Sodium-filled hollow stem valves are available for higher heat applications, and are typicallyused for the exhaust valves. The sodium inside the valve stem melts and absorbs heat from thevalve heat. As the valve opens and closes, the sodium sloshes up and down inside the valve totransfer heat from the valve head to the stem. This helps the head run cooler to reduce the risk ofvalve burning, pre-ignition and detonation. The difference in cooling is quite dramatic. With aconventional solid stem exhaust valve, 75 percent of the cooling takes place across the valve seatand 25 percent through the stem. In a sodium-filled exhaust valve, 40 percent of the cooling isthrough the stem so the valve can tolerate more heat.