The following represents my guesses, not backed up by data!

In designing a tuned length manifold, I suspect it is best to choose the
length that corresponds to a speed about at the middle of the speed range at
which full throttle is used. In an automobile, that's probably about
two-thirds or three-quarters max engine speed. American cars have torque
curves that max out at about 2500 RPM. This allows you to have responsive
acceleration without downshifting. I don't know airplanes, but I'd guess that
you only use full throttle during take off and climb -- and that the RPM is
closer to 85% or 90% of max engine RPM.

I'd further guess that the effect on efficiency of a tuned length manifold
decreases rapidly as the throttle is closed. Ram tuning, after all, increases
efficiency by reducing pumping losses, and, secondarily, by allowing an
engine to operate at higher BMEP which is generally more efficient. When the
throttle is partially closed, the BMEP effect goes away completely, and the
benefit of reduced pumping losses might be much reduced.

It's therefore my guess that the manifold should be designed to maximize
torque at take off or climb RPM. At cruise, you're not operating on the
torque curve anyway, so the torque improvement created by the tuned manifold
is not of any benefit at that condition.

Just my .02

Jon Lauter
Rotary Power Marine Corporation

One difference Jon. At 8,000 feet and up the throttle is fully open due to
lower air density yet the engine is generating only about 70% or less of
max power. It is common in a normally aspirated A/C engine to cruise at wide
open throttle at anything over 8000 feet.

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org