Complex sentence: The BMW IIIa followed the SOHC, straight-six configuration of the Mercedes D.III, but increased displacement, higher compression, and an alititude-adjusting carburetor produced a marked increase in speed and climb rate at high altitude.
Split into simpler sentences: The BMW IIIa followed the SOHC, straight-six configuration of the Mercedes D.III, but incorporated several improvements. Increased displacement, higher compression, and an alititude-adjusting carburetor produced a marked increase in speed and climb rate at high altitude.