Date: Thu, 8 Mar 2012 00:32:41 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: How lifters work and Two turns to set valves?
In-Reply-To: <DC6015B02E0340A0A73D2597551073E0@customerPC>
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At 09:10 PM 3/7/2012, HotelWestfalia wrote:
>But how do I know, the valves open enough or they close
>fully? Somehow it is foggy for me why to do two turns, or any for
>that matter.
A hydraulic lifter has a piston in a very closely fitted
cylinder. It is meant to have the space under the piston always full
of oil. If it isn't, the lifter is soft and will not open its valve
properly until it gets the air worked out.
There's a weak spring under the piston, and if nothing stops it the
spring pulls oil in through a check valve as it extends the piston
fairly rapidly. However the only way to retract the piston is by
applying strong steady pressure which forces oil very slowly out
through the gap between piston and cylinder.
Now think about the valve train. Camshaft pushes on the lifters,
lifters push on the pushrods, pushrods push on the rocker arms which
reverse the direction of motion and push on the valve stems, with
strong resistance from the valve springs.
The springs push back and are strong enough to slowly force the
lifter piston back until the valve is closed. At this point the
spring has no more travel so it can't push the piston back any
farther. You adjust the rocker arm adjusters so that this point
happens somewhere in the middle of the piston's possible travel.
At that point, in theory, you're finished. The valves will maintain
correct adjustment at all temperatures, operating and wear conditions
for the life of the engine. Any time a clearance opens up in the
valve train, the spring under the piston quickly extends it a little
more, taking up the slack. *The amount that it can do this is the
amount of the "preload" that you cranked in when you set things up
originally. When that amount gets used up the lifter cannot extend
itself any farther.*
Any time the clearance becomes too tight and doesn't allow the valve
to close fully, the valve spring shoves the piston slowly back into
its bore until clearance is correct again. This action occurs as the
engine warms up but also all the time, because any time a valve is
opened its spring will be trying to force the piston into its bore,
thus opening up the clearances. It can't do it much because the
valve doesn't stay open long, but it does a little. This tiny
clearance is taken up by the extending spring, and round and round we
go. *If the piston bottoms out (too much "preload" then it can't
allow the valve to close entirely.*
As the valve train wears the parts become shorter and the correct
position of the lifter piston gradually extends until the piston is
fully extended. At that point it can no longer perform its function
and it is now operating like an expensive and very slightly faulty
solid lifter.
The trouble we have with our WBX lifters is that (effectively) the
valve springs aren't strong enough to keep them properly (and quickly
enough) compressed, so they tend to not let the valves fully
seat. That wrecks your compression.
So you set the Type 4 lifters to .006 clearance cold, right? As the
engine warms up that clearance decreases to somewhere near
zero. Every n thousand miles you set them again because the
clearances open up from wear and the valves start tapping.
You can do the same thing with the hydraulic lifter. Treat it like a
solid and give it .006 cold, and readjust on a similar schedule to
the Type 4 engines.
Or set it up touching, depending on the lifter to compress that .006
as the engine warms. You still have to adjust it as the clearances
open from wear.
Or at some reduced "preload" - but since the extending spring is not
very strong, I frankly doubt if it makes any practical difference
other than reducing the total amount of wear that the lifter can
compensate for. I don't know that for certain, though.
I would love to see someone take an engine that was losing
compression because of the lifters and put stronger valve springs in
it. I would *really* like to know what effect that has. But in the
mean time, treating them like solid lifters works, at the expense of
having to...treat them like solid lifters.
Yours,
David
