The picture on the left is an indication of the temperature range of an exhaust valve. This valve will be visibly glowing red-hot while being used.  This is the reason that fuel has an octane rating, which is the measure of how far the fuel-air mixture can be compressed in a standard test engine before 'pinking' (detonation) starts to happen.

If the heat conduction away from a poppet valve head can be improved, an engine can be run at a higher compression ratio.  This is extremely important in aeroengines, since higher compression ratios allow higher fuel efficiencies and thus a greater range and payload.  Sodium-filled poppet valves were first tried in WWII.  The pair of drawings below show these valves.

A spring-closed poppet valve is the element which normally limits the revving speed of a motor.  Above a certain limit, the spring cannot hold the valve against the cam mechanism and valve 'float' or 'bounce' start to set in, with the chance of the valve hitting the piston greatly increasing.