Supercars continues to work on equalising its new Gen3 cars with Ford and its teams convinced the Mustang package is at a disadvantage to Chevrolet's Camaro.
The chief suspect in the alleged disparity is the engine, with the 5.4-litre Ford V8 thought to lack performance and drivability compared to the 5.7-litre Chev, despite their power outputs being the same.
However there have been whispers from Ford drivers that the Camaro is better in other areas too, suggesting that elements such as aero could still surge back into the limelight at some point.
As it stands there is ongoing tweaking with engine maps, which is likely to continue in Darwin next month, while Supercars waits to run a beefed up testing programme with the two engines.
That is set to include a transient dyno testing programme and torque sensor fitted to the cars.
According to Rushbrook, until that data has been collected, and shared, by Supercars, pinpointing the exact issue with the engine, or any other factors that may be affecting parity, is impossible.
“I think that comes back to the data transparency," he said when asked by Motorsport.com if the engine was the only concern.
"So, in every racing series outside of Supercars we have full transparency into the data and we know what we are doing relative to our competitors on-track. We know where we, as a manufacturer, may be falling short or succeeding, or our partners – whether it is an engine partner or a team – we have that as a dashboard in front of us.
"We don’t in Supercars.
"So to be honest with you, we can make observations based upon what we see on track and what data is available from our teams, but until there is data transparency, we can’t draw clear conclusions of engine parity or anything else."
Rushbrook admitted that it was frustrating that transient dyno testing hadn't been part of the initial development of the motors, but said Ford is willing to be patient in the pursuit of parity.
"There’s a different level of patience required in every series around the world to work through issues that we see there," he said. "Certainly I would say that there has been some progress and there is a sense of urgency to continue to make improvements, both near-term in terms of calibration changes, which I would say are more on the interim containment plan, as well as commitment from the series for torque sensors and transient dyno, which is unfortunately longer term.
"Those things are all long lead and we wish they were in place already before the start of the season but at least there are actions in place to have them on-hand."
Both Ford and Chevrolet did run their Supercars engines on their own transient dynos during durability testing last year, something Rushbrook suggests Supercars should have paid attention to.
“Every engine that we develop in every series, we run on a transient dyno, usually both a transient engine dyno and also a transient driveline dyno which includes the full engine and full driveline out to the wheelhouse," he said.
"So I think it should be an indication that if automakers that are involved in motorsport on a global basis are using transient dynos to develop the engines, we’re using them for a reason – because there is a lot of available information that you get on the transient dyno that you don’t otherwise.
"We ran on a transient dyno but that doesn’t do anything to help parity within the series if the two engines, or as installed in the driveline system, if they’re not run on the same dyno in a comparative way."
When asked what reason Supercars had given for not doing transient dyno testing earlier, Rushbrook said: “They had confidence in their parity process".
