Originally posted by Scoubidou
Hello Gerald,
With a two blade APC 17x8 I got 8900-9000 RPM readings.
1)Did you tach a 18x8 APC or 18x7 prop 2 blades?
2)Which tune pipe model did you use and what is your airplane model and weight please? I plan to use it on 24% CA Extra but that long tune pipe can bring dead weight! 
Scoubidou
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.