I guess tanstaafl cant answer my question

My sincere apologies! Somehow, I missed the last five posts on the other thread, and did not see your question until Tony pointed it out to me. No fair! My question was a lot easier to answer than yours!

I have a question for you. What is the benefit of valve overlap at high rpms and what is the benefit of a stepped exhaust manifold and why? These question are related feel free to discuss flow velocity versus flow volume in your discussion.

Valve overlap at high RPM is required in order to compensate for the inertia of the gases involved, both burnt and unburnt. One might think that having both the intake and exhaust valves opened simultaneously would cause problems, but in actuality when the intake valve opens the burnt gases have been flowing out the exhaust valve for the duration of the exhaust stroke and have developed considerable inertia flowing in that direction, while at the same time the intake charge is virtually stopped because the intake valve has only been open for a short time and it takes time for the inertia of the intake gases stacked up behind the intake valve to start pushing the intake charge into the cylinder. This means that there will not be excessive mixing of exhaust gases into the incoming intake charge, and not much of the intake charge will be lost out the exhaust valve. The more overlap you have, the more mixing you will get, although this is very much a function of engine RPM. Note that this is before the piston begins its downstroke creating the partial vacuum that will then continue to draw the new fuel-air charge into the cylinder.

I am afraid I do not know what a "stepped exhaust manifold" is. This is entirely my shortcoming, I am not accusing you of making a trick question, I am just not familiar with the term. Does it have something to do with different cross-sectional areas of the exhaust manifold at different points along the length of the exhaust manifold runners?

If so, then a stepped manifold would set up standing waves which, in accordance with Bernoulli's principle, would have maximum flow velocity at the areas of lowest pressure which would (unintuitively) be the areas of smallest cross sectional area; and maximum flow volume in the areas of highest pressure which would be the areas of largest cross sectional area.

Did I do good?

tanstaafl.