Bill, Vacuum has nothing to do with it. It is air flow and fuel flow. And to a lesser extent the fuel "sloosh" in the float bowls. As the air/fuel flow leaves the bottom of the carb, it has to turn, several times to get to the cylinders. Fuel can drop out of suspension. You want the oriention of the carb to give the most equal air/fuel distribution. Even the amount the throttle blades are open effects how the air/fuel flows. Look at the old Direct Connection books, where they talk about "Dams" in the manifold runners, and different radius on the entrance to the runners. With a slant six I don't Know which way is better, as I have never tried back to back tests. On my 170 I run a 500 2bbl mounted side ways. On my 225, I have a 465 4 bbl mounted "normally" like a V-8.
