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  A revamp of the Forester and Outback has improved fuel economy, emissions and broadened the engine range of peak torque.

Lower thirst and broader torque spread, but no auto transmission.

The Japanese car-maker has undertaken a revamp of the flat-four "Boxer" turbodiesels in its Forester and Outback model line-ups that has improved fuel economy, emissions and broadened the engine range of peak torque.

The 108kW (110kW in the Outback) and 350Nm outputs remain unchanged but is now available a little earlier in the rev range - 1600rpm instead of 1800rpm.

Subaru says the fuel use has improved by up to 6.2 per cent, which puts the Forester and Outback diesel models at a combined-cycle figure of six litres per 100km .

The flat-four turbodiesel has been given a new exhaust camshaft profile, more efficient exhaust-gas recirculation and some lightened engine internals to reduce fuel consumption.

Multiple transmission changes - low-friction bearings and gearshift system among them - have produced lower friction and greater efficiency, says Subaru, but there's no sign of an auto.

Subaru Australia's David Rowley says there are no immediate plans for an automtic transmission to accompany the diesel powerplant.

"There are no immediate plans for an automatic, we would certainly love an auto and we will be at the head of the queue when one becomes available, but that's not an option presently," he says. "We're very pleased with the response to the diesels in both the cars since we launched them, so we're keen to build on that when an auto becomes available, but there's no timeframe."

Mr Rowley says the natural disasters and engineering and development focus on the new XV and Impreza models would have taken priority over development of an automatic.

Pricing has been left untouched despite the extra development - the Forester diesel starts from $36,490, rising to $40,990 for the Premium model.

Subaru Australia managing director Nick Senior says diesels represent up to 20 per cent of respective Forester and Outback range sales.

"These significant refinements to engine and transmission make an even more compelling case. The smoother gear change helps more efficient driving and better fuel economy, which is a major focus for Subaru diesel customers," he says.

The Outback diesel is priced from $40,990 for the manual an d $46,990 for the Premium model.