Crankcase Checkvalve Article

Have you seen the hose hanging off Geoff Jones’s R1? It runs between the oil filler cap and the airbox, supposedly relieving the crank case pressure and gaining some power. Geoff, the tinkerer from way back, couldn’t help himself, and was inspired by the following (heavily edited) technical discussion he gleaned from one of the newsgroups on the web. Read on and be enlightened! ..Ed

On Apr 15, 1996 Andy Scheen wrote: I've had a lot of problems with oil leaks (most of them minor) the last couple years and I'm starting to get paranoid. It's not much fun wondering if your tire is sliding because of excessive throttle or an oiled rear tire! Any advice on this subject would be welcome as well.

Hello Andy, Are you running any kind of crankcase check valve on your bike? You may be having your minor leaks caused by excessive internal crankcase pressure. You need to run a spring-loaded check valve, preferably with an exhaust sucker to help pull the crankcase pressure. If you are running an open breather you are losing BHP as well as forcing oil out of the motor from the high internal pressure. You may also be having problems with ring sealing, causing blowby past the rings. Michael Moore

Various inquiring minds wrote:

Hi Michael,

I wonder if you could explain the situation with breathers and loss of power. I've never considered that angle, and I'm not even sure what factors come into play. Stan

How specifically does one install one of these spring loaded check valves and how is the 'exhaust sucker' implemented? Cheers, Randy

There I go again, letting out all the hot engine building secrets. Oh well, now that the cat is out of the bag I guess I'll have to tell you about breathers and suckers.

An engine may be considered as an air pump. The piston(s) pump air on both sides of the piston crown. If you have a single, the piston is pumping 600-800cc of air in and out of the crankcase breather. Pumping air expends energy. If you are pumping air in and out of the engine through a tiny little breather it takes even more energy, because the small hole is more restrictive. If you build up the crankcase pressure by the air not being able to easily escape, it will try to escape wherever it can. This is the cause of many of the small oil leaks - oil isn't seeping out of the engine, it is being forced out. The internal air also tends to have a lot of oil in it, and oil suspended in the air isn't being pumped by the oil pump.

If you install a check valve on the engine breather(s) the air that is pushed out by the falling piston can't be sucked back in through the breather. It has to come in past the oil seals/gaskets/rings. The amount of air that come from these places should be much smaller than the amount the piston can force out past the check valve, resulting in a reduction of air in the cases after a few revolutions of the crank. The car guys found out that if they ran the breather hose from the check valve to the exhaust pipe, the exhaust rushing by the attached hose would help to pull the air out past the check valve, adding an additional extractor effect to the check valve. Exhaust sucker sounds so much more entertaining than an exhaust extractor.

I use a Standard Motor Products AV13 check valve. This is a metal bodied, spring-loaded rubber diaphragm automotive PC valve. The engine side has a hose barb of about 7/8" OD, and the exhaust side has a big heavy nut brazed to the body. You could screw a pipe fitting into the nut, but I cut them off and make a lighter hose barb and weld it to the valve body. This must be done carefully as you don't want to melt the rubber seal. I set the valve in a bowl of water and make a bunch of short TIG welds, allowing everything to cool down between welds. The exhaust pipe attachment is a bit of steel tubing of a size to match your hose barb. Weld this to the head pipe near the manifold/collector and drill a .125" hole through the pipe. Melting of the hose doesn't seem to be a problem. The car folks found that you can overdo things here, as with larger sucker pipes pointing "downstream" in the exhaust they were able to get enough of an extra suck to where they started to pull oil from the engine.

The car guys have reported this type of system developing 4-5 inches of mercury vacuum. When Bill Jenkins wrote about this in 1976 in "The Chevrolet Racing Engine" he said that the only thing stopping them from pulling more vacuum was the lack of a better main seal. Sometimes engines require the seals to be reversed, as the problem is keeping air from coming into the engine, not keeping oil from going out.

I figure that even if this just breaks even on power, it is worth the time to reduce the likelihood of the pesky oil leaks. My TT500 road racer once developed enough internal pressure at Sears Point to where it pushed the countershaft seal out, liberally lubricating the left side of the rear tire going through Turn 1.

My friend Craig also thinks it is worthwhile to eliminate rocker cover breathers, and just use a crankcase breather. He theorizes that air rushing to get out of the rocker cover breather may slow the return of oil from the head to the crankcase. We don't have any proof of this, but it sounds reasonable, and if you run the valve/sucker you will only need one breather outlet anyway. I hope this answered your questions.

