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I'm saying that the increase in clearance that you measure between cold (0.1mm) and hot (0.25mm-0.35mm when stationary) is all but taken up by the valve head and stem expansion when it's running at 500°C+. Are we on the same page ???
John,
Your theory seems to be that there is a sudden, disproportinate, expansion of the length of the valve during running that closes up the clearance and disappears as soon as combustion stops ? And it is kind of like Schrodinger's Cat, if I want to measure it, it's not there.
My questions would be , how much how and quickly ? Since this stem expansion would have to be greater than the actual measured hot stationary clearance, why doesn't the same expansion take up the guide clearance and ruin the seat fit ? Around 75% of the heat goes through the seat not the stem and the stem tip would not be +500 C. I have stood next to engines on a dyno that have had the clearances checked immediately after shut down, so your expansion would have to disappear in seconds. I think this is a complex issue involving the net expansion of several components. It is somewhat counterintuitive that an air cooled engine would have such a small exhaust valve clearance spec. I have other engines with sodium cooled valves that are water cooled and they have much wider (0.020") exhaust clearances. Guide clearance spec however, is wider for the air cooled engine. I do not know of any auto engine with a tighter exhaust valve clearance spec that the Porsche factory spec of 0.1 mm, have never seen a range published by Porsche, and have never seen an OEM recommend diffferent clearance specs for minor changes in ambient temperatures. Have You ?
I think the main variable in the different clearances spec is to prevent closing up of the gap to zero in between the service interval.
Paul
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Paul
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