In 1906, while still doing general development work on the flat experimental engine, the Wrights started two new engines, and for the first time the brothers engaged in separate efforts. One was "a modification of the old ones" by Wilbur and the other, "an entirely new pattern" by Orville. There is no record of any of the features of Wilbur's project or what was done in connection with it. Two months after the experimental operation of the two designs began, an entry in Wilbur's diary gives some weight and performance figures for the "4" x 4" rebuilt horizontal," and since Orville's design was vertical the data clearly refer to Wilbur's; but since the output is given only in test-fan rpm it does not serve to indicate what had been accomplished and there is no further mention of it. Orville's design became the most used of any model they produced. It saw them through the years from 1906 to 1911 or 1912, which included the crucial European and United States Army demonstrations, and more engines of this model were manufactured than any of their others including their later 6-cylinder. Although its ancestry is traceable to the original 1903 engine, the design form, particularly the external configuration, was considerably altered. Along with many individual parts it retained the basic conception of four medium-size cylinders positioned in line and driving the propellers through two sprocket wheels. From the general tenor of the record it would seem, despite there being no specific indication, that from this time on Orville served as the leader in engine design, although this occurred with no effect whatsoever on their finely balanced, exactly equal partnership which endured until Wilbur's death in 1912. The first major change from the 1903 design, putting the engine in an upright instead of flat position, was probably done primarily to provide for a minimum variation in the location of the center of gravity with and without a passenger. Whether or not it had any influence, the vertical cylinder arrangement was becoming predominant in automobile powerplants by this time, and the Wright engines now began to resemble this prevailing form of the internal combustion engine--a basic form that, in a wide variety of uses, was to endure for a long time. Over the years, the Wrights seem to have made many changes in the engine: the bore was varied at different times, rod assembly methods were altered, and rod ends were changed from bronze to steel. Chenoweth states that on later engines an oil-control ring was added on the bottom of the piston, necessitating a considerable increase in the length of the cylinder barrel. This arrangement could not have been considered successful, as it apparently was applied to only a limited number of units and was not carried over to the later 6-cylinder engine model. There was much experimentation with cam shapes and most probably variations of these got into production.