In this picture I've begun to insert the center tube into the exhaust pipe. Unfortunately this picture makes it seem as if the plumbing cap nearly fills the ID of the exhaust pipe. This was not the case, there was quite a bit of room for exhaust gases to pass. Add to this the cap will be located in the bend of the pipe. Once assembled there will be even more room for exhaust gases to pass. I had approximately 1/4 inch on each side at the straight part of the tube. However, once it arrived at the bend the area increased due to the shape of the plumbing cap mis-aligning with the curve of the exhaust tube at that area. If the area would have been too tight, I would have abandoned the idea of creating a venturi effect and just gone with plugs. But I was happy with the way this fit. If you try to duplicate this, you'll have to test fit in your application to determine what seems reasonable.

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