<VV> Lifters and other topics
Sethracer at aol.com
Sethracer at aol.com
Sat Feb 26 16:40:55 EST 2011
In a message dated 2/26/2011 12:33:51 P.M. Pacific Standard Time,
vairguy at echoes.net writes:
I also don't see how the tappet being flat would really make such a major
difference, and if it did it seems all it would do is wear the lifter
and/or pushrod which are easy to adjust and fairly easy to replace. So please
enlighten me!
I'm not trying to argue the point, I'd just like to be educated in the
mechanics involved.
The "Tappet" is just another name for the "lifter" and they can be solid
or Hydraulic. The cup end holds the pushrod and the movable, internal
plunger, is kept in place by a clip or spring. Internally, the lift acts as a
passageway to feed oil from the gallery, located in the case on the Corvair,
up the pushrod to lubricate the valve spring and the rocker. The hydraulic
lifter also uses this oil - fed under pressure by the engines oil pump - to
take up the clearance, by pushing up the plunger, in the valve train to
help provide quiet operation. A solid lifter engine, in order to allow for the
growth of metal under temperature change, has to have extra clearance
pre-set in the valve train to make sure the valve can close - absolutely - when
hot. If no clearance was pre-set, the growth of the dimensions of the parts
would hold the valve open causing mis-fires. Most engines have gotten away
from solid lifters because the noise can be distracting (Let me tell you
about a Duntov .030" inch clearance cam I had in a Corvette! You could hear
me coming for blocks) Almost all Corvairs have hydraulic lifters - a few
racers have installed solids. Flat tappet cams are called that because they
present a flat face on the bottom of the lifter to ride against the lobe of
the cam. Roller lifters present a cylindrical mounting interface. In both
cases, a line of material contacts the cam. That line, in theory, changes as
a roller cam follower spins at the bottom of the lifter, and in a flat
tappet system, the cam is ground with a slight taper which is supposed to
"spin" the lifter around a bit during operation. Most importantly, a flat
tappet lifter presents a heavier (as in lbs per square inch) wiping contact load
against the cam, than a roller lifter. It is generally regarded as the
highest load applied to any surface inside the motor. The wear on this surface
is mitigated by the oil on this surface and the "high-pressure" additives
of the oil. The ZDDP - essentially a zinc-phosphate compound, provide a
cushion in that load. Supposedly that compound is a contaminator of the
catalytic converter on modern cars so it is being removed - or greatly reduced on
new-car oils. Thus the controversy for older cars that, supposedly, need
that assistance on their flat tappets. Almost all new cars have roller
tappets for several reasons. Wear on the tappet to lobe quickly results in low
or no valve opening, not good for performance. And the cam, far from being
easy to replace, is just about the last thing that falls out of the motor
when you disassemble it.
Seth Emerson
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