vacuum discussion/give us more

[RandalPark@xxxxxxx]

I think that we all dig it when you explain things that way. I learned so much 
when you explained the the causes of spark knocking, pre-ignition, engine "run 
on", and dieseling. If I tell you more, will you keep going:

1) The car that had a hole in the top of the piston was spark knocking quite 
badly when that happened. That is another reason why I do my best to prevent an 
engine from doing that.

2) Another time that I lost vacuum during a massive compression drop was when a 
valve keeper fell apart, allowing the valve (I believe it was an intake valve) 
to drop into the combustion chamber. Luckily it didn't fall far enough to ruin 
the engine, and I was able to fix it. Vacuum wise, I think that you explained 
this one in your post, but maybe it will inspire additional conversation from 
you.

3) I had a friend who also developed a hole in the top of a piston and 
continued to drive the car, even though it was difficult to do. Finally, the 
engine was destroyed when the crankcase exploded and blew off the oil pan. I 
was under the impression that this occurred as a result of an accumulation of 
gas fumes in the crankcase that were ignited when the spark plug in the bad 
cylinder fired.

Many of us have worked on cars for years, and some of us can  tear down an 
engine and rebuild it, but few of us truly understand the science and 
engineering behind it. I think these posts are extemely interesting.

I did realize that it would take one heck of a compression drop to lead to no 
vacuum, and suspected that the lack of vacuum wasn't neccessarily attributed to 
the lack of compression as much as what would have caused it in the first 
place. Please tell us more.

Paul

In a message dated 10/30/2003 5:58:17 PM Eastern Standard Time, 
dardal@xxxxxxxxxxxxxxx writes:

> 
> 
> Quoting RandalPark@xxxxxxx:
> 
> > A serious lack of compression can also cause a lack of vacuum that might
> > effect the way the brakes operate. I once had  a car that developed a hole 
>in
> > the top of a piston. This condition messed things up enough that, if I could
> > make it run, none of the vacuum operated accessories functioned properly.  
> 
> OK, Paul, time for a bit more engine theory.  The vacuum in the manifold is
> generated by the combination of piston suction during the intake stroke and 
>the
> closed throttle.  During compression and expansion, the intake manifold is
> isolated from the cylinder, so even a major compression leak will not directly
> affect the vacuum (unless of course the intake valve is leaking, but that's
> very rare).  A way the loss of compression can indirectly affect the vacuum is
> as follows.  Due to loss of compression and therefore loss of engine
> efficiency, at idle the engine may need more air/fuel mixture to compensate 
>for
> internal friction and maintain idle speed, so the throttle may have to be held
> a bit more open with the idle set screw, which means reduced vacuum.  However,
> even this is somewhat stretched, because the power loss due to worn out rings
> is going to be very small at idle.  Your case of a hole in the piston is
> unique, as that cylinder will have a huge vacuum leak directly into the crank
> case every time that particular intake valve is open (which is almost 1/3rd of
> the time).  In other words, the contents of the crank case will be sucked in
> the cylinder and intake manifold (I guess, you would not need a PCV valve in
> that case!).  A worn out piston ring will never allow that much leakage for
> appreciable loss of vacuum in the intake stroke (for various reasons).
> 
> Speaking about vacuum.  We all know about engine braking (i.e., downshifting 
>at
> a lower gear helps you slow down).  Many people think its the engine
> compression that helps you slow down.  In reality, its engine decompression
> that slows you down.  By down shifting, you increase the engine speed, meaning
> that the suction during the intake stroke increases.  Since the throttle
> remains closed, the vacuum in the engine gets very high.  The braking effect
> comes during the intake stroke, where cylinder pressure is well below
> atmospheric, meaning that the crank case pressure resists the piston downward
> motion.  Diesels with no throttle cannot achieve engine braking, unless a
> certain device is installed (called jake brake or something in big trucks).
> 
> D^2
> 
>