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 led to 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. However, the BMW engine combined increased displacement, higher compression, and an alititude-adjusting carburetor to markedly increase speed and climb at high altitude.