I have the rr12l3 engine and as the posts above stated, low end acceleration is fantastic!!! I have an rs4-3 converted to just run short track parking lots. I would highly recommend this engine as once its broken in, it runs like a clock!! I run mine with no restrictor at 30% blue thunder with the #5 spark plug leaned more for low end than high. No flameouts and gets great run time.
Currently waiting for the OS 12TZ that may perform same as the rr12l3 but much cheaper...