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  The S400's boot space remains the same shape and size as that of the S350 donor car. Photo Gallery

• BMW claims 13 litres/100km.
• Mercedes-Benz cuts down CO2 emissions to a mere 186g/km.
• An S-Class model and the BMW 750i will arrive September.

The new Mercedes-Benz S-Class and BMW 7-Series flagships will face off later this year.

One has two turbochargers, an eight-speed auto and a sub-5 second sprint time — but it isn't a sports car.

The other has a 3.5-litre petrol V6 mated to an electric motor and, at 7.9 litres/100km, will be the cleanest, greenest luxury saloon to hit Australia.

The bi-turbo car is BMW's new 7-Series luxury flagship, the 760Li, which arrives here in September with an expected $375,000 pricetag.

The flagship gets a new all-aluminium 6-litre V12 engine with direct petrol injection and variable-valve timing and lift — all enough for 400kW at 5250rpm and 750Nm of torque from 1500rpm.

BMW claims 13 litres/100km.

But Mercedes-Benz almost cuts that consumption in half — and CO2 emissions to a mere 186g/km — with its S-Class hybrid, the S400 Hybrid.

This car, one of 10 new S-Class models that may start coming into Australia in September, is the world's first standard production hybrid drive with a lithium-ion battery.

The S400 Hybrid's modified 3.5-litre petrol engine has 205kW and the electric motor produces 15kW and has starting torque of 160Nm.

Mercedes-Benz claims the major advantages of the lithium-ion batteries over conventional nickel-metal hydride batteries include a higher energy density, greater electrical efficiency, compact dimensions and a low battery weight.

The S400's boot space remains the same shape and size as that of the S350 donor car.

The S-Class range also includes S350 CDI BlueEFFICIENCY, S350 CDI 4MATIC all-wheel drive; the 4-litre V8 S450 CDI; S350 and S350 4MATIC; the 4.7-litre V8 petrol S450/S450 4MATIC; the S500/S500 4MATIC with 5.5-litre V8s; S600 bi-turbo 5.5-litre V12; and the two AMG versions, the 386kW 6.2-litre V8 S63 AMG and the 450kW 6-litre V12 S65 AMG.

Further details of the S-Class range will be revealed next month.