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-   -   Sportster Crankcase Pressure / Engine Breathing / Wetsumping and Mods (

bustert 31st October 2019 15:53

you are dealing with hydrostatic
the pressure you feel at sea level is about the same pressure you feel at around 35 feet under water.
pascal's formula can shed some light
the hydro-static pressure is a constant at an elevation, that said, the smaller id and larger id will be the same except with the lager id having more resolution lines since it surface area is greater
according to him, the pressure across the capillary tube is the same

Hippysmack 31st October 2019 21:31

That supports what this site shows only I didn't get why from it.
Atmosphere has it's own weight which affects the air above the liquid in the tube thus affecting the movement of the liquid.... I think.
The pressure reading is always the difference between fluid heights, regardless of the tube sizes. With both manometer legs open to the atmosphere, the fluid levels are the same (A). With an equal positive pressure applied to one leg of each manometer, the fluid levels differ, but the distance between the fluid heights is the same.

Tomcatt 31st October 2019 21:58

That's not a good drawing, you'd have to lose fluid for the botom right to be correct, but that is the right idea.

Hippysmack 31st October 2019 22:19

I see your point (their drawing).

I'm sure they are just giving example but the water level would have had to begin level on both legs at a lower than zero value in order for that to be correct.

Hippysmack 1st November 2019 01:54

This is suppose to be the way you measure height.
Take the reading from the left leg between zero reference and the height of the liquid and multiply it by 2 to get total rise in vacuum.
Likewise you can add zero to liquid height on both tubes and get the same result.

In the drawing (mine, or ours now),
A rise in vacuum pressure of 6“ of water (left leg) has a total of 12” in rise as the water has traveled in both legs.
Then an addition of 3" of positive pressure exerted (6” in actual height) on downstroke still yields a balance of 6“ of total vacuum from the engine (showing 3” on left leg).
In this example, the total pressure in the crankcase is still negative but with not as much pressure as it was at the last upstroke.

That leaves me with 13-3/4" in the left leg at idle which translates to 27-1/2" of water column, correct?

Buster you had 15" in the left leg making 30" of water column, correct?

Just making sure I'm reading this right.

Our readings probably won't match but your instrument would be right for all future readings you take from that tube.
My readings are 'right' for my tube and it also will be right for my future readings from it.

Hippysmack 1st November 2019 02:35

This may help explain why breather valves designed for autos are not sifficient for use in Harleys.
According to this PDF, 1994-2007 BMW engines had a crankcase pressure requirement between 10 mbar to 15 mbar (4"- 6" WC)
The breathers have a spring and diaphragm.
Long way from 30" WC in a Sporty in thinking about it but it's not really that big of a pressure difference.
I assume more pistons make the pos to neg swing not so big.

It goes on to say,
The values on both sides of the slack tube must be added together to get the proper reading.
Example: 2.5 + 2.5 = 5.0 inches of water.
(The readings may fluctuate 0.1 to 0.2 during the measurement which is normal and does not indicate a defect).
If a crankcase ventilation valve is defective (damaged diaphragm) the column of water will indicate an off the scale reading (all of the water on one side of the slack tube) which indicates a high crankcase vacuum.
A higher than normal crankcase vacuum will also cause the crankshaft seals to leak outside air into the crankcase during engine operation.
A whistling or howling noise is usually heard coming from the seal areas (front or rear) at idle.

GM states normal vacuum is between -12 to -18 inches of water.
Based on GM technical service bulletins, vacuum at idle should be no higher than -30 inches of WATER. I encourage anyone with a squeaking crank seal or dipstick to vacuum check their engine for an excessive vacuum condition.

bustert 2nd November 2019 15:38

not following the cruze thingy.
rice vehicles do things diff and some american follow to an extent.
american has a tendency to just skate by with old tec add on while rice designs floor up.
a lot of power plants gm uses are not american, even though made in the usa.
haven't really followed things except what i buy. the last nissan i had sorta was a semi-closed system, no longer air inlet from the cleaner, through the block and through the pcv to manifold.
it used a fixed orifice bleed from the c/c to the manifold and when this was exceeded, c/c was vented through the block to the intake, gone the hose to the air cleaner.
the new honda turbo, have gotten to it yet.
so i can see the leak to the manifold in addition to the fixed orifice causing high c/c vacuum at idle.
remember the old days when an engine would pull 28 inches? todays engines pull at the lower range you stated, all controlled by camming unless there is a mechanical vac pump, that said, c/c vac cannot exceed what the engine can pull. again, have not kept up but the emissions era in the beginning was shinola with 18" a norm with low compression to boot.

Hippysmack 2nd November 2019 15:56

I've just been doing research on using a slack tube for testing and keep coming across these lower figures on cars.
Then I see basically 2 stage systems restricted for low RPM and a bypass for WOT.

In testing breather flow (pressure) with a slack tube, I need to create a restricted tee.
Any idea on how to gauge the restriction for a Sporty engine?

bustert 2nd November 2019 16:35

there is a formula but do not remember. see if i can find it.
probably be in a diesel test page as that is where i ran across it.

Ireeman 3rd November 2019 01:34

General gas law...for christ sake.

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