Conundrum alert gearheads! Does your lust for diesel outweigh your desire for speed? Is a sexy BMW drop-top more or less sexy when you drop in a more powerful motor for a top-o-the-line diesel fourbanger? Would you rock AC Shnitzer's 99d concept car?

Let's start with the facts: a brand new BMW Z4 went under the knife to have the stock six-cylinder powerplant replaced with the diesel four-cylinder from the BMW 320d. And because it's AC Schnitzer they bumped power to 190 hp and 310 lb-ft of torque. This is good for a 0-to-62 mph time of 6.9 seconds. Not great, but not awful.

If they snagged the twin-turbo 335 horsepower six-cylinder from the sDrive35is we'd consider this sacrilege, but in Europe they also offer the wimpier 201-hp six-cylinder version. If this was the swap they made they're a little lower on horsepower, but they've boosted torque by 130 lb-ft without sacrificing much in the way of acceleration.

And what of emissions? If the point is to be green, dropping emissions from 109 g of CO2 to 99 g isn't anything to sneeze diesel particulates at (99 g is roughly the equivalent of 63 mpg US). As an enthusiast there's something sexy about being able to out-mileage a Prius in something this attractive (paint aside).

Unfortunately, the conversion was a one-off vehicle and AC claims the cost of building one of your own is something like $210,000. At that price it's probably better for you wallet and the environment to buy a tuned AC Schnitzer Z4 and to sometimes take the bus.