Michael: It seems to me that by putting in an exhaust sucker, you are creating something of a vacuum in the cases. As the piston goes from bottom to top, it wants to fill the space under it with air. If it can’t suck air into the cases somehow, it is much harder to lift the piston, and reduces power. Imagine a plunger type air pump. If you push it all the way in, then plug the hole, it is very difficult to pull back out. I would think that a large, open-to-the-air breather would be the best, allowing air to come and go as it pleases with little restrictions. David.

David: I think the amount of vacuum pulled is much less than the atmospheric air pressure the piston compresses and forces out the breather. Rest assured that Cosworth, NASCAR engine builders, etc would not have spent literally hundreds of thousands of dollars on this technology if there wasn't something to it. Mike.

Crankcase pressure has no effect on power output through forces on the underside of the piston(s), as long as it remains constant. It does absorb power if the pressure is higher on the down-stroke than on the upstroke. A completely sealed crankcase would not cause much of this effect, but it would allow blow-by past the rings to raise the average pressure to the point where oil was pouring from every crevice. A SMALL crankcase breather will not allow enough flow between up and down strokes to make much difference, and the pressure will pretty well follow the changes in crankcase volume. A sufficiently LARGE vent (e.g. open crankcase) will allow air to flow so freely that the pressure remains constant, again eliminating losses at the pistons. The worst case is when the vent is neither small enough for the one nor big enough for the other. Quantitatively, this happens when the vent tube and the crankcase volume, acting as a Helmholz resonator, are tuned to the lowest frequency at which the crankcase volume is changing. For a single cylinder, 360 deg parallel twin or any V-engine, this is just crank frequency. For a 180 deg twin, it's twice crank frequency, and for an in-line four, four times. But the effect is much less with more cylinders. I don't think any modern engine has a breather anywhere near big enough for this to happen, so we're always in the range where the crankcase pressure is following the volume. I'm not so sure about old Italian racing engines, some of which had vents shaped like (and almost as big as) a ship's ventilator.

Where crankcase pressure apparently IS important is through its effect on the windage losses caused by all those cranks and rods whirling about, and this seems to be the reason that the F1 car world goes to such trouble with sealed crankcases and vacuum pumps. I forget the pressure they aim for, but 28" of mercury vacuum (= 2" absolute) comes to mind. An exhaust sucker is just a small but helpful step in this direction. Ian Gunn.

The dry-sump Nascar engines use several scavenge pick-up pumps, which, when they aren't picking oil up, are evacuating the crank-case. One other positive side-effect of the lower crank-case pressure is that the extra pressure differential across the oil ring, really helps oil control. The later Ducati crankcase breather is a reed valve that is probably pretty effective at keeping the crankcase negative. This reed valve works just like the PCV valve described. Doug L

That is essentially how it is already done on late Ducs; there is a reed valve (a form of check valve) inside the breather housing (the one that seeps oil on nearly all bikes!) that lets air (fumes) out but not back in. It is plumbed through a labyrinth separator and into the airbox on the downstream side of the air filter (the low pressure side).

Some bikes have timed breathers (early Commandos) that try to do the same thing without check valves: reduce parasitic pumping losses. This would not work on a 4 cylinder (or V twin?) as well since on a Norton both pistons rise and fall together. Tom Terry

What if your crankcase breather is routed to the airbox, as in a production machine? Could you still install a check valve, and receive similar results? Tony Pagliaroli

Yes Tony, it would work just the same as venting the hose to atmosphere. You lose a little extra extraction from the exhaust, but at least you are pumping the cases down some with the check valve.

In 180 degree cranks, multis etc:

While the total volume of the cases remains constant as one or more pistons go up while others go down, the air has to move back and forth between the cylinders. You still have losses from this. I think that when the Yamaha FZ750 came out, an article pointed out that Yamaha had put holes in the crankcase webs between each pair of cylinders to allow the air a more direct path for moving back and forth. It may be one of those things where it doesn't help a great deal, but since it is pretty easy to drill some good sized holes, it certainly couldn't hurt. We even trim away some of the cylinder sleeve between the two cylinders to try and make the hole as close to the pistons as possible. Michael Moore

Food for thought:

Yes, you get a major pumping effect in the crankcase when you have got a single cylinder motor; that’s how a two stroke works. And if you have got a four stroke 360 degree twin with the pistons rising and falling together, the same holds true. But, if you have a 180 degree two cylinder, two-stroke, you have to seal each cylinder from the other, or the intake charge displaced by the dropping piston is moved into the area evacuated by the rising piston. And if you have an opposed Boxer twin, with the pistons moving out and in together, you get a MAJOR pumping effect. Then again, if you have a four-cylinder motor, with two pistons rising while two pistons are falling, the pistons pumping action cancel each other out.

