Ahmed worked hard to reduce the gap to Green in the second half of the race, but the latter held on to win on his debut.
Two cautions and subsequent restarts, first for a collision between Salvador de Alba and Reece Gold and then for Ahmed colliding with the wall, helped Foster to rise to second ahead of Eves, but Siegel remained in the lead to win.
He led from start to finish, remaining unchallenged throughout a caution-free race, with the only incident being a collision between de Alba and Jack William Miller.
Siegel claimed pole for race two, leading Eves, while Foster gained three places at the start, rising from sixth to third.
The weekend started in chaotic fashion when surprise polesitter Jonathan Browne locked up in turn one, producing a slide which collected Foster and Miller, retiring both cars.
The final race of the weekend saw Miller start from pole again, with Foster cleanly overtaking him for the lead this time.
A caution brought the field back together and led to multiple changes in the order, but Foster held on for his first win, ahead of Eves and Miller.
Miller started seventh, but rose quickly enough to also pass Green and begin attacking Eves, albeit without success.
Porto resisted Foster's pressure for the whole race, with the gap never growing beyond a second, and held on for his first win, with Sundaramoorthy in third.
Race one saw a flag-to-flag victory by the Englishman, holding of Ahmed at the start and surviving two caution restarts when Browne lost his front wing and de Alba retired.
In the end, de Alba won in dominating fashion, Siegel managed to hold on to second, and Eves was third after Green ran wide with ten laps remaining.
He was challenged by Gold in the opening part of race one, but remained in first place as an incident between Siegel, Porto and Miller promoted Ahmed to third behind the leading pair.
Behind him was Marcos Flack, who surprised in just his second race by jumping from 16th on the grid to fourth place in the span of a single lap.
Foster was still battling for second despite having already sealed the championship, and eventually passed Porto, but Gold had pulled out a gap to the rest of the field by then and won the race.