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 cylnder 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
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