Originally Posted by 1slow370

they don't run off back pressure, to much is still a bad thing as then you start to trap exhaust in the cylinder instead of more fresh air mixture. the breathing mechanics are complicated. basically what you want is enough duration and lift on the exhaust that the engine pumps out most of the exhaust gasses, but on our cars which are tuned for n/a the valve overlap between the exhaust and intake valves allows the now compressed air charge to escape with the exhaust before the valve closes. all that is really needed is to bump the intake cam timing backward to reduce the overlap but with vvel you have to do both or nothing or else you risk the valves hitting the piston due to the high lift.