[Healeys] Fwd: New post on my blog

[Michael Salter]

I think you are absolutely correct Bob.

M

On Mon., Oct. 31, 2022, 3:56 p.m. Bob Spidell, <bspidell at comcast.net> wrote:

> 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.
>>
>>
>> [image: 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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