Jay
I have 3 OS 160 twins, one of them in a DP CUB (see some of my exhaust pipe pics in earlier posts) and I dont use muffler pressure on any one of them, no problems at all with fuel draw. I do use an on board glo lighter to make the starting easier with both plugs lit at the same time and no cowl holes. The Keleo exhaust pipes that have been shown on other posts bring the two pipes down to the same side of the cowl, I bent pipes and brought them both down to the middle of the cowl, more like the full scale. You will love the engine (and the plane)
Ric