Crankcase Breathing

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Crankcase Breathing

Post by FJCamper » Sun May 11, 2008 4:56 pm

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This is a long post, but worth it.

The stock VW Type 1 crankcase breather can't handle racing pressure. The engine comes with a single crankcase breather on the oil filler tower so small you can't stick the tip of your finger into it. Four cylinder Porsche 356 and 912 engines were a little better, evolving through the years from vents on the valve covers, to hoses coming off the heads and the oil filler tower to the carb intakes, just barely enough for light competition.

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High performance engines create a storm of high internal crankcase pressure as the crankshaft spins, and the pistons fly back and forth. Add some piston ring blow-by, and power goes down as pressure resistance to the moving parts go up. Reliability also drops as oil blows out of seals, gaskets, and even the engine's crankcase breather.

Racers of all types resort to "dump cans," a can or tank the crankcase breather vents into, keeping oil off the track. At screaming win-or-break RPM's, air pressure is forcing oil out of every hole in the engine it can find.

What Causes Crankcase Pressure?

Two things. Moving parts, and piston ring blow-by. The "pressure" we talk about is made up of air resistance, and splashed oil droplets suspended in gases that made it past the piston rings, including unburned gasoline and water vapors.

Pistons push air and gasses on both ends of the cylinder. Moving anything takes energy. Pumping pressure out of an engine through a small breather also takes energy, because the breather hole is so restrictive.

When crankcase pressure builds because the air and gasses inside can't easily vent, it will escape wherever it can. This is the cause of many small oil leaks.

To fix this, you have to exhaust the area inside the crankcase, removing the combustion products, which blow by the piston rings. After only a couple of power strokes the case pressure rises. The perfect breather would expel crankcase pressure and blow-by gas while simultaneously providing low pressure inside the crankcase.

How Much Pressure Is In A Crankcase?

A vacuum/pressure gauge connected to a port into the engine (not the intake, but the crankcase itself) will show you how much pressure your engine creates as it runs. Most engines don't have a handy crankcase vacuum port, but sometimes an oil dipstick hole can be a good improvised location.

A rough rule of thumb is that for every one litre of engine displacement, it will blow out (by volume) about 1.25 cubic feet per minute of air at high (5000+) rpm.

That means that even on a modest 2-litre engine turning six grand, you can have 2.5cfm trying to blow out, at 30psi (or more) of internal pressure.

Air-Cooled VW and Porsche Engine Breather Problems To Overcome

The VW Type 1 engine breathing system changed very little from the old 1100cc's to the 1600's. VW provided a half-inch diameter vent hole on top of the generator stand near the oil filler cap. A tube ran from the vent hole to the air cleaner, providing a low-pressure path for the vented gasses to follow to the intake.

That small hole was okay in 1949, but became increasingly more marginal as horsepower and RPM's went up. And VW never improved it.

The only venting changes were the clean-air mandated elimination of the "condenser tube" that ran downwards from the oil filler housing atop the generator stand, to allow drip-off of condensed water and oil contaminants.

A problem the VW (and VW-derived Porsche 356) engine has is oil being sprayed by the spinning cam gear, which is just in the right place to constantly flood the generator stand. VW compensated for this with a louvered splash plate between the case and generator stand. With no splash plate, a VW engine running without an oil cap would blow oil out the filler hole like a fountain.

Porsche's spray problem was not as bad, because the 3-piece 356 case enclosed the cam gear in a separate front cover housing with a cast-in deflector above the cam, blocking most of the cam gear spray.

This oil spray in the VW and Porsche significantly adds to the oil vapor and droplets in the crankcase, making the crank assembly rotational resistance even greater, and the need to improve case venting even more important.

"Sucked-In" Valve Cover Gaskets

VW Type 1 and Type 4 engines, and Porsche 356 people have all experienced valve cover gaskets that appear to have "sucked in," causing oil leaks. This seems to indicate low pressure, not high, inside the crankcase.

