I would like to know how much the length of the tailpipe effects thrust. I am thinking of moving a P120 about 8" forward from it's current mount, but not if it's going to cost me noticable thrust. Anyone have experience here?

Thanks in advance

Very good question! Pipe length?.... I think it has a LOT to do with turbine performance, length and inlet/outlet diameters, and also position of turbine distance from your pipe inlet... vs your turbines total thrust out put! Some serious testing is needed to really learn more.

What is important to remember is not just pipe length but the inlet and outlet diameters as well are just as critical...

We can really go on also about the pro's and con's of bypasses and how efficiant they maybe, and their relation to pipe performance....
Very interesting subject! - maybe Tam can chime in on this one. He will be the best guy for you to ask, for your specific install.
url- http:www.tamjets.com

I have two of Tams pipes! one in an AD F16 with a 27 lb thrust turbine, (TJT3000) this one was custom ordered to suit engine/airframe combo... It sounds awesome and performs exceptionally well. The other pipe is a bi-fircated pipe for a Avonds F15 ( yet to try! for same engine ) but Tam has flowen this pipe (see pics. ) with your engine type.
I find this pipe performance stuff very interesting....

Another view of the Tam- bifurcated pipe

another view... ( Inlet ).

F15 pipe. (outlet)