Sennan Fielding fought through to take third and close the gap to the championship leaders, taking a podium position after a last lap pass on Gaetano di Mauro, who finished fourth.
Struan Moore took fifth for Lanan Racing, ahead of Raoul Hyman who starred by rising from 18th to finish sixth.
George Russell originally started the race from pole position, but finished seventh ahead of team mate and championship leader Arjun Maini.
At the original start, Menchaca made the best getaway and was able to out-drag Russell on the run to Riches on lap one, while Gaetano di Mauro was able to move past Russell also and take third.
Further back, Diego Borrelli and Rodrigo Fonseca retired at Palmer corner on lap one. A further tangle between Will Palmer and Jordan Albert at Montreal hairpin brought out the red flags with the cars interlocked on track, with Albert handed a five place grid penalty for tomorrow morning’s race.
After a lengthy delay, the race resumed over six laps, with Menchaca lining up on pole, the grid positions taken from the order when the red flag came out. The Mexican repeated his good start and held the lead, while Eastwood climbed into second. The pair lapped nose-to-tail throughout, with Menchaca keeping his team mate at bay by just 0.247s at the flag.
Behind the Douglas duo, the championship contenders hit more trouble. Russell made a better start from third place on the restart, and challenged di Mauro. As he did so, Eastwood, lining-up fourth was able to move past the pair to take second. Russell was also passed by team mate Struan Moore, and was down to fifth at the end of the first racing lap.
On the fourth lap, Moore challenged di Mauro through Riches and at Montreal. Moore was delayed and allowed Russell to get a run on him heading to Palmer corner, but Russell was squeezed out and fell to seventh with Hyman moving into sixth, while Fielding moved past the Lanan duo to take fourth.
With the Lanan drivers suffering, Fielding was well placed to make inroads into the championship lead, and was able to move past di Mauro on the last lap to take third place.
Menchaca commented: “Charlie made my life quite hard but I’m so happy to finally take my first win. I really want to thank all the guys at Douglas Motorsport, they’ve been amazing all year.
Menchaca, Eastwood and Fielding were presented with their trophies on the podium by BRDC member Joey Foster.
Maini still leads the championship by a slightly reduced margin of 19 points, while Fielding in third is now just 22 points behind Maini and three behind Russell. Hyman stays fourth and is 34 points adrift of Maini. Moore and Palmer can no longer win the championship. Maini can win the championship in tomorrow morning’s race, which starts at 10.20, should he outscore Russell, Fielding and Hyman by at least 16 points.