gkull

No I did not forget the header collector motor evacuation system. I used that on my dragster with open collector extensions.

They are designed for OPEN exhaust systems using the Bernoulli's principle. The typical header collector motor evacuation system has a one way check valve (flow in one direction) It doesn't open until a relatively high vacuum is created. Which doesn't happen on a closed exhaust system with back pressure. I know because we tried and tested with vacuum gauges. Even on a super free flowing dual 3 inch pipe system like I have on my vette.

REELAV8R

Like gkull said every thing from the pan to the valve covers is an air chamber.

If the air chamber were sealed air would still get into the chamber through the intake system since rings are not a perfect seal and blowby allows air into the chamber.

Holes connecting all these chambers together allows them to be considered as one chamber.
The holes to the valve covers are small in comparison to the chamber size in the oil pan area. Since air is compressible a pressure gradient exists where as the pan pressure is higher than the valve cover pressure due to ring blowby into the crankcase.

This is why using vents only, still allows pressure to exist in the system, particularly in the area of the pan.

We like to think of air as light and easy to move. It in fact can create significant resistance to movement and create significant problems in particular to ring seal for the pistons. This is why evacuating the pressurized air is desirable.

The PCV system evacuates this air pretty well at high vacuum levels and reintroduces it into the intake, as you already know. The oil mist in this air is inevitable due to the agitation and high local pressures inside the crank case.
So the catch can is used to help separate the oil from the air to prevent introduction to the induction system and then contamination to each fresh charge of intake air.

As gkull also pointed out once vacuum levels fall and the engine is at WOT the PCV system does little to nothing. It's beyond it's design limits.
So again we are left with high pressure in the crankcase causing problems.
The fresh air intake that is on the passenger side for your PCV system is now venting crankcase pressure and not introducing fresh air to be drawn through the crankcase by the now non-functioning PCV. Given this, if you spend enough time at WOT your intake charge is now being contaminated by this fresh air intake that typically is piped into your airbox. If it's not piped into your airbox then it's just venting under the hood.
A vacuum pump is the ideal high performance solution.

However if there were vent on both valve covers, one on the passenger side, flowing both directions, and one on the drivers side, where the PCV is located. The vent on the drivers side could incorporate a one way valve to allow no air to pass into the valve cover and open when the pressure became positive under the valve cover. This may be a simple, and not completely ideal, but still better than the stock, configuration solution.

If the intake on the passenger side valve cover is piped into the airbox and you spend some considerable time at WOT or very high throttle settings where the PCV effectiveness is minimal, then it may be advisable to put a catch can on that side as well. Typically the catch can is only on the PCV side.

gkull

IMO - there is no difference between the pan and valve cover internal pressure or vacuum how ever you look at it. Look in the cam valley of a chevy v-8 block. You have all the open oil return openings (lots of square inches). Then to the heads you have all the square inches of oil return holes and open area around all of the push rod holes.

On race cars we tapped the intake directly into the manifold above the cam valley to setup the variable bleeds to regulate maximum vacuum when using dry sump oil systems

REELAV8R

If the air were static I would agree with you. Once it stops moving and more pressure is not being generated then the system would equalize.

Since in an engine more air is being added to the system all the time then a pressure gradient has to exist between the point of origination (rings) and the exit of the system. Air is compressible. If it were hydraulic fluid then no. Anything compressible, yes.
Lots of restrictions to flow and no direct uninterrupted large volume line between the crank case and the exit on top of the valve cover.
An air hose for example. Tank pressure is 90 psi and a compressor that can maintain it at that pressure. Pressure at the end of a 100 ft 3/8" hose is 90 psi when no air is flowing.
Now hook up your air tool or just open the end of the hose, watch the pressure at the end of the hose drop due to flow restrictions, creating a pressure gradient between the hose end and the air tank.

My point being for the engine, if there were no restrictions to crankcase pressure exiting at least there would be a net zero pressure with simple vents. The situation however is not that, therefore creating a situation of pressure in the crankcase with vents only.

gkull

In my case I posted some pictures of my leak down testing showing less than 2% ring leak that the cubic feet of internal engine air space would not generate negligible pressure with twin one inch K&n air filters breathers

Like I said substandard to pumps

David@MMS

the air flow capability from the mains, up around the cam, into the valley & last into the head is more than any fitting or filter you will see atop the valve covers.

i have this conversation with customers assuming that both valve covers need symmetric venting.
they really just don't in general; more of a packaging requirement at a certain power level.

i have found the highest, driest place is the best place. i have several accounts of trouble venting from the valley moving out unnecessary volumes of oil.

resdoggie

Anyone ever accurately measure how much pressure/vacuum builds up in the "crankcase" at cruise and wot?

cv67

David/gkull

have the vents around the cam tapped and plugged

whats your opinion on a longer stroke high rpm engine using

-either 2 breathers on one side, pvc on the other (not convinced the typical breathers do much and may cause leaks)
-or no pvc and breathers on both sides picking brains here;id think the less internal pressure the engine is fighting the better?

Doesnt make more than 7 in of vacuum at 1000-1100 rpm on a good day

REELAV8R

I would guess that number would vary significantly depending on many factors. CR, ring type and seal, cylinder distortion, running temperature, RPM, and probably several others I'm not considering.

It would be interesting to see a graph of that pressure though and where exactly the transition from vacuum to pressure could be expected. And just how much pressure it is.
We've seen the posts about guys popping their oil dipsticks out just from blowby, that's got to be a fair amount of pressure.

David@MMS

cuisinartvette. why are your crank vents at the valley plugged, and what did you do elsewhere to compromise for this so crank flow can get from the bottom to the top.

for me pcv is fumes control and contaminant filtration for the system. not a method of crankcase vacuum control.

while you can use pcv to affect crankcase vacuum it is not really there 'when you need it' or of significance to cause tangible benefit.

I have done on-car crank flow measurements. the results are all over the place and would need to be also done car to car basis.

the existence of crankcase vac or pressure is much more relevant to system sealing than it is to power, where you are talking a maximum of around 5 percent using an external mechanical vac pump in a race only application.

some relevant info contained:

cv67

David, its a solid roller motor machinist insisted it was the thing to do...less dripping on the crank I guess who knows. Hes been at it for 50 yrs so I didnt question him.
Toyed with the idea of drilling a hole in the intake (single plane) with an oil separator/breather never got around to it

resdoggie

Yes but what was the state of the crankcase ventilation? Anyway, it would be nice to see some solid numbers rather than just "crankcase pressure" which can be anywhere between full vacuum and maximum cylinder pressure on the power stroke. Obvious exaggeration but just what is it on a HP street engine? 0-5 psi? Maybe a FI guy can drill/tap his fuel block off plate and install a pressure gauge?

David@MMS

it is +/- 1psi for the 99%

REELAV8R

David, some interesting tests.
Hooking up a catch can to the intake tube to draw a source of vacuum from is laughable.
An unrestricted intake tube or box has less than 1 1/2" of water vacuum much less anything readable in inches of mercury. If you can detect any vacuum in an airbox with a standard vacuum gauge it's waaay too much restriction from the filter and/or air supply size.

I've considered hooking up a water manometer to the dipstick tube in order to measure crankcase vacuum/pressure. Have not gotten around to that yet.
A water manometer vs vacuum gauge would show the vacuum or pressure much better and be many times more sensitive.

george2066

what did we determine here, scrap the pcv valve and just run breathers?