The purpose of the valve guides is to help move heat from the valves to the cylinder head. This material was chosen of it's high strength, effective heat conductivity, and it's high resistance to abrasion. The stock valve guides are made from cost materials that are chosen for cost effectiveness over performance reasons. Which is fine for the stock power making factory engine. But not suitable for the high end tuned race engines, especially with tuned engines that are used for high engine speeds and high power outputs. The TOMEI phosphor bronze valve guides have efficient heat dissipation properties that allow the cylinder head to absorb the heat from the valves during high thermal conductivity. This increases the strength of the valve train even at higher temperatures, helps minimize the valve guides from cracking and will also extend the valves life.