Though I haven't fired up the engine yet to be able to voice my opinion of their performance. but I will tell you this. The machining on these heads was excellent. Unported as cast port were very clean. I had no problems with any of the work on them. As a matter of fact my machinist whom I've worked with for many years was very reluctant to even take the job in cause of his experience with Edelbrock aluminum mopar heads, he insisted I just stick to the X-heads I had. Once he saw them he marveled at the work on them and said it was the best Mopar aluminum he had worked on. I'm happy and Bruce is second to none in my opinion.