Curt,


This was not the main issue of my post. The blow-down period was, saying exhaust duration is not alone here.

As to your question; I was saying that a tuned pipe of a given design (divergent-convergent cones F.E.) must be longer for low RPM than it needs to be for high RPM.

Please see this web page: http://www.mh-aerotools.de/airfoils/...pe_main_en.htm .


GPSpacewalker,

What you wrote about the Nelson UltraThrust surprizes me.
Its Jett counterpart, the Jettstream, is admitedly biased toward the 15K-18K RPM range.

When mounted on Jett .40-.50 engines, its performance with larger 10x8 to 11x7 props, could at best be considered average.

In his tech-sheets on tuned pipes, Dub Jett wrote that on the Jett .46 with a 10x6 prop, a full length tuned pipe, may realize a gain of 300 RPM over the already tuned Jettstream. This, while on larger props, about 1,500 gain is realistic, when changing to the full length pipe. With the Jettstream this prop is turned at only 12K.

This would be a direct indication that the Jettstream is not tuned to resonate in the 12K-13K RPM range.
It surprizes me that that you wrote that the Nelson UltraThrust is.