> Hello list ,could someone explain what is meant by engine
> negative pressure
> and how it might affect oil leaks? Is not some pressure exhausted
> via the oil
> filler cap and crankcase vent tube ? TIA Randy
Hi Randy :
Joe was talking about crankcase pressure. The issue gets more complicated
on the later cars, but basically you are right, most of the pressure is
exhausted by one means or another. On the TR2-early TR4, there is a fairly
large bore "road draft" tube that is open to the air, plus air passages in
the filler cap.
However, even so, there can be a fair amount of positive pressure built up
in the crankcase. After all it takes some pressure to push the gases out
through the vents. How much depends on current operating conditions,
usually wide-open throttle is the worst case (and of course race engines
like Joe's operate at WOT for extended periods of time). Although I'm not a
racer, I have the same problem during high speed runs.
What Joe is talking about is a system that literally pumps the gases out of
the crankcase, so the pressure is below atmospheric pressure (zero for our
discussion) all the time.
Crankcase pressure has a large impact on oil leaks. Basically if the
crankcase pressure is positive, then it will try to blow oil out of the
engine wherever it can, like through the seals and gaskets. This is
particularly a problem with the scroll-type original rear seal, since it
does not provide a positive barrier to oil flow. Rather it functions
somewhat like a pump, so that any oil that starts to run out gets pumped
back into the engine. But with pressure against it, the oil will run out
faster than the pump will pump in, and the result is a leak. Positive
pressure also makes it much harder to get the rocker cover to seal, and so
on.
My tired old TR3 engine develops a fair amount of positive pressure even at
idle. If you take off the filler cap and hold your hand above the opening,
you can feel the air being forced against your hand. Might be an
interesting experiment to hook up a simple manometer to see how much
pressure there is, but I've never tried it.
Many people think that the "PCV" system on later Triumphs keeps a slight
vacuum on the crankcase, but in fact it only does so at idle and (hopefully)
cruise conditions. At WOT there is no manifold vacuum, so the pressure in
the crankcase builds up until it can blow enough air through the relatively
small passages involved. Thus the later engines tend to leak oil even worse
than the earlier ones. I've even heard of them blowing the oil filler cap
off (which is why the later caps have a positive locking mechanism instead
of the slide-on used on earlier cars).
Randall
=== This list supported in part by The Vintage Triumph Register
=== http://www.vtr.org
|