Originally posted by GeraldO
I used the MVVS tuned exhaust designed for this engine. Morris Hobbies has it. It doesn't add "dead" weight, it adds a major power increase. The bigger problem is where do you hide that great long thing? On the Four-star it runs down the outside of the fuselage. I just tried the plane with a K&S "can" type smoke muffler last weekend. Still runs good but the power loss is quite noticable. It's in a Sig Four Star 120 which weighs in at about 13.5lb. Went from unlimited vertical with the tuned muffler, down to unable to hold a hover for more than a few seconds with the K&S. It does run a little smoother through the midrange though with the K&S.
I havn't measured the RPM or even ran it yet with an 18x8 APC but I have one and intend to try it soon. I bent the aluminum landing gear down farther on the Four Star to make it sit taller for the big props.
The MVVS tuned pipe is in resonance between 7000 and 9000 rpm.
Below 7000 the pipe falls off the edge slowly