There is more to exhaust tuning and performance than I will ever know - no matter how hard I try. There will always be results that tend to make you re-think what you
think you know.
If you want to learn more about all things related to performance/racing internal combustion, I would recommend becoming a member of the SpeedTalk forums - There is WAY too much good information there from an amazing number of good sources.
http://www.speedtalk.com
Then check out the following link in the Advanced Tech forum and read the thread from beginning to end -
http://www.speedtalk.com/forum/viewtopic.php?t=8305&highlight=log+manifold
There are many pages to wade through, but it is well worth the time.
To give a brief overview, a tubular log manifold with relatively short primaries, built by one of the best exhaust system builders in the industry, gave results (on a full out NASCAR engine for a well known team) within 1.4% of the
very best stepped merged headers they had tested, and better than many others. The low end results were good as well.
If you really want to screw up your thinking on exhausts, do some research into the work and patents of Dave Piekarski -
http://patents.ic.gc.ca/cipo/cpd/en/patent/2417289/summary.html
His work with
square tube headers in 4-1 and 4-2-1 designs has a lot of smart guys saying it will never work. Mr. Piekarski's work and testing in racing environments has been going on for several decades, in several forms of motorsports. I had a chance to meet the man and see some results of over 200 back to back (A-B-A) dyno tests at a respected (and neutral) friend's shop several months ago, on two different engines. More power, more torque - peak and average - at 8% (or more)
lower rpm. There is definately something there. The results completely changed what I thought I understood.
Something to think about.