> All my life experience with 4 cycle engines have been based on this
> theory ...
>
> Intake Stroke: piston is going down intake valve open
> Compression Stroke: piston is going up both valves closed
> Power Stroke: piston going down both valves closed
> Exhaust Stroke: piston going up exhaust valve open
That's accurate enough, as far as it goes. But it is a simplification and so
leaves out some details. On a 'modern' engine, the exhaust valve actually stays
open slightly beyond the exhaust stroke, as the momentum of the gases flowing
through the valve will continue to carry them out even though the piston is no
longer pushing them. The intake valve also opens slightly before the beginning
of the intake stroke, for a similar reason. The same things happen at the other
ends of the intake and exhaust strokes. Plus of course it takes time for the
valves to move so the camshaft starts moving them well before the point where
they "open". The result is that it takes almost a full crank revolution from
when the valve clearance starts to go down to where it quits going up.
Without these changes, the engine would only run at very low rpm and hence would
not make much power relative to it's displacement and size.
If you find the time, go visit the Antique Gas and Steam Engine Museum in Vista,
CA. There you can see many early internal combustion engines that don't have
any valve overlap. They typically turn at only a few hundred rpm (150 is
common), and a 2 liter motor might only make 10 or 20 hp.
> Now it seems to me that TDC is very close to either the end
> of the compression stroke or the beginning of the power
> stroke, with both valves closed this should be when they can be
> correctly adjusted.
TDC stands for "top dead center", it refers only to the movement of the piston
in the bore. There are actually two places where the piston reaches TDC during
a single cycle. One is between the compression and power stroke, the other is
between the intake and exhaust stroke.
> My new TDC indicator (neat whistle the
> screws into spark plug hole) allows me to determine TDC (I
> think).
What the whistle indicates is that the piston is coming up on the compression
stroke. It won't whistle at TDC, so it would more correctly be called a
compression stroke finder. And of course it only finds it for the cylinder that
it is installed in.
With the "on rock" method, both valves closed at TDC between compression and
power is indeed the position where the valves are adjusted. However, it uses
observation of the opposite cylinder, the one coming up on TDC between intake
and exhaust (where the valves overlap), to find that point for the cylinder to
be adjusted.
> >As any true TR mechanic who has used the true "on
> >the rock" method will clearly recognize, this latter definition is
> >obviously utter poppycock and will not produce nearly as effective a
> >result.
I do hope everyone recognizes that Scott was making a joke !
Randall
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