Excellent post Mr. Horton
Dear Kevin Horton:
Thanks for that nice post, it has all the facts and numbers and was much shorter and better than I could do. There are many "tall tales" about how much one gets in RAM rise. Here is the interesting thing, RV inductions are pretty good in stock form, with Van's designed parts (scoop, airboxs and so on). A Cessna, not withstanding it has less RAM air to work with (slower air speed), it also has a filter/airbox set up that produces 1 to 1.5 inches less pressure (inches-Hg), all from the poor filter/air box design.
If you want a "down-N-dirty" quick test, Van wrote this procedure in the RVator years ago. Here is my rough recollection:
On the RAMP, engine shut down, note the MP - manifold pressure (static air pressure).
Take off and fly a wide open throttle (WOT) low pass** over the runway (approx ramp elevation or altitude).
The difference from ramp to low pass MP is the total ram rise. It will most likely, typically be about the same +/- a few 1/10ths.
I've thought about ways to improve Van's set-up; its pretty hard beat it. The BIGGEST negative with Vans stock set-up, which Van knows, is the soft connection between the cowl scoop and the air-box. That is where there is most likely loss of incoming air. Van recognize this but for practical purposes current design (at least for vertical induction) meets his design goal of simple and efficient. A solution (as Van suggested long ago) is a little access door in the cowl you can affect a sealed duct connection between cowl and air box, with a clamp for example. With out the access door the scoop/air box connection is not possible with the lower cowl in place.
The other improvement are in the Lyc induction it self with those cold sumps.
A "Urban Legend" is that forward facing induction is soooooo much better. Not really, all the air has to make a 90 degree turn from the free air stream to the intake lower intake ports in each cylinder at some point. Whether the air makes the 90 degree turn before the throttle body/carb or after does not matter. In fact fwd facing induction give less room to develop a good air box to slow the air down and build up pressure and volume reserve. Remember a piston engine is not a turbine that is constant flow. Piston engine induction starts & stops in pulses. That is why a plenum is so critical to act as an air reservoir. Many fwd facing inductions just have a small tube and than BAM, engine. The horz air-box van sells for the RV-7/8 is very efficient and well engineered. It has been tested and actually has slightly more RAM efficiency than the Vert air-box. Also the elimination of the external scoop may add 1 or 2 mph.
** For goodness sakes be very careful doing high speed passes down the runway. I have had the good fortune to have uncontrolled fairly low use long runways way from population to do test in the past. Also the potential bird strikes thru the canopy scares me. Do this at your own risk and please be careful. Flying low to the ground, fast and looking at your MP gauge may not be for everyone. Nothing wrong with that.