Motorsport.com understands both manufacturers came into today's two practice sessions with a fresh engine map at their disposal.
For Ford it is the third new map in as many rounds amid ongoing questions marks over the performance of the Blue Oval's powertrain compared to the Chevrolet.
The Chevrolet V8s were also given an upgrade focussed on improving drivability.
The new Chevrolet map was trialled by Triple Eight during testing at Queensland Raceway last week where it reportedly performed well, as it did when sampled on a Brad Jones Racing Camaro at Winton.
However it hit a snag when implemented across the GM field for practice 1 in Darwin today with poor idle characteristics and throttle response the typical feedback.
As such Supercars elected to ditch the new map for the Chevrolets and return the motors to the same specification as they ran in Tasmania last time out.
They will continue to run in that spec for the remainder of the Darwin weekend.
The Ford upgrade appears to have been more successful, with all Mustangs running the new map across both practice sessions at Hidden Valley, some with a 30 millisecond shift cut and some with a 50 millisecond shift cut.
Ford drivers locked out the first three positions on combined practice times from the two sessions, with Anton De Pasquale (Dick Johnson Racing) leading James Courtney and Cam Waters (both Tickford).
But while the times looked promising, another Tickford driver in Thomas Randle, who was fifth fastest for the day, told Motorsport.com that there was no noticeable difference from the cockpit.
"To be honest it's hard to feel the change," he said. "Maybe if we'd run the old map for a lap or two and then put the new one in, it would be easier to tell the difference. Because the last time I drove this car in the dry was Tasmania and that's a very different style of track.
"It's hard to give you a definitive answer, but it's not like, 'oh my goodness, it feels like I've got 20 extra horsepower'. You can't notice much."
Under the Gen3 rules teams no longer have control over engine mapping, with changes implemented by the category in consultation with the homologation engine builders.
That proved to be a major problem for Triple Eight in practice 1 earlier today when attempts to trial a safety car speed limiter on Shane van Gisbergen's car went awry, and the team was unable to rectify it alone.
The Kiwi was still complaining about throttle response in the second session even once he was back on the Tasmania map.
A more accurate portrait of the competitiveness of each brand will be painted tomorrow when the field qualifies for the first of three sprint races.