The gaskets have indeed been sucked in, and low pressure in the head was the problem, but as a side effect of very high pressure elsewhere in the engine.

Pressure highs move about inside the engine, displaced from one area to another by the pistons. There is an instant in which the heads contain very high crankcase pressure, then another in which it is very low. Due to the way VW and Porsche valve covers fit their gaskets, the gaskets can't be forced outward as easily as they can be sucked in, so sucked-in wins.

Porsche engineers tried different venting methods on racing 356 valve covers, to help out the small generator stand vent that even the early 356 engines had. One try was to design vents directly on the top center of the valve covers. These were raised openings covered by metal filter mesh, but oil tended to blow out past the mesh and soak the outside of the valve covers.

Racers sometimes rigged vent hoses directly off the valve covers, routed to dump cans. These reduced crankcase pressure, but the problem was the valve covers had to be removed a lot. Having to always disconnect the hoses was extra work, not to mention the cause of oil leaks if the frequently disconnected hoses loosened or came off accidentally.

Porsche finally refined its system (in the 912) to hoses that came directly off the heads, not the valve covers, each head venting to its individual cylinder bank carburetor air cleaner.

So Why Not Just Run a Bigger Breather?

While the total interior volume of the crankcase remains constant as one piston goes up while another goes down, the displaced air has to move back and forth in the case. You suffer power losses from this.

A too-small crankcase breather (relative to crankcase volume and rpm) will not allow enough equalization between pistons strokes to make much difference.

The old school racer's fix, motorcycle to race car, was to fit (if possible) a big, open breather pipe, and let the case suck in and blow out air as the engine runs. There is no power loss with this system as long as the breather is big enough, so that no significant pressure difference builds up between the crankcase and the outside atmosphere.

But the problem with this is there is usually no easy place to put such a big hole.

So, what then must we do? Another post follows.
Last edited by FJCamper on Thu Nov 17, 2011 8:12 am, edited 1 time in total.

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Vac pumps

Post by FJCamper » Sun May 11, 2008 5:05 pm

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A Formula 1 & Drag Car Speed Trick: Vacuum Pumps

Lowering crankcase pressure with a vacuum pump will increase horsepower because the rotating crankshaft and piston assembly meets less drag.

Low pressure in the crankcase means the piston doesn't have to act as an air compressor on its downstroke.

F1 racers run sealed crankcases and vacuum pumps. They try to pull at least 28 inches of vacuum in the case, and they build their engines specifically for this.

Dry-sump NASCAR engines use multiple scavenge pick-up pumps. When one pickup point isn't sucking oil, it is evacuating crankcase pressure.

Some V8 drag racers also run vacuum pumps, either electric or belt-driven. Dyno tests on V8 engines usually show at least a 2% power improvement with a crankcase vacuum, and torque increases of about 2% to 5%.

These pumps can draw about 10 inches (or more) of vacuum in the crankcase. They run the oil fog they suck up into an oil/air separator, vent the air, and drain the liquid oil into a dump can. Oil seals begin to suck in air at about 10 inches of vacuum, unless the seals and sealing surfaces are specially made to resist it.

The only thing stopping some of the V8 drag boys from running more than 10 inches of vacuum is their stock main seals. Some engines actually benefit from reversing the main seals to keep air out of the crankcase.

Crankcase Vacuum = Better Piston Ring Sealing = More Power

You may also be having problems with poor ring sealing in your engine, causing more blow-by. A good side effect of lower crankcase pressure is how the extra pressure differential affects the piston oil ring, helping oil control.

With high pressure in the crankcase, when the piston reaches the top of its stroke, the oil ring is flexed in the piston groove, which allows excessive combustion chamber pressure leakage.

When a crankcase vacuum is present, the piston compression rings seat better, rather than wobble in their grooves. Piston ring to piston groove sealing is as important as piston ring to cylinder wall sealing.

