I believe I posted these in another thread some time back, but I thought they might be useful here too.
Photos here show a Jett SJ-90L engine installation with the standard jett-stream muffler. Extention pieces are required. Dub can provide these additional pieces modified with an additional angle cut (as shown) to allow the muffler to clear the fuselage. The angle is provided by requesting the "Toni installation"
As you can see, the installation is similar to a few others that have been posted here.
Recommended props for a bit of speed from this combination range from 11x8, 11x9, 12x6, 12x7, 12x8. So far folks have told me that the 11x9 has worked out very well.
Of note, the Jett-Stream muffler and extensions shown here are sized for 47mm bolt spacing, which is the same as the ST 90. Just specify the ST90 and Toni installation when ordering if you are interestest. The jett-stream is also available for the OS91FX, but the custom extensions are not available for the bolt configuration.