The main reason to change the valve angle is to flatten and/or reduce the volume of the combustion chamber. Also, with a flatter valve angle, an engine is apt to produce a flatter, broader torque curve. With a smaller combustion chamber you don't need as much of a piston dome to produce high compression. That lower (smaller) dome enables better flame travel across the combustion space, the ability to make more power with less spark advance, and a more complete burn across the entire combusion space.
The first modified-angle heads were angle-milled small-block castings that reduced the size of the combustion chamber and stood the intake port up slightly in relationship to the bore. Then came aluminum cylinder heads. The valves began to get flatter (e.g., 18-, 15-degree, and so forth) and the intake ports get taller. Most modified-valve angle heads require specific inlet manifolds because of the raised inlet ports. The exhaust ports were also raised.
When you start moving big-block valves around, you're messing with the valvetrain angles and the geometry, so you can quickly screw up the valvetrain if you're not careful. In order to stuff the largest valves possible into the combustion space, the big-block head has compound angles between the inlet and exhaust valves and the angles of the valves on a secondary plane. The best numbers for a stock big-block are 18 degrees off the bore centerline for the intake, with a cant of 9 degrees. The exhause comes in at 10 degrees off bore centerline and 13 degrees of cant. Most performance aluminum heads have the valves "rolled" 1.5 to 2 degrees from stock.
With all this momvement, you must change the angle of the valve pockets in the pistons. Most aftermarket pistons are machined to stock valve angles unless you specify something different. Just one more reason to make sure that all your components will work together. CHP
If you have technical questions for Kevin Mcclelland, send him an e-Mail at chevyhi@Primedia.com.