Valve Sizes
In Cup racing today, very large intake valves are common. Most people think of valve sizes in terms of diameter, but a more important characteristic is valve area. Top Cup engines today have an intake valve which is between 26 percent and 29 percent of the area of the engine's cylinder bore. In these same engines, typical exhaust valve areas are between 50 percent and 59 percent of the intake valve area.

All Cup engines have intake valves that have a compound angle. This means the valve is "rolled" from vertical and canted toward the side. This is the arrangement pioneered by big-block Chevrolet ("porcupine") cylinder heads. It allows the valve to un-shroud itself from the cylinder wall, during the valve opening cycle. The intake and exhaust valves are also rotated (from a common line), and are therefore not aligned.

NASCAR mandates that all Cup heads do not have canted exhaust valves. If canted exhaust valves were legal, all manufactures would use this method. When the rules in a given series do not specify a cylinder head configuration, canted-valve heads should be considered.

The regulations for most circle track racers, outside of NASCAR's top series, require the use of small-block engines with traditionally inline valves and no side-canting. However, this valve layout may restrict the size (area) of the valves that can be packaged. Remember to consider the top ring position in the pistons, with relation to the valve pockets, when changing valve size.

Valve lifts of 0.900-inch are being raced in Cup today. As you suspect, large cylinder bores allow increased valve areas. Many times, it is not possible to run large valves, but is it possible to increase valve lift? Increasing lift only provides a benefit if the heads continue to be flow-efficient at high valve lifts. Don't overlook the fact some port configurations actually begin to flow less at very high lifts (see the baseline port in Chart 1). In this case, increasing valve lift may not have much effect on either flow or power.

One way to understand the relationship between the valve events and air flow potential of the cylinder head is to plot air flow vs. cam or crankshaft angle. To generate this type of graph, make a table of airflow vs. valve lift. Next, create a table of cam or crank angle and valve lift. Table A shows an example of this type of table. Finally, calculate the airflow at each valve lift. Chart 1 is an example.

Table A
Cam AngleNet Valve LiftAir Flow
1700.8222389
1710.8263388.8
1720.83388.7
1730.8333388.6
1740.8361388.5
1750.8385388.4

Valve Head and Throat Size
Intake air or exhaust gases see the valve head as a restriction. That portion of the port under the valve seat should have the smallest cross-sectional area in the port. This area is called the "throat." One trend in Cup that may improve the performance of your cylinder heads is to increase the throat diameter until the valve seat has just enough area to seat the valve. Increasing the back-angle on the valve, in conjunction with this technique, has proven to be effective.