The Haas drivers have been switching between aero packages during the final few races of the first half of the season as Magnussen had the upgraded package at Silverstone with his team-mate using an older set-up.
The Dane also ran the upgraded package in Germany and Hungary as Grosjean kept the status quo.
However, both drivers will run the same aero package at Spa.
“For Spa we will go with the upgraded car, which we ran at Hockenheim,” Steiner told Autosport.com.
“We’ve learned quite a bit out of it and tried to improve the car by changing some of the components.
“Both cars will be the same spec, and hopefully we can still learn more about it.”
After the highs of 2018 which saw them finish fifth in the Constructors’ Championship, Haas have struggled this year as they are down in eighth with only 26 points.
Although they are hopeful of moving up the grid by making improvements to the current car, Steiner says they have started work on their 2020 challenger.
“We need to see what we can come up with in the windtunnel, with the small developments we do there for the 2019 car, and if we bring any upgrade kits,” he said.
“We don’t know yet.”