20 oz will be the very very minimum that you will want. I have a 2.10 with a regular muffler in a Byron's Glasair and I can fly safely for 7 minutes on a 24 ounce tank. That leaves 2-3 ounces in the tank so that I can have a go-around, or loiter and low throttle if I am waiting to land. I almost never use full throttle since the plane is moving plenty fast at 50-60% throttle. Take offs, verticals, and loops are about it for WOT.