Nothing spectacular about any of the letter heads, big or small valve. The differences are so minute, you'd likely not even see it on the dyno. I always chuckle when guys have an X or Z and think they have something rare and experimental. I think the Chevy boys started it with the X casting Chevy stuff. Also, almost every single Ford Cleveland, 351M and 400 have a series of X's cast in the lifter valley. You should hear those Ford guys talk. lol It's just another letter. That's all.
