solitude).

Rocketlike Power

Too many convertibles are claustrophobic with the roof up – -in the Northeast that can mean nine months of not-so-great driving — yet the 335i feels like a regular coupe. My head didn’t brush the ceiling, nor did I feel as if a clam had snapped shut on me.

Then there’s that new power plant. With 300 horsepower and 300 pound-feet of torque, this is the kind of engine that you’d find Prince singing about: something about rocket in your pocket, or maybe a ditty about a kitten who turns into a tiger at night. Pushed gently, the engine responds in kind: quiet and easy-going. Given the go, it responds with sudden, slightly irrational torque and not a bit of turbo lag.

I drove both the 335i convertible and coupe over several weeks, equipped with six-speed manuals (the coupe, as tested, was $45,720). The convertible adds some 400 pounds of weight, and if you really pay attention you’ll notice it doesn’t turn in quite as neatly. With the added bonus of driving in the open air, though, it’d be my choice.

Functionality Meets Banality

All is not perfect in the Land of the 3 Series, however. The interior may be comfortable and ergonomic, and the leather has a special feature that’s supposed to keep it cooler in the sun. The interior is also joyless. There is nary a hint of playfulness or visual interest — functionality at its most banal. One wishes they would have taken a look at the interior of the Volvo S80, with its sexily curved headrests.

The fact is that no matter how pretty the exterior — I had a wonderful shiny blue on mine — over the hours you’re driving, the interior is what you’ll be seeing most. At a premium BMW price (the convertible starts at $49,100 and quickly spirals upward), buyers could, and should, expect more.

Last, while the BMW is more practical than many convertibles, don’t try to sell it as an “everything” car to your significant other. A run to Target ended up with me stuffing purchases into every cranny, and the leg room in the rear seat precludes a double date with another couple — unless they’re little people or really accommodating.

Still, those are forgivable offenses, I tell myself, as I burn back down the mountain roads. After all, the sun is sure to come back out — if not today, then soon.

The BMW 335i Convertible at a Glance

Power: 3.0-liter inline six-cylinder engine, with 300 horsepower and 300 pound-feet of torque.

Drive train: Six-speed manual or ZF six-speed automatic.

Speed: 0 to 60 mph in 5.5 seconds.

Price as tested: $54,450.

Best feature: A great driving experience, top up or top down.

Worst feature: The depressing interior.

Target buyer: Someone looking for the joys of a convertible without completely sacrificing practicality.

(Jason Harper writes about autos for Bloomberg News. The opinions expressed are his own.)