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Going with the Flow Nelson decided to try a 30-degree back-cut on the valves. As he told us, “Air is a tricky thing. The goal in a port is to control the air around the valve. As a matter of fact, at high lifts, if you were a mini person standing next to the valve stem there would be very little air passing you next to the stem. The busy areas are on the seat and valve angle. The way we cut the back of the valve we were able to bend and direct the air in an efficient path around the valve.” The Mast heads flowed an incredible 390 cfm at .750-inch, but because the lift of our cam maxed at .624-inch, we would never see a benefit. The back-cut traded some flow at the higher lifts for more air down low where a street engine like ours spends most of its life. Yeah, it’s a trade off but one we felt suited how the engine would be ran.