Though it doesnt tell the entire story, if you can find each ones flow numbers and compare them, that usually gives a good idea on how the head can perform. A lot has to do with waht you pick as a supporting combo. Are you throwing in a cam and intake to compliment the heads? What size throttle body do you have? Motor size? NA? Forced induction? I know the Edelbrock E - series aluminum heads (170CC) arent that far behind the 165s in terms of flow. Toward the end of the cam lift range, they were pretty damn close. Close enough for me that I am strongly considering the pair of assembled E- streets ($920) over the AFR 165s ($1400). For one, I drive my car maybe a few times a week tops lately so I dont care if I squeeze every fathomable ounce of power out, and not to mention I can think of a couple other things I could use that extra 500 dollars toward. Solid steering shaft, Kenne Bell, supporting mods for Kenne Bell, etc. The site selling the heads usually posts the flow numbers at the bottom of the page.