...and this applies to us VW types how?

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The VW fix

Post by FJCamper » Sun May 11, 2008 5:21 pm

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The VW Breather Fix

Not many modifications give you both more power and more reliability at the same time, but a good crankcase venting system is one of them. Reducing crankcase pressure buildup gives better piston ring sealing, reduced oil leaks, less intake charge contamination, and overall better engine performance.

Our Problem: Oil Leaks

What prompted our search at RetroRacing for a better breather system for our road racing 1970 Karmann Ghia was the fact that after fifteen to twenty minutes of constant 7000rpm racing around the track, we were literally smoking.

You would see it as a wisp of white just behind the car, rolling in the draft. Then, for a minute after we stopped the car and shut off the engine, white oil vapor would gush out of our breather box as if we were on fire.

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We used a conventional breather system with valve cover vent hoses running to a small foam-baffled breather box mounted high center on the fan housing. A drain hose allowed oil collected in the bottom of the breather box to run down to drip back into the engine case.

The vapor and mist that escaped came from the breather was coming from under the vented cover on the box itself, the cover slightly elevated on low standoffs, to allow ventilation.

Worse than smoke, we had vent hoses sometimes blowing off the valve cover fittings and creating oil gushers, we had oil mist from the breather covering our engine, and we even had oil blowing out the front sand-seal behind the belt pulley.

We were simply overloading the breather system with the intense heat and pressure generated inside our case by racing extremes. We could have gotten a larger breather box and larger dump can, but that wasn't the real fix.

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We found a simple and effective solution in a small device named the KrankVent, sold by ET-Performance of Santa Barbara CA.

The KV is actually a "crankcase vacuum valve," a very sensitive device, without PCV-like springs and valves to overcome to vent pressure from an engine, so the KV has no poppet valve flutter at high RPM like a PCV valve.

To create crankcase vacuum, the KV allows pressure pulses out but nothing back in. Outside air pressure will try to come back in past the oil seals, gaskets, and piston rings, but the amount of air that can come from these places is much less than the volume the pistons can force out through the KV, creating low pressure in the case after just a few moments of running.

In the above drawing, the breather box has been sealed to become an air/oil separator. Filled with foam, the breather box now acts as a baffle in the flow stream to separate cooling oil vapor from unburned gasoline and water vapor in the bypass gases. The oil is drained to a dump can. For smog purposes, the gasoline and water vapors are returned to the carburetor intake for reburning. For racing, they can be routed to the dump can.

KrankVent VS. Positive Crankcase Ventilation Valves

The KV was granted a patent as a PCV-type valve for any internal combustion engine, but the similarity ends there. The KV patent claims include the unique property of both releasing crank case pressure, and producing and maintaining a vacuum in the crankcase.

A PCV valve is designed to work with a partial vacuum being pulled on its low-pressure side, but the KV does not need the presence of a vacuum "pull". The internal crank pressure "push" is enough. The engine must be airtight in order to maintain the vacuum. Loose oil tank caps, dipsticks, and leaky seals are often overlooked. A kinked hose between the engine and KrankVent, or using too soft a hose that collapses under a vacuum is another problem to avoid.

High oil temps (even above 260° F.), will not harm the KV's valving. That makes it safe for racing applications. The KrankVent valve material is the same as used in the diaphragms of pressure regulators used to control the flow of coolant in nuclear reactors. The material has to be good or there will be a meltdown.

Dr. Ted Shrode of ET Performance already had one high-powered VW dune buggy racer as a KV user, but wasn't sure about how that car had its KV connected. He provided us with a single KV, and advised us to "seal up" our engine, and connect the KV off the standard vent hose location.

With no valve cover vents, and a new sand seal, we were tight. We were in fact so tight, and the KV so effective, that liquid oil was sucked right out of the generator stand starting at 3000rpm, and would fill a one-quart dump can in two minutes. Wow. Shrode was not kidding about the KV being efficient.

