Just a plenum with ports cut into it? What are the physics here?

 

I am going to wing it here, but the runners help to guide the air into the head. If you just had a plenum then the air would have to be sucked in from the plenum and that could create turbulence for the next cylinder that required air. Remember runner design plays a great role in the amount of hp your motor will produce. In NA motors long runners are good for torque while short runners are better for horsepower. The column of air in the runner is fed into the cylinder, depending on the mass of the column and shape of the runner, you can affect where the HP and TQ peaks are and how high they are. Thats just a begging, you shuld probably search the net for a more precise and better explained answer.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Atheist &raquo;</TD></TR><TR><TD CLASS="quote">What are the physics here?</TD></TR></TABLE>More like acoustics...

It's non-steady flow, there's pulses from the valve opening & closing. They reflect back from the plenum & either help or hurt the next intake stroke. The 'best' length of the runners depends on the speed of sound, & your choice of what range of rpms you want some 'boost'. Longer runners help out at lower rpms. Short runners help out at higher rpms, but at lower rpms the reflected pulses sorta act like a restriction. That's why the short runners (on a GSR) are closed at lower rpms.

It's sorta similar to exhaust headers. Someone chooses the length of tube before the first 'feature' (branch) that reflects an exhaust pulse.