Granstra was the only double-figure scorer for the Leathernecks (5-6), finishing with 14 points, nine rebounds and five blocked shots. Clark scored nine points and had game highs of 11 rebounds and six assists.
David Nurse added nine points for Western Illinois.
The Bears (5-7) got 11 points from Mitch Rueter.
Central Arkansas held its last lead with 6:22 left in the first half when Rueter made it 18-17. But the Bears were held scoreless the rest of the half, and the Leathernecks were on their way.
Western Illinois overcame 15 turnovers by outrebounding Central Arkansas 44-28 and holding the Bears to 26.4 percent shooting (14 of 53).
This was the Leathernecks' second victory over Central Arkansas this season. They also won 56-46 Nov. 21 in Conway, Ark.