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I understand that the excessive backpressure will hinder VE. In this webinar, the data log shows that the exhaust pressure is significantly lower than the intake pressure until a certain rpm, in which they become equal, then exhaust pressure rises past the intake pressure. My question focuses more on the area where the engine has a much lower exhaust pressure than intake pressure. Camshaft overlap takes advantage of this in terms of scavenging, but in my mind, it would occur to me that the exhaust pressure can be so much lower, that the fresh air just goes right through the exhaust port. I want to know if my thinking on this is true and if so, would this be a case of having a camshaft that is just too big or has too much overlap?
I don't think you'll ever have a scenario where the intake air/gas mixture will blowby straight to exhaust, since you'd need negative pressure a.k.a. vacuum on the exhaust line to do so. No matter how good your exhaust system is, there's no way to achieve enough vacuum for this to occur, without attaching some air pumps with higher capacity to move air than a combustion cycle can produce. This is obviously not gonna happen in a car.
Think of overlap like this. Air is lazy, and it doesn't want to move. So after you invest some energy, let's say you combust some mixture, air (gases) starts to move through exhaust port, and builds inertia. Once enough inertia is created, in this case, by gases moving from combustion chamber to exhaust, engine will start to develop a pressure drop inside the cylinder volume (at least relative to exhaust line, if not relative to atmosphere), and if your intake valve is opened at this point, fresh air mixture will pour in to fill this low pressure volume. So your lowest pressure is in the cylinder at this stage, not the exhaust. This is how overlap helps improve intake efficiency.
Well, at least this is how my mind's eye sees it, and also English is not my first language. Someone feel free to correct me.