So, unless you have a special case, what we are talking about is not building pressure but moving air back and forth. A possibly significant effect, especially if you are spending upward of a million dollars a year for between 50 to 4,000 horsepower (you do the division). But it is not the missing piece to the puzzle. Then again, if you learn how to hold it wide open a half a second later before each time you brake, learn how to brake harder once you put the squeeze on, and dial it on a second sooner once you're on the way out, you WILL notice the difference.

But if you do want to play around with this, and why not, cuz if your racing, you have time and money on your hands, what is being called an "exhaust sucker" is properly called an eductor. The most basic eductor in our experience is either a carburetor or perfume atomizer. They operate on Bernouille’s principle; that is, as the speed of a fluid (gas) increases, the pressure associated with the flow drops.

So, when you cut into your exhaust system to tie in your eductor port, try to do so in a straight run, favoring a location closer to a downstream bend and farther away from an upstream bend. Avoid locations where the cross section increases, these locations trade velocity for pressure and are inimical to eductors. If you have to locate your port in a bend, locate it in the throat and avoid the heel. Avoid locating the tube so that it protrudes into the exaust flow. And finally, cut the tube that you are using for your port tie-in square, and locate it perpendicular to the long axis of your exhaust pipe and at the side or top of the pipe, not the bottom. Joseph Facer

Typically the crankcase breather is routed into the intake tract. Isn't this a "low pressure" area as well, and therefore generate some "suck". Matt

Might. I think the exhaust pipe probably has a higher speed in the gas flow, creating more vacuum in the line. The other thing to consider is that you are contaminating your intake charge with oil vapors when routing the crankcase breather into the inlet. This is probably not the best of all possible setups for performance, although it does reduce emissions. Michael Moore

My experience:

1) I never run check valves in my engine breather lines, early in my career whenever I did I had oil pumping problems. My current theory is I want the motor to be able to suck back the oil it has just pushed out (at least half the time).

2) I run only vented valve covers, and because I run no check valves, I know half the time my motor is pulling oil down from under the valve cover area back to the sump.

3) I run my valve cover vents to a labyrinth breather box which allows the oil mist to condense and has a oil return line to the sump (must return to the sump below oil level or the motor will breath through this line). I then vent the output of this breather box to a catch can.

4) Any motor that ran the output of its engine breather (or breather box as in my case) to a negative pressure venturi port on their exhaust system would be in violation of rules stating such hoses must be vented to a catch can. Also I'm sure your competitors behind you will just love being pelted with the fine atomized oil mist coming from your exhaust (think its burning? guess again).

5) I have seen pictures of Guzzi endurance racers from the 70's that had exhaust driven crankcase suckers. These pictures are usually annotated with something like "and they retired in the 11^th hour with oil problems". While I may be losing HP with my setup, I would rather finish a race with a full sump, a dry bike, dry catch can and a dry track behind me than experiment for a few potential HP and oil every damn thing except my crank. [email protected]

When dealing with exhaust system "suckers" you have to understand why this was done in the first place. The negative pressure (i.e. vacuum) created in the crankcase of engine was, and is, sought after not to reduce oil leaks or oil suspended in the crankcase air, but to help provided a better piston ring to cylinder wall seal. Without the vacuum created in the crankcase when the piston reaches the top and bottom of its stroke the ring is cocked in the piston landing, which allows excessive combustion chamber pressure leakage(loss of power). The vacuum in the crankcase helps prevent this problem, which in turn means that the piston ring is sealing better and there is less leakage past the rings at top dead center. Once this was realized, the engine builders then discovered they could run a piston ring with less tension, which reduced friction and helped to make more horsepower. The reduction in friction is not even worth mentioning on a motorcycle engine. The decrease in leakage past the piston ring is significant. The position of the tube placement in the exhaust system is critical. The size of the hose from the crankcase to the exhaust system is critical. The type of check valve and tension of the check valve spring is critical. There is a lot of R+D that has to be done to make such a system successful. Calducati