Some great reading here on back pressure:
http://www.google.co.uk/url?sa=t&rct...ZXjwugSHi3jtxQ
Also a research paper on marine diesel engines which tests and proves that less exhaust back pressure is a good thing on turbo diesels.
http://www.google.co.uk/url?sa=t&rct...6ftTzqrFO90QRw