[Healeys] Fwd: New post on my blog

[Bob Spidell]

I think it's the difference between reading pressure/vacuum at the 
crankcase (dipstick tube) vs. pressure/vacuum at the intake manifold 
(where a typical vacuum gauge is mounted). PCV valves close under vacuum 
at the intake (small) side and open when the vacuum decreases--at part 
throttle or WOT--where the 'leak' is less consequential. High neg. 
pressure, at idle and overrun, at the intake side closes the valve--at 
least partially, they all seem to 'leak' some--else you'd have a massive 
intake leak. Hence little or no vacuum in the crankcase at overrun. At 
idle, the low BMP makes the blow-by less of an issue.

As always, I stand ready to be corrected.



On 10/31/2022 11:48 AM, Michael Salter wrote:
> I think there is something to what you suggest Bob but, when I first 
> observed the only very minor negative pressure on overrun I  was 
> somewhat confused because my thinking was that as there is minimal 
> blow-by on overrun the PCV valve (vacuum source) should have 
> encountered no problems keeping the crankcase evacuated. Further study 
> however has provided what may be an explanation.
> Apparently the PCV valve selected, and this applies to most versions, 
> actually throttles down the air flow at high vacuum aka overrun.
> I suspect that I was observing the results of this on the manometer.
> Closing off the original valve cover breather tube probably meant that 
> crankcase pressure dropped further because there was less "make up" 
> air available during these conditions.
>
> M
>
> On Mon., Oct. 31, 2022, 2:26 p.m. Bob Spidell, <bspidell at comcast.net> 
> wrote:
>
>     Michael,
>
>     From your blog post:
>
>     "occasionally on overrun very little if any negative pressure was
>     evident in the crankcase so, in order to improve things a little I
>     installed a plug in the hose which runs from the valve cover to
>     the rear air filter and this resulted in a significant improvement"
>
>     Probably because you're pulling air (and sump oil) into the
>     crankcase through the rear main 'scroll' seal (and dipstick tube
>     and oil filler cap hole). This is how the typical, owner-installed
>     PCV installation on the 6-cyl cars functions as the PCV valve
>     meters gasses both from the crankcase and the valve cover. But ...
>     with fresh, filtered air coming in from the carb air filter you
>     essentially had a true PCV 'system,' where the gasses--fresh air
>     and blowby--are cycled back into the intake manifold to be burned
>     in the cylinders.
>
>
>     Video: The Importance Of Using A Catch Can System
>
>     On 10/30/2022 10:22 AM, Michael Salter wrote:
>>
>>
>>     Sorry forgot the link...
>>
>>     Adding a PCV to a 100.
>>
>>     https://precisionsportscar.com/austin-healey-100-pcv-system/
>>
>>
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