Quote:
Originally Posted by jr_ss
The stem of the valve isn't the concern with the P12 heads, it's the neck of the valve(4.5mm) which can stretch with the added heat of a turbocharged engine.
Inconel or SS valves are the best when it comes to heat management. Sodium filed play a role in longevity, typically used in OEM applications but do have a limit to heat tolerance, usually 1600*F.
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Jr has it spot on! The neck is actually 4.5mm. This is all to aid in flow and valvetrain weight which ends up being a downfall when dealing with heat.