John, I have not seen any vacume at WOT on the engines I've dyno tested between the carbs and the blower rotors , same thing below the blower when at part throttle going to full throttle before boost is made It would (vacume signal)read vacume at part throttle in the blower intake and then go to zero and then start making boost This was with two 650 Holleys, two 1050 Dominators and two 750 carters, I have not dyno tested a roots blown motor with a single carb so I don't know if they wll make vacume under the carb at the upper RPM or not