Next, how to do it. After several attempts, the best setup for us we found were two KV's, one connected to each valve cover, as shown in the illustration. Having dual KV's allowed each head to be individually vented, and reduced the volume that would have fallen to a single vent.

By connecting a vacuum gauge to a port created by screwing a fitting into the cover plate over the mechanical fuel pump location, we detected about 2" of vacuum at 3000rpm. This puts us in the comfort zone of an anticipated 4" to 6" of maximum vacuum at a steady 6000 to 7000rpm. That's enough vacuum to be efficient, but not enough to lower our oil pressure by reducing pump resistance at the bearings, or to cause main or sand seals to suck in.

We did swap over to modified valve covers with a welded-in strip along the lower edge to stop the gaskets from sucking in.

The only perceptible external change we experienced was a smoother idle. But a half hour road test at variable RPM showed a big difference. Only vapors, not liquid oil, escape up to the air/oil separator now. The vent hoses to the carbs revealed no oil residue, and the dump can stayed dry.

Just this much improvement alone was terrific. We had become so accustomed to smoking, oil leaks, and making excuses to track safety officials that if we'd gain nothing but cleanliness and reliability - forget extra torque and power - we'd be happy.

For KrankVent info, contact:

Ted Shrode, Ph.D.
ET Performance Products
email: ET@et-performance.com web site: http://www.et-performance.com
Last edited by FJCamper on Thu Nov 17, 2011 8:09 am, edited 1 time in total.

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Re: Vac pumps

Post by Stripped66 » Sun May 11, 2008 5:27 pm

FJCamper wrote: ...and this applies to us VW types how?
How doesn't it? A vacuum pump isn't difficult to install on a VW.

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Reply

Post by FJCamper » Sun May 11, 2008 5:32 pm

Hi Stripped66,

Actually, we're both on the same page. I thought we were going to have to go the pump route until we got our KrankVents.

We get the advantages of a pump this way, without the extra appendages.

FJC
Last edited by FJCamper on Sun May 11, 2008 5:35 pm, edited 1 time in total.

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Post by Stripped66 » Sun May 11, 2008 5:35 pm

I'm going the pump route, probably shoot for 12-14" of vacuum.

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Post by sideshow » Sun May 11, 2008 6:22 pm

I have been kicking the idea of a PVC system (as a side effect from type-4 rays postings). It is my understanding that in the 914 configuration you bring in clean air at the heads and pull on the oil tower.

Have you tried only letting air on the 3-4 side and pulling crankcase pressure out the oil tower?
Yeah some may call it overkill, but you can't have too much overkill.

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Worth a shot

Post by FJCamper » Sun May 11, 2008 8:13 pm

Hi Sideshow,

With KrankVents, experimenting is easy. We tried several configurations until coming to the one I illustrated.

Drag race people can fill up a dump tank in one pass from oil froth and pressure. The KrankVent really helps. But only once you find the right arrangement for your engine.

FJC

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Post by Scott Novak » Mon May 12, 2008 6:39 am

A Very informative article Mr. Minimalist Don't Fix What Ain't Broken.

The only thing I don't like about this breather system is having to cut a sand seal and weakening the pulley. I like the idea of a sand seal in general, if it doesn't weaken other parts.

As blow by is a major issue and the root cause of most of the problem, what is your opinion on Total Seal piston rings on the track and also for street engines?

Scott Novak

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Total Seal

Post by FJCamper » Mon May 12, 2008 7:19 am

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Dear Scott,

Okay, you caught me. Sometimes I do pay attention. This is one of those things necessary to win.

I use Total Seals on my racing 94's, but not the 85.5's. And I have since 2002 used the verboten 87mm and slip-in 88mm big bore kits (standard rings) just to see if they caused Apocalypse, and they do cause blowby if ragged to the Road Atlanta limit.

