See Roberts.

This company made two-stroke engines, the 4-cylinder, the 6X and the E12.

-The 6X had 6 cylinders in line and gave 100 HP. The E12 was 12V and gave 350 HP.

“The Roberts with 6 cylinders in line”

-For both engines the speed was between 400 and 1,500 rpm and the normal cruising speed was 1,200 rpm.