<VV> adjusting valves - I want to believe!

[Kenneth E Pepke]

On Sun, 2 Nov 2008 21:23:47 -0800 "Craig Nicol" <nicolcs at aol.com> writes:
> Jim wrote:
> A little math is in order. 

snip

> Craig replies:
> I like where you are headed with this, Jim!  I do have a little 
> correction to consider. 
> 
snip 

> Here's how that shakes out:
> 
> 1/4 turn = .015 at the lifter (ignoring the pushrod angle to the 
> lifter) In  > an environment where the engine grows .006" 
> per 100 degrees and the lifter has an available range of 
> .180 to .200., this theoretically doesn't offer enough lifter 
> stretch for higher engine temperature and uses very little of
> the lifter's range. (.15" out of .100" to center of lifter's range)
> 

snip

 
> So the questions are: Why does 1/4 turn seem to work so well? 
> 
> If the preload is set to 1/4 turn when cold, according to this 
> analysis it should clatter when hot. Perhaps 1/4 turn works 
> only if adjusted at operating temperature. I wonder if this 
> unspoken need to have the engine at temperature is why we 
> hear so much cold/hot, 1/4 turn-1/2 turn-1-turn controversy?  
> Maybe 1-turn is the right answer if valves are adjusted cold
> and 1/2-1/4 turn is the answer if adjusted hot.
> Craig Nicol
> 
++++++++++++++++++++++++++++++++++++++++

All these methods are for an engine at assembly ... so are not
at operating temperature.

Probably everyone has experienced at least one of their 
Corvairs, after a long high speed run, idling poorly to very
rough to stalling at the top of the exit ramp.  In a short period
of running things smooth out and seem fine again.

This is caused by lifter 'pump up' when the leak down rate of
the lifter is less than the oil supplied so the lifter piston has 
moved up in its bore.  That keeps the valve from fully seating.
Once the 'leak down' has caught up the valves seats and seals
again.

The quarter turn in at assemble is meant to simulate a 
mechanical follower.  By having the lifter extended to the top at 
operating temperature the lifter cannot 'pump up.'  Because  
the hydraulic lifter cam has no ramp to take out the lash setting
the quarter turn adjustment should net out at zero to .001
lash.  In practice it may not be quite that precise resulting in
increased valve system operational noise.
Ken P

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