Trying to improve on the best factory small-block cylinder heads ever built is no small endeavor. And yes, GM's 12-degree LS7 castings are the best small-block heads of all time-regardless of engine make-trumping even the legendary Ford Cleveland heads. With a full CNC rubdown, monster 2.20/1.61-inch valves, and raised 270cc intake ports to go along with a flat 12-degree valve angle, factory LS7 heads flow 360 cfm out of the box. That's big-block territory, and at a hair less than $3,000 for a set of fully assembled LS7 heads from your friendly GMPP distributor, they're an exceptional value as well. So naturally, we were quite intrigued when Mast Motorsports announced it had just developed its own LS7 castings that outflow the factory design.
Mast's LS7 heads are a clean-sheet aftermarket design featuring CNC-ported 274cc intake runners, a thick 3/4-inch deck, and extra boltholes that make them compatible with six-bolt aftermarket blocks. According to the company's advertised flow ratings, the Mast LS7 castings flow 395 cfm through the intake ports at 0.700-inch lift and 249 cfm on the exhaust side. Considering the Mast heads utilize the same-diameter valves as the factory castings and that their intake ports are only slightly larger, the increase in airflow is very impressive. That is, as long as they can back up those claims on the dyno.
To find out, we baseline-tested one of Mast's 427ci LS7 crate motors on its SuperFlow 902 dyno. The combo features a factory standard-bore LS7 block, a Callies crank and rods, Mahle 11.4:1 pistons, a custom 246/260-at-0.050 hydraulic roller cam with 0.659/0.666-inch lift, factory LS7 heads, and a stock LS7 intake manifold. With the stock heads, the 427 produced 662 hp at 6,900 rpm and 560 lb-ft at 5,300 rpm. With no changes to the motor except swapping out the stock LS7 heads for the Mast heads, the motor put out 686 hp at 6,900 rpm and 589 lb-ft at 5,300 rpm. Just as impressive as the peak gains is the fact that the Mast heads picked up power and torque throughout the entire power curve. This is exactly what you'd expect, considering the increased airflow of the Mast heads through ports that are only a hair larger than the stock LS7 castings. Throw some more ci and a bigger cam at these heads, and they will easily support 750-plus horsepower without breaking a sweat. For heads that perform just as well on the dyno as they do on paper, we give the Mast LS7 cylinder heads two big thumbs up. -Stephen Kim