The conventional wisdom on crankcase pressure is if you have good rings you don't have crankcase overpressure, but that's not true. You've got the air-compressor effect as pistons move.

I use a cheater sand seal, the kind where you need a special pulley. When you sand seal the pulley, you are messing with Mother Nature and Dr. Porsche, because they designed the system to use that open crank-nose area for ventilation.

But, the KrankVent won't work with such a hole, so I use the seal.

FJC

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Re: Total Seal

Post by Stripped66 » Mon May 12, 2008 12:12 pm

FJCamper wrote:When you sand seal the pulley, you are messing with Mother Nature and Dr. Porsche, because they designed the system to use that open crank-nose area for ventilation.
I don't buy it. VW eventually went to a sealed pulley in the Type 1's successor, the Wasserboxer. The Type 1 already had two avenues of venting crankcase pressure out of the case (the breather hose and draft tube) that would be more effective that through the nose of the case, and venting air INTO the crankcase certainly isn't a hallmark of the design. The labyrinth seal used throughout the Type 1's manufacture was arguably due to its simplicity: 1 part is less than 2 parts.

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The Big Hole

Post by FJCamper » Mon May 12, 2008 3:34 pm

Hi Stripped66,

Think for a moment. VW guys began sand-sealing their engines because sand and grit was being sucked in through the hole, not blown out.

Actually, the reason for the VW's unsealed hole behind the pulley really is case ventilation -- inward. I know the fact of the unsealed hole is just accepted in the VW world, but it was changed in the Porsche world. From the 356A onward, the redesigned engine cases used a genuine oil seal behind the pulley.

To be able to block off that hole, which has the purpose of allowing enough air in to try and counter sucked-in valve cover gaskets, when the engine pumping pulls that momentary low-pressure on one side, Porsche had to modify its case vent system to compensate.

The reason oil doesn't just gush out of the VW hole is the oil slinger and reverse pulley base threading to return oil back into the engine ... and the fact there is a slight inward airflow at the hole.

When you turn off a VW, only then does some oil seep out of the hole. On the VW, a little oil seepage was allowable. The Porsche customers who had spend four times as much or more on their cars didn't want to see an oil drip spot.

VW didn't leave the pulley hole unsealed as an economy measure. It was by design.

FJC

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Post by Piledriver » Tue May 20, 2008 8:13 am

I will be looking into these check valves, nice.
(OK....Freaking $Ouch, maybe for racing use...)

I discovered last year (and apparently Bob Hoover figured it out ~30 years ago) that SS pot scrubbers make excellent air/oil separators.

I run 2 "Chore Boys" (SS, woven type, no loose bits) stuffed in the T4 breather tower. A T1 tower might not have room for 2, but then again it might...

With otherwise the same breather setup, I went from using considerable oil and messy engine to much lower consumption. (ancient probably factory motor with toasted rings/jugs 7 years ago @30K/year but still gets 35 MPG... It's more than due for a rebuild)

I rearranged the breather setup and started running a very slight vacuum in the case (~ constant 4-5in H20, CIS meter pressure drop, measured at dipstick) and the rings actually seem to be sealing again, and consumption is essentially zero now.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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Small world

Post by FJCamper » Tue May 20, 2008 8:14 pm

Hi Piledriver,

I thought I was the only one using a soft woven steel pot scrubber in my oil filler tower, no kidding.

I did it for the same reason, to better control the oil storm and give the oil something to condense to. It works.

We're either both very cheap, or both very smart.

FJC

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Post by Piledriver » Mon May 26, 2008 7:22 pm

FJCamper:

Given how the 3/4 side rocker box fills up with oil due to windage, have you tried PURGING that side with restricted fresh air feed (1mm or so) and just sucking on the other 2 ports?

That way you'll be more likely to suck that ~quart of oil back into the sump where it belongs. Keeping it actually circulating will also be a win.

The fresh air purge will also help sweep blowby crud out of the engine.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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