Valve Angle - Make More Horsepower

This 30-degree back-cut idea works so well, even the very popular Vortec iron small-block Chevy head incorporated this 30-degree technique for both the intake and exhaust valves on all production heads. It's one reason this head works so well on mild street small-blocks.

This is the Edelbrock Performer RPM head for the small-block Chevy. These heads make good power right out of the box, but if you're not in too much of a hurry, yank the valves and have your machine shop back-cut them. We used a Powerhouse valvespring compressor to remove the springs.

Flow Curtain Area
The combination of valve diameter and valve lift creates a flow window or flow curtain area. This area increases with valve lift. The area can be calculated by first determining valve circumference (3.1417 (pi) times valve diameter) and multiplying it by valve lift. For example, let's say we have a 2.02-inch intake valve with a valve lift of 0.500 inch. 3.1417 x 2.02 = 6.34 inches of valve circumference. 6.34 x 0.500 = 3.17 square inches of valve curtain area.

Low-Lift Flow
In a majority of the heads we tested, we found the gains with a 30-degree back cut were greatest at the lower lift values between 0.100 and 0.300 inch. At first, this may seem of marginal value, especially if your camshaft is capable of valve lifts of 0.500 inch or more. The reality is that all the valve lifts contribute to filling or emptying the cylinder.

On the intake side, increasing low-lift flow is especially important for mild street engines with max valve lifts of less than 0.450 inch. At 0.225 inch, the valve is already at half total valve lift, so low-lift flow is important. Plus, the valve will be at this lift twice during the complete valve-lift cycle, both on the opening and closing sides of the lift curve. Perhaps most importantly, as the intake valve is closing, a more efficient low-lift flow curtain area will increase volumetric efficiency just as the piston is rising in the cylinder and the intake valve is about to close. Better flow numbers at these lower lift values will allow the heads to fill the cylinders better and therefore make more power.

Jim Grubbs even went so far as to try two back cuts on a small-block Chevy valve, but the results were inconclusive.

On the exhaust side, the picture is a little different. Cylinder pressure is at its highest point in the exhaust cycle just as the exhaust valve opens. With more efficient low-lift flow, this reduces the cylinder pressure at these lower lift values, which means the engine does not have to expend power to force exhaust gas out as the piston rises during the valve-closing portion of the exhaust cycle. This is called negative work and is subtracted from the power the engine is already making.

In both intake and exhaust applications, low-lift airflow improvements will improve power. It doesn't get much easier or simpler than that.