General Motors' Cadillac-only Sigma platform (CTS, SRX, STS) proved too expensive for cars like the Pontiac G8 and Chevrolet Camaro. Now GM is merging the Sigma architecture with its other RWD platform, the G8/Camaro's Zeta, in time for the urgent 2011-model-year Cadillac STS/DTS convergence. Cadillac has locked in the new flagship's design. We hear designers are reworking the nose because the flashy headlamp design turned out to be too busy.
A rear-drive Chevrolet Impala is all but off the table. GM is unwilling to give up sales of the front-drive full-size sedan, which isn't ceding sales to the popular new Malibu. The Impala competes directly with Ford's Taurus, and the division already has a Zeta-based model, the Camaro, on the way. Because the only other car on the Impala's aged W platform, the Buick LaCrosse, will soon be replaced with the Epsilon-based Invicta, the next Impala probably will ride on a stretched Epsilon II platform. Will an optional V-8 fit? Doesn't matter, as new Corporate Average Fuel Economy standards start to take effect in the 2011 model year, same as the next Impala's arrival.
The converged Sigma/Zeta platform will encompass six versions on three wheelbases:
Coupe: Next-gen Chevy Camaro, although Holden is still pushing hard for a Monaro version, and Pontiac may want a G8 coupe, too.
Coupe premium: Next-gen CTS. Same wheelbase, but gets forged-aluminum suspension for premium road feel, handling.
Midsize: 114.8-inch wheelbase, including Pontiac G8/Holden Commodore and possibly a Chevy sedan, but not the next Impala.
Midsize premium: Next-gen CTS. Also shares 114.8-inch wheelbase (a 1.4-inch stretch over the current car), but gets the upscale forged-aluminum suspension.
Large: 118.5-inch wheelbase for Holden Statesman/Buick Park Avenue (China), Holden Ute/Pontiac G8 ST, and maybe RWD Lucerne replacement.
Large premium: New STS/DTS replacement. Same 118.5-inch wheelbase as the large sedan, but with forged-aluminum suspension and longer overhangs.
But that's not the entire GM rear-drive story: The company is also working on a small rear-drive platform, primarily for Cadillac's forthcoming 3 Series-fighter, desperately needed to establish the brand in Europe. Speculation suggests this may be split into premium and regular versions, as well, with Holden/Pontiac/Chevy getting the regular models.
