On Thu, 1 Jul 1999, Robert Duquette wrote:
> My PCV is still springless. However, while searching for 'something else'
> in my junk box, I happened upon a spring that looked like it might fit, so
> ... even though I believed it to be too firm, I had to try it.
>
> The RPM went up by about 700 rpm and I thought that maybe I had better take
> the thing out. So I took the top clip off and the top metal piece that I
> was expecting to fly off due to the heavy compressed spring ... didn't.
> (car still running.)
>
> So, all this to ask how the PCV really works.
>
> Here are my thoughts:
> The intake sucks from the bottom and the front hose provides pressure from
> the crankcase. The intake would be pulling down on the diaphram and the
> crankcase pressure would be pushing up on the diaphram. The stronger
> pressure wins (with the right spring and a good diaphram, etc. ) and the
> result is that the plunger moves up or down and the opening that allows air
> to pass through the PCV increases when the pressure is greater than the
> vacuum.
>
> How did I do?
>
> Now, why the jump in RPM in my example?
>
> Robert Duquette
> Ottawa ON Canada
> http://www3.sympatico.ca/robertduquette
> RobertDuquette@Sympatico.ca
> '65 RHD BRG Sprite
Not bad! :-)
To see why the PCV valve is necessary, think about hooking up the hose
from the timing cover directly to the intake manifold.
We know that vacuum is highest at idle and decreases as the throttle
plates open. So the amount of crankcase air sucked in would also vary
with the amount of vacuum. This means that you would get a varying amount
of air into your intake manifold, that the carbs would have no chance
adjusting for.
The valve now works like this:
With little vacuum, the spring pushes it all the way open.
With increasing vacuum, the valve slowly closes against the spring,
keeping the amount of air being sucked into the manifold approximately
constant for all vacuum conditions! If you think about this, it is clear
how critical the stiffness of the spring is. If the spring is really
stiff, it holds the valve open all the time and it will be like the
straight connection discussed above.
So you have a roughly constant amount of air being added to the intake
mixture, which is accounted for by a richer mixture setting of the carb
than would be required if you disconnect the PCV.
The jump in rpm was caused by the added air going into the engine.
After you get a right spring, retune the carb mixture and idle speed.
Ulix __/__,__ ___/__|__
..............................................(_o____o_)....<_O_____O_/...
http://students.washington.edu/~ulix/ '67 Sprite '74 X1/9
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