<VV> adjusting valves - I want to believe!

[James Davis]

Looks good to me.  I stand corrected.
Jim Davis

At 11:23 PM 11/2/2008, Craig Nicol wrote:
>Jim wrote:
>A little math is in order.  The threads on the rocker studs are
>3/8-24, so each turn of the adjusting nut is 0.0417 inches.  Since
>the slack take up is on the pushrod and valve tip at the same time,
>the adjustment is 0.0833 inches each turn.  The the adjustment range
>in most lifters is 0.180 to 0.200.   The engine grows about 0.006
>inches for each 100 degrees Fahrenheit.  So now you can make an
>informed decision as GM did.
>Jim Davis
>
>Craig replies:
>I like where you are headed with this, Jim!  I do have a little correction
>to consider.
>
>The notion that a turn of the adjustment nut causes .0833" at the pushrod is
>possibly incorrect; here's my thinking:  I don't think the adjustment nut
>equally affects the valve stem and the pushrod as you stated.
>
>The valve stem does not move when the adjustment nut is turned, the valve is
>stationary and is effectively the fulcrum of the lever.
>
>The rocker supposedly has a 1.58:1 ratio but let's imagine for a second that
>it was a 1:1 ratio, which would put the adjustment nut in the exact middle.
>At THAT ratio, .0417" movement at the nut would cause .0834" movement at the
>pushrod. To simplify the math for our actual ratio, consider for a moment
>that the rocker arm ratio was 3:1. In that case the pushrod would move 4/3
>the distance of the nut movement. (4=overall units of length, 3=units of
>length at nut, relative to fulcrum, hence the 4/3 effect)
>
>At the actual ratio of 1.58:1, the overall units of length = 1+1.58 (or
>2.58) so using the same concept as the 3:1 example, the pushrod would move
>2.58/1.58 (1.63) times the nut movement. Using this factor, I think a .0417"
>movement at the nut would cause.060" movement at the pushrod. (.0417" x 1.63
>= .060") rather than the .0833 you calculated.
>
>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)
>
>1/2 turn = .030" (which should accommodate a 500F degree engine temp change
>but still uses only a fraction of the available .100" "center of lifter
>range")
>
>3/4 turn = .045", not even halfway to the center of the lifter's range, but
>clearly able to accommodate well beyond any conceivable engine growth.
>
>1-turn = .060", still well short of the lifter's center of range.
>
>I'll also add observations that the more the preload there is the less time
>it will take a collapsed lifter to refill and the less time it will take for
>the pushrod to fill and deliver oil to the upper valve train; both worth
>considering.
>
>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
>
>Note to math wizzes (which I am not): please check my math on the effect of
>the rocker arm ratio when the fulcrum is changed to the end when the ratio
>calculates to 1.58:1 with the fulcrum is at the middle. ;-)