Complex sentence: The Mk IV range was launched, not at an October motorshow, but in the Spring of 1966 with new V format engines, the 4 having a V4 and the 6 a V6 unit.
Split into simpler sentences: The Mk IV range was launched, not at an October motorshow, but in the Spring of 1966. The engines new V format engines, the 4 having a V4 and the 6 a V6 unit.