MG Rover has replaced the 160PS 2.5-litre V6 engine in its MG ZT saloon and ZT-T estate car models with an equally powerful turbocharged and intercooled 1.8-litre K Series engine to meet UK market demands for low CO2 emissions.

UK company car drivers are taxed according to a complex formula that takes CO2 emissions into account so the move makes some sense though a similar Rover-branded 75 model with a similar turbocharged engine attracted a poor review from the consumer magazine Autocar.

MG Rover, of course, calls the latest model a “tax-beating car with a sports appearance and excellent chassis characteristics” that “provides even greater all-round performance with significantly reduced fuel consumption and a crucially better CO2 rating”.

The new 1.8-litre turbocharged versions of the ZT range have a CO2 rating of 194 g/km and a Euro III standard combined fuel economy figure of 34.9mpg compared with 225 g/km and 30mpg for the V6 160Ps.

All well and good for company accountants, maybe, but we’d love to see the reaction of US or Australian buyers if a similar trick was tried on a popular V6 model (think Camry, Accord, Taurus and a whole line of GM mid-size models) in their respective markets.

The 1.8-litre K-series engine is widely used – with and without turbocharging – in other MG and Rover models, including the F roadster and, newly installed in the MG ZT, is claimed to reduce the 0-60mph time from 8.8secs to 8.5secs, and the fifth gear 50-70mph time from 10.7secs to 9.8secs.

MG Rover says the 160 PS turbo unit has been specifically developed for high torque in the key driving range, with 215Nm available all the way from 2100rpm to over 4500rpm while final drive giving 22.2 mph/1000 rpm in fifth gear (the 160PS V6 was 23.4mph/1000rpm) produces an ideal performance/economy compromise.

The company also claims handling improvements as the four is 50kg lighter than the V6.

The turbocharged MG ZT models use a Garrett GT20 turbocharger with T100 turbine wheel design which is claimed to have been developed specifically for petrol engines, rather than as an adapted diesel unit.

In order to retain good off-boost efficiency, a balanced combination of moderate boost pressures and a relatively high compression ratio is used.

The engine has a cast stainless steel exhaust manifold able to withstand temperatures of around 1000 degrees C, new pistons and connecting rods, and an uprated oil pump.

Suggested further reading: Global market for automotive turbochargers and superchargers: Forecasts to 2005