After a four-year run with the Mazda 6 on the market, Mazda is finally upgrading its midsize family sedan. Our latest spy photos show a development mule undergoing tests on the streets of Detroit. It's wearing current sheet metal, but we suspect it's riding on the chassis of the 2008 Mazda 6 sedan.

The prototype might not give away any styling details, but its strange appearance does give a few hints to some of the car's expected changes. For one, the next-generation Mazda 6 is going to be bigger. Originally designed for Japanese and European markets, the current model's narrow cabin and limited backseat room have been a constant source of criticism.

This prototype clearly has a wider track as the rear tires barely fit underneath the extended wheel arches. You can also see a cutline running over the rear-quarter panels and wider B-pillars which suggest this prototype is several inches longer than the current model. More width and length would be welcome upgrades as the 2007 Toyota Camry and 2007 Honda Accord both have significantly bigger cabins than the current Mazda 6.

Another major upgrade is expected under the hood. With only 215 horsepower, the current model's 3.0-liter V6 is no match for the V6s from Honda and Toyota. Even the Hyundai Sonata makes the Mazda look weak.

For 2008, we expect the next-generation Mazda 6 will upgrade to Ford's new 3.5-liter V6. This engine debuts in the Ford Edge and Lincoln Mark X crossovers this year as well as the Lincoln Mark Z sedan, a car which shares the Mazda 6's CD3 platform.

With 250 hp and 240 pound-feet of torque, the DOHC V6 won't exactly put Mazda at the top of the family sedan pecking order, but it won't be looking up from the bottom either. A six-speed automatic transmission should help make up for some of its lack of power versus the competition. Ford also claims the new engine can be easily upgraded with new technology like gasoline direct injection, a hybrid drive system, or a turbocharger for more power.

Although Mazda could use the help right now, we don't expect to see this new Mazda 6 for at least another year, probably longer. With two new crossovers hitting the market and a Mazdaspeed 3 on the way, the small Japanese company will need some time before it's ready to launch such a crucial car.

Hopefully the extra year will give Mazda engineers some time to tweak the V6 to give the new sedan the power it needs to compete. Expect to see the results sometime next spring.

With only 215 horsepower, the current model's 3.0-liter V6 is no match for the V6s from Honda and Toyota. Even the Hyundai Sonata makes the Mazda look weak.