The following series of comments are from a thread on the Advanced Engine Tech section of the Speedtalk forum. The writer is a professional camshaft designer, whose clients include major race teams in multiple forms of racing. I found them interesting—or in some cases, "interesting"...
I have separated individual comments with a double line, In some cases I have edited for spelling and structure, or have paraphrased or simplified statements, but I do not think I have altered the intended meaning—at least as I thought I understood it..
This is just one more example of why I really like Speedtalk Engine Tech. There is always something way above my head to think about. If you have not been in there, join and lurk (and learn). These conversations are so far above most of the stuff you’ll find in any Harley forum, it’s almost embarrassing.
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An excerpt from the attached document:
I have delayed answering your question because the answer is very long, and there are many different interpretations of it. Each Cam designer has his own theory, and often they contradict each other. Here are some of my thoughts and practices on cam lobes versus airflow.
First--AFAIK, in a normally-aspirated (NA) engine there is NO airflow into the engine before-top-dead-center (BTDC). Before that point the piston is pushing exhaust gases out of the combustion space. These gases have positive pressure and block the intake port as the intake valve opens. (In essence, this is reversion). I believe no intake airflow can start into the cylinder until this pressure has dissipated. This is why I design all my cams to minimize reversion by opening the intake at that point. I have done it this way since 1977, and I have seen that it works.
Note to page viewer: I really hesitated to put this out here, because in it I admit that I'm too stupid to understand what this guy is trying to get to. But, that's the idea behind this section of the site: This is the "...let's think about this..." section, where there may not be complete answers---just more questions.