So I just had this thought on the way to work today. In the 1984 Transformers Movie, Megatron throws a spiked piece of rubble at Optimus' abdomen, Prime is visibly hurt. Later Megatron slashes the same spot with a laser blade (as if to confirm this weak point) and finally he shoots him in the same spot with what are meant to be kill shots "Fall -FALL!" (Ignoring the fact that the writers hadn't yet come up with the concept of the spark) Prime was outfitted for a Matrix this means that his spark had to be relocated. I am suggesting that Primes spark casing is in his lower left abdomen and Megatron knew it after the first penetrating attack. Basically Megatron's precision attacks ruptured the spark casing leaving Optimus with only moments to live.