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Old 06-11-2005, 11:28 AM
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Crankcase pressure effect on oil pressure?

In a recent thread HR.Com members helped my diagnose a bad pcv valve which was causing oil to blow out the dipstick tube, breather cap, and a few other places as well. I replaced the pcv valve and just drove 50 miles without any signs of oil leaking in the suspect places. So hopefully the problem has been solved.

However, on this shake down run I noticed my oil pressure gauge showed about 5-8 lbs less pressure than normal (at highway speed with engine warm). Can the build up of crankcase pressure due to a bad pcv valve actually boost engine oil pressure at the gauge - or might something else be going on? I'm running about 37 lbs of pressure so I'm not worried too much, but I had been running from 42-45 lbs before. (Ford 460 BTW with a high volume pump.)

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Old 06-11-2005, 07:38 PM
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Although it will not really increase pressure it could read as a pressure increase on the gauge if the crankcase is sealed from the atmosphere and the gauge is vented (like most every mechanical gauge i've ever seen).
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Old 06-11-2005, 08:53 PM
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Quote:
Originally Posted by NXS
Although it will not really increase pressure it could read as a pressure increase on the gauge if the crankcase is sealed from the atmosphere and the gauge is vented (like most every mechanical gauge i've ever seen).
The gauge itself is electronic, not mechanical, so unless the sending unit (stock 460 unit) is vented...there must be some other explanation for the variation in my oil pressure reading. Like I mentioned in the original post, I don't feel it is disastrously low, just lower than before swapping out the pcv valve.
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Old 06-11-2005, 10:17 PM
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When you pull a vacuum on the crankcase the oil pressure goes down. When we run vac pumps we drop 10 lbs at least, this is at 15 inches of vac.
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Old 06-12-2005, 03:39 AM
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Quote:
Originally Posted by Rick WI

When you pull a vacuum on the crankcase the oil pressure goes down. When we run vac pumps we drop 10 lbs at least, this is at 15 inches of vac.
Now this is interesting. I have never run a vacuum on a crankcase. Can someone explain the theory? The only thing I can imagine is the vacuum is pulling the oil out of the galleries after the pump and pressure registering device (allowing less flow resistance to maintain a certain PSI).

The 460 (as does the 335 Series) has it's pressure sending device at the end of the galleries (as opposed to immediately after the pump/filter).

That is one heck of a PCV valve...
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Old 06-12-2005, 05:33 AM
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Quote:
Originally Posted by KULTULZ
Now this is interesting. I have never run a vacuum on a crankcase.
I could be misunderstanding the terminology here, but isn't the pcv system "pulling a vacuum" on the crankcase - even if it is not a very strong vacuum? I'm curious. I have about 19 inches of vac from the carb to the pcv valve...how much of that is actually introduced into the crankcase by the pcv valve...and does it change as rpm goes up and down? (I have a vague recollection you answered this once before Kultulz - so sorry if I'm covering old ground.)
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Old 06-12-2005, 06:43 AM
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nothing to worry about. lets say your building 20lbs of presure in the crankcase. this 20 lbs is fighting the oil pump since the oil galleys are open to the crankcase. never seen it before but when racing it really shows up if your not evacing the crankcase well. high volume pumps show drops in presure more than others. engines dont leak oil from main seals, they blow it out with crankcase presure. run a fat carb for a while and you'll have a leaker due to washing down the cyclinders and losing comp to the crankcase.
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Old 06-12-2005, 07:37 AM
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Quote:
Originally Posted by cboy

I could be misunderstanding the terminology here, but isn't the pcv system "pulling a vacuum" on the crankcase - even if it is not a very strong vacuum? I'm curious. I have about 19 inches of vac from the carb to the pcv valve...how much of that is actually introduced into the crankcase by the pcv valve...and does it change as rpm goes up and down? (I have a vague recollection you answered this once before Kultulz - so sorry if I'm covering old ground.)
The PCV System can in no way draw the vacuum as can a vacuum pump in a race engine. The PCV is not so much drawing a vacuum as providing a ventilation path.

-HERE IS A GOOD TECH ARTICLE ON CRANKCASE VENTILATION-
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Old 06-12-2005, 08:04 AM
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PCV valves draw very little vacuum on your crank case. The filtered air inlet balances anything the PCV draws out resulting is a cleaned out but essentially balanced-pressure system. Th only pressure drops possible are due to the restriction in the crank case inlet filter (virtually zero if you keep it clean) and to the restrictions in the various block and had passages and drain holes between the air inlet filter and the PCV valve. Any conceivable pressure drop you may have altered is surely way smaller than the resolution of your pressure gauge. Your oil pressure drop is from some other coincidental cause.
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