I've been reading that 2 1/4 is the ideal size for these. Was just curious because the stock exhaust manifold is 2". Wouldn't that make the extra 1/4" ineffective?
I wouldn't know about ideal size or manifold size. My 73 came with a 1 7/8" exhaust pipe. The Feather Duster had a 2¼" exhaust system from head-to-tail. As far as I know the same exhaust manifold was used.
When I replaced the exhaust on the 73, I went to a 2¼" pipe. There was a slight improvement in mpg. The sound through a "turbo" muffler was a little bit deeper. To answer the question, I would say that the /6 responds favorably to the larger pipe.
The physics of the change suggest that there would be at least a little loss of scavenging due to the lower velocity through the larger pipe. I have no way to measure the difference, so at this point the difference is theoretical.
FWIW: A gas contracts as it cools not expands. (see Charles's Law). If this were not true, a lot of money has been wasted on ram air hoods and intercoolers.
