[Vwdiesel] Fwd: Hydraulic Lifter Question

[Sandy Cameron]

At 10:27 AM 31/05/2006 EDT, you wrote:
>In a message dated 5/30/06 8:39:16 PM US Mountain Standard Time, 
>george.allison at gmail.com writes:
>
>
>> it should not affect
>> compression, because the valves will just be closed for more of the
>> time.
>
>That does significantly affect compression.
>
>Andrew

Probably not much at idle.

The compression "ratio" does not change.

The ability of atmospheric pressure, (NA),  (or the turbo for a TD) to stuff
air in to the cylinder in the brief time the valve is open, determines the
"inhalation volume" , the amount of air to be squeezed to raise the
temperature to the flash point.

An increased gap just puts you at a "rocky mountain high" situation until it
closes up.

If the valve does not open, no air, no bang.

Valve opening is expressed in degrees of rotation, and also determined by
the shape of the cam lobe. On race cars, the cam may be shaped to allow both
valves to be open at the same time for a few degrees, to improve flow and
scavenging. Called, not surprisingly, "overlap" A racing cam lobe  may have
a very steep ramp, and a flattened tip on it  to get the valve wide open
sooner, hold it wide open longer and powerful valve springs to slam them
shut quickly as the ramp goes away.

Italian racing motorcycles go one step further, using, in effect 2 tappets
or followers, one to open the valve, and the other to slam it shut, reducing
valve "float".
I think it's called "Desmodromic" valve gear.

In our dowager diesels, I should think a slightly reduced "dwell" [inverted]
(from old cam type distributors) at idle would not affect it much, and
eventually they will pump up.

When ever I have the lid off, I try to remember to dump some oil on the
lifters, to protect them until the pump can lift enough from the pan to wet
them, and to avoid scuffing the cam.

I think the lifters are designed to hold a little puddle of oil to avoid dry
starts, but in the tilted engine, I'm not sure how effective that is. Dont
leave parked uphill for long periods of time :~)

Sandy