Many research groups have attempted to overcome the short travel distance of electric cars by using RE-EVs (range extended electric vehicle) with small engine generators for battery charging. Engines for RE-EVs need to be optimized for maximum power at around medium engine speed or less because a higher engine speed causes higher noise. In this study, basic research about intake and exhaust flow was conducted to find the design point of high power at around medium engine speed. Test parameters are intake valve open (IVO) timing, the shape of the exhaust manifold and the intake runner length for a 0.6-L naturally aspirated (NA) gasoline engine. The firing order of the test engine is from the first cylinder to the second cylinder and cylinders have 180 degrees CA (crank angle degree) phasing difference. Test conditions are from 1200 to 6000rpm at full load operation, 12, 22, and 32 degrees CA BTDC (before top dead center) of IVO timing, and an exhaust manifold to reduce interference of exhaust gas between the first and second cylinder; there are also 200, 340, and 440mm length intake runners. The effect of the IVO timing on the engine performance was found not to be significant in this study. The shape of the exhaust manifold and the lengthened intake runners affected the torque and the BSFC (brake specific fuel consumption) and helped to improve the engine performance at around medium engine speed.