Thought I would post a blog for those of you interested in using dual exhaust on the Jabiru powered XL, 650 and 750. I believe if you're using the stock Jabiru cowl there will be no modifications, the pipes should have enough clearance. I went down a different path with a custom cowl.
I used a Hooker Reducer as the collector, 2.5" to 2" and then had the
muffler shop make the pipes. The pipes are identical and I rotate to get the position I like. The Hooker Reducer needs to be shaped to match the exhaust pipes. I used a radiator clamp to squeeze the pipes together to fit the Reducer.http://www.summitracing.com/parts/HOK-11125HKR/
The exhaust wrapped. There are two different schools of thought on wrapping the exhaust. Not that I needed it, but it is approved by Jabiru USA.
Fabricating exhaust cuffs. I used some foam around the pipe, covered with duct tape and used fiberglass. Version 2 of this setup will be to reshape the bottom cowl into a flat surface.
I cut the pipes so that they are just inside the cuffs to help push air out of the cowl. This tip came from "Speed with Economy", an excellent resource if you're thinking of increasing the performance of your airplane. I'm pleased with the results.
And finally, the mockup I used to have the pipes fabricated. In the picture of the pipes exiting the cowl, the welded tabs are visible, used for a rear mounting strap.
My original thought was a 2 into 1 system, but the muffler shop had limited capabilites. The mockup is made of 1-1/2" PVC, this had an OD a bit smaller than 2". The pipe will fit into the Hooker Reducer. To make a 2 into 1 mockup you will need 5 - 45 degree bends and 1 y pipe. If you're just interested in straight pipes you will need 2 - 45 degree bends. 3-4' of 1-1/2" PVC.
When it became obvious that the 2 into 1 couldn't be fabricated easily, I decided to go with straight pipes and was able to use the mockup since the left side went straight back.
The key measurement is 200mm between the center lines of the 45 degree bends marked in red. This can be longer and the pipe will extend farther under the fuselage. The reducer end will have to be expanded to 2" ID and as close as possible to the first bend after cutting some length. The tail will be long and I had mine cut at about 24". Have 2 identical pipes made and then fit on the airplane. Rotating the pipes will give you a close fit. Mark at least 1" in from the firewall for the location of a tab to be welded.
Shaping the Hooker Reducer to match the pipes can be difficult. Strapping 3 1-1/4" pipes together would help. When installing the reducer use a radiator clamp to squeeze the exhaust pipes together, a C-clamp helps. I don't like the idea of loosening the exhaust pipes to install the muffler or reducer. I felt it was better to tighten the exhaust bolts to specs first, then using clamps to squeeze.
One final very important item. I left the Reducer/Collector open where the 3 exhaust pipes enter. It is possible to seal with when wrapping, but mine is still open and I use it as a variation on exhaust augmentation.
That's my story.