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Old 01-19-2020, 05:00 PM   #561
Kingtal0n
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Quote:
Originally Posted by mechanicalmoron View Post
Vacuum's good in the motor, but there's going to be diminishing returns past a mild point, which is why production cars don't come with hoovered out PCV systems. Cars with excessively positive OR negative crankcase pressure develop all sorts of weird problems, like seals flipping and wearing, or other solenoids or systems not doing what you'd think because they can never bleed vacuum as fast as they need, or whatever.
production engine crankcase vacuum range in wet sump applications is 1 to 3" of Hg. 1" of Hg is about 3300Pa or 3kpa, so a general guideline for WOT production engine crankcase pcv is approx 10KPA below atmospheric pressure. (103KPA - 10KPA = 93KPA crankcase pressure, absolute)

It varies depending on the engine's current state and pcv orifice diameter but somewhere within that range is acceptable to majority of production engines...

Performance engine vacuum range is actually the same, 1 to 3" of Hg in a wet sump application. So your statement about excessive vacuum is accurate whether we are discussing production engines or high performance engines with oil in the pan.

The difference you neglected to mention however is in the flow rate of each situation. A production engine which has been turned up (say, a sr20det turned up to 400whp or a 4.8L truck engine turned up to 600rwhp) to around double of the intended output is going to require around double the pcv flow rate to maintain the same vacuum (roughly 10KPA) given all else equal, due to the roughly double pressure integral experienced by the piston area.

This is a classic example of pressure vs flow, just like in oil systems we are familiar with. What it really means is that when attempting to hit production pcv standards (10KPA pressure drop ranges) in highly modified (often production) engines, the requirements for adequate pcv performance change as much as the power does, you can't always re-use the factory pcv components and expect to achieve a factory level of PCV on highly modded engines. The orifices likely need to be larger and the filter situation needs to be addressed, since the air filter essentially drives WOT pcv action.
In other words, crankcase pressure should be discretely measured, to ensure that phenomenal range of 1 to 3" Hg is being hit at WOT and cruise.
If that can't be done, then at least adequate paper filtration and correct pcv orientation should be used to give it a fighting chance at least.
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