McLaren could yet decide to keep the F-duct on its car for the Italian Grand Prix, despite the team saying earlier this week that it would be removed for the Monza event.
McLaren's managing director Jonathan Neale said on Wednesday that the team will wait until it has analysed the performance of the F-duct at Spa this weekend before making a final decision.
"We have the option to run it or not [at Monza]," Neale said. "At the moment we're just looking at all the options. We'll watch carefully where the end-of-straight speeds come out at Spa and make the decision in the following week."
He explained that although the F-duct gives a straightline speed advantage, the team is unsure at the moment whether the gain on such a low downforce circuit will offset the compromises of running the device.
"It all depends on where you want to be for total aero efficiency," he said. "There tends to be a herd instinct in Formula 1 of where the downforce and end-of-straight speeds are set.
"Other teams have F-ducts and are performing well in straightline speed. If we think there's an advantage from it, we'll run it. If taking the weight of running it out and making the rear wing more efficient is effective, we'll do it. But it's all just fine tuning, it doesn't make a big enough difference to win or lose you a race."
Neale added that McLaren is no closer to understanding the situation with the flexi-wings of Red Bull and Ferrari.
"I've seen the footage and we can't explain why those cars operate in the way they do," Neale said. "We've spent a lot of time looking very hard at the cars, but it's just speculation on our part. It's a discussion for the FIA and those teams to have.
"We presume that because Charlie [Whiting, FIA race director] has issued a clarification for the coming races, there was an issue. We're all working with the FIA on it."