Look at the flow chart for the heads. That is positive air flow through the heads and it all goes into the exhaust. There have been many ways devised to lower the back pressure an exhaust system makes. There will never be a vacuum when you are blowing air into a pipe. Not unless you hook a pump to the other end that sucks more exhaust than flows through the heads. Just think about it. If there was vacuum anywhere in the exhaust that would mean negative flow. Positive flow means no vacuum. Positive flow with any restriction at all causes pressure.