<VV> valve lifters

[BobHelt at aol.com]

On Fri,  1 Aug 2008 23:08:46 -0600 Dennis PLEAU <ddpleau at msn.com> writes:
>  One of my friends who is a well known Corvair mechanic once told me, 
>  he tightened the valves on his cars and his performance customers at 
>  1/4 turn past zero lash.  Non performance customers got 1/2 turn as  
> they complain about the noise until the engine gets warm.  You  pick 
> your driving style, you pick your  prelload.



Hydraulic lifters have an internal piston that self adjusts itself so as to  
maintain zero valve train clearance. All Corvair and SBC lifters have at least 
 0.125 in. movement of this piston which translates into at least two full 
turns  of the valve adjustment nut. So the adjustment can be set anywhere from   
zero lash to one full turn in and still be fairly good. It takes more than 
two  turns to bottom the piston which results in a bad adjustment. 
 
The reason one might want to pick one adj setting over another is strictly  
to avoid the effects of lifter pump up. Lifter pumpup occurs mostly at high  
engine speeds when the lifter is unable to follow the lobe profile due either to 
 a poor lobe design, over revving, or weak valve springs. What happens is  
that as the lifter is raising the valve and reaches the top of the lobe, inertia 
 lofts the lifter above the lobe peak (floating the valves). When this 
happens  the internal piston tends to rise to take up the slack in the valve train. 
Then,  when the valve does close the internal piston is too high in the lifter 
and  tends to keep the valve from seating properly until the piston returns 
to its  previous position. By setting the adjustment nearer to the zero lash 
point the  amount of pumpup is limited since at zero lash the internal piston is 
already at  the top of its travel. So that is probably the only reason one 
might want to set  the adjustment nearer to zero lash. Otherwise the amount of 
adjustment is  totally irrevelvant as long as it doesn't bottom the piston or 
cause valve train  noise due to the expansion/contraction of the engine and 
valve train. (That's  why making the adjustment at 1/4 turn or making it at one 
full turn makes no  difference in normal engine performance.)
 
There is a designed-in balance between the oil pressure pushing the  internal 
piston up into the top of the lifter and the valve spring pressure  trying to 
keep the valve closed and seated pushing back on the internal piston.  If 
either of these two pressures is too strong or too weak the other will  overcome 
it and cause valve performance problems. So it is wise to check the oil  
pressures at all engine speeds to see that they are within specs and similarly  
check for weak or improperly set valve springs. Weak or improperly set valve  
springs are a hidden "gotcha" since they are seldom suspect when problems  occur 
and they are hard to check/verify as being good.  But they do cause  valve 
train problems.
 
Regards,
Bob Helt



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