"www.rcvengines.com" The SP versions turn huge props at one half normal rpm and have the cylinder pointing straight forward and the cylinder turns the prop, the CD series has the cylinder in the normal position and turns props at normal rpm. The cylinder rotates in both series. They are fine, totally reliable engines. 10% nitro and 15% lube are recommended. A Hangar 9 Super Plug will let them idle like a clock. I have run 15% nitro with 18% lube in mine with no difference in performance. WildCat fuel in both instances.