It depends to some extent on the design of the bypass, the diameter of the inner and outer exhaust ducts, the space between them, and especially the size of the bellmouth/splay at the front of the inner exhaust duct.

In most models the dimension from the back of the Mercury exhaust nozzle should be between 13mm - 15mm from the front edge of the bellmouth of the inner duct. Incorrect positioning of the turbine will give a noticeable loss of thrust, higher EGT's, and poor throttle-up. Incorrect positioning will DEFINATELY give higher exhust duct, and therefore fuselage temperatures. In extreme cases you could damage the tailpipe and/or fuselage.

I spent a day with Philip Avonds testing this in his F-15 with interesting results, and Bennie and I have also done considerable testing on this in many customer models as well as pour own.

For reference, in the Avonds F-16 with a Mercury HP the distance from back of turbine exhaust nozzle to front on inner tube bellmouth should be 13mm exactly.

If you can tell me which exhaust and bypass you are using I can confirm excat dimensions that will work best.

Needed: diameter inner and outer ducts, size of bellmouth (normally 1/4" x 45 degrees), relative location front of outer and inner ducts at front, relative position outer and inner ducts at rear of model - and also distance that outer duct is inside the rear of fuselage. Also what type/manufacturer of bypass are you using?

Best regards,

Mike C
(AMT Netherlands/Holland)