Quote:
Originally Posted by psalt
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.
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Paul - You've captured the crux of my argument (although give me a bit of latitude on "sudden" and "disproportionate"), Certainly Schrodinger's Cat applies.
I've not seen cold clearances set any tighter than 0.1mm, and the valve head temperature pic I posted is only indicative - I've not measured it myself - so can't speak as to its derivation, but taken from
G&S Valves Ltd UK. I'm only talking about valve/stem length not diameter (and hence not valve to guide clearance)
Here's a rough model of what I'm talking about (these are NOT ACTUAL MEASUREMENTS) but simple estimates. (and I've omitted impact of the rocker/and rocker ratio).
The datum for the model is the valve seat/head interface (top edge)
Sign indicates whether the expansion of the element increases (1) or decreases (-1) clearance.
Lengths are estimated effective heat path lengths, in the line of valve action.
Note that the valve has been broken into 3 distinct parts, and the sign of each parts contribution to lash. (The valve head actually adds clearance as it is located on its conical seat, but the stem parts decrease clearance with increasing temperature) I had not previously considered the sign of the head as different to the stem parts)
Of course this is a limited model (and I know it shows negative lash by 0.01mm) but you get the impact of the very very hot valve in the running engine, which loses its heat very very quicky when the engine is stopped.
Again, if it were as simple as "lash expands with increasing temperature", then try the acid test (set "near to zero" cold valve lash - the follower will still hit the opening ramp at the design velocity - and run the engine and you'll burn all your ex valves.)
As Walt picked up - if it's valve cooling you're after, widen the seat and reduce your duration.
John D/camgrinder - interesting REAL data on the 906 profile....which I'm still digesting !!!
John