6 We had a set of factory 516, 902, and 906 casting heads at the shop, so we measured the flow of the intake and exhaust port of one cylinder on each head for a comparison. The untouched (except for hardened exhaust valve seats) 516 had a best flow of 228 cfm at .700 inch valve lift. These heads are a closed-chamber design and are a very similar casting to 915 heads. Our factory 902 casting big-block cylinder heads flowed very much like the 516s at low lift numbers, but didn't have the high lift flow capabilities of the closed chamber 516s. As expected, the 906 castings had the best flow numbers in stock form, nearly 238 cfm on the intake side, when compared to the other factory heads we tested. These numbers also apply to the later 452 casting cylinder heads, which flowed identically and have the added benefit of hardened exhaust valve seats. Realistically, most moderate street or bracket engines won't have .700 lift, so the flow at .500 and .600 lift is more important to most people.