wowbagger
http://www.pspec.com/
It's Performance Specialties. They are the importer for YS engines over here. If you go to the engines, engine rep forum. You can ask Dave Shadel direct about them, or send him an email. There price is in the 50 to 70 dollar range and the performance increase is generally over 1000rpm.
They are what is attached to the exhaust of a Nelson.