With no restrictions on testing in the DTM, top teams usually hold private tests at all rounds on the calendar, with the exception of Norisring, incurring significant expenditure in order to do so.

It contributes to a third of their annual budgets, and means competing in the DTM costs significantly more than other GT3 championships of similar scale.

Due to high costs, many people in the paddock have called for a ban on testing in the DTM, including former motorsport chief Norbert Haug.

So far, the ADAC has remained sceptical about the prospect of imposing restrictions on private testing as GT3 cars are used in a variety of series around the world, making it hard to monitor any illegal running.

But SRO has shown that it is possible to put limitations on in-season testing in GT3 championships, and DTM could do well to follow in its footsteps and impose a ban of its own. 

“There is definitely less testing [in GT World Challenge],” Plentz, who moved from the ADAC GT Masters to GTWCE in 2023, told Autosport's sister title Motorsport-Total.com. "We didn't have a single day of testing during the year at tracks where we raced."

In GTWCE, teams are free to conduct as much testing as they want until eight days before the start of the season. However, after that point, teams can only hold tests on tracks that are not on the calendar (or, as the season goes on, on circuits they have already raced on). The only exception to this rule are official SRO tests, such as the one before the Spa 24 Hours.

While policing such a stringent ban on testing is not easy, SRO relies on agreements with circuit owners to ensure no team is disobeying the rules.

"Nobody in the world can control a test ban 100%, unless we were to work with manufacturer control units [ECU] that are taken away from us [after every race]," said Plentz, referring to how teams in customer racing get to keep the ECU as they own the cars.

"If [the circuit owners] see a suspicious car there, they may report it and then it will be investigated.

"[Or] someone would see it in today's digital world, take a photo and report it. So that's something. And if everyone sticks to it to a certain extent, because it's in their intrinsic motivation, then it can be regulated."

However, Plentz believes restrictions on on-track testing won’t solve all the problems, as teams could use dynos to gain an edge on their rivals.

"There's a big but. Money isn't just spent by physically driving the car. I can also put my car on a seven-post rig," said Plentz, who withdrew from the post of team principal to take up a behind-the-scenes role at Rutronik.

"A day can easily cost me 7,000 to 10,000 euros. And if I do it for three or four days, then I also have data and results that I can use for my development and my performance. 

"In other words, you'll still have the budget versus low budget component [battle between top teams and smaller teams], even if it shifts towards off-track."

For a cap on testing to work, Plentz believes it is important for the ADAC to work with teams and come up with a set of regulations that is suitable for all parties involved.

“What Stephane [Ratel, SRO chief] does really well is there are team committees, and there were discussions with each team to get feedback from each individual,” he explained.

"You sit together and discuss the issues critically and try to define a path for the future together. That's the only way it can work, because if only one person makes a commitment, then you can forget about it. That has to come from everyone.

“If the ADAC says tomorrow that we’re going to do it now, then that would be difficult.

“The teams have to agree on a regulation with the ADAC. If everyone signs, then you can definitely find a rule that goes against cost development."