CRANKCASE CHECKVALVE ARTICLE


This article is a collection of posts from the Thumper, Race, and Brit Iron lists. I originally responded to a post on the Thumper list and crossposted the answer to the Race list. The topic recently came up on the BI list too. I've attempted to present the posts in chronological order, and make it clear where inclusions from previous posts begin and end. Do be aware that the email addresses of the different writers (all of mine in particular) may no longer be correct.


My Original Post

On Apr 15, 1996 05:55:22, 'scheen@ix.netcom.com (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 excesive throttle or an oiled rear tire! Any advice on this subject would be welcome as well.

Andy

**************

From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Mon, 15 Apr 1996 21:54:47 -0400

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 checkvalve, 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.

Keep us posted,

Michael Moore


From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Tue, 16 Apr 1996 23:56:10 GMT

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. I guess I really should read those books asap, eh.

Stan

**************

Are you sugesting a pvc valve or something like that? I've heard of people adding another one on the clutch side.What the heck is an 'exhaust sucker'? I'm having a hard time picturing how a crank breather is supposed to work.

Andy

**************

How specificly 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.

To start, you must realize that your engine is an air pump, and the piston(s) pump air on both sides of the piston crown. If you have a single, the piston is pumping 6-800cc 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, eh what?

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 mainseal. 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! (Yes Stan, you should have bought the book and read it - then you to could represent yourself as an engine guru by passing along someone else's hard won information).

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 use just 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.

Cheers,

Michael Moore


From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Tue, 16 Apr 1996 20:59:25 -0400

On Apr 16, 1996 17:37:47, 'DACHOWEN@raychem.com' wrote:

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 cant 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 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.

Even though you might be mistaken (hey, even I'M mistaken now and then) at least you are giving some thought to the subject, which is more than many people can say. Of course, if you keep thinking, you might get dangerous, so for the sake of humanity, stop now before your race bike gets any faster.

Cheers,

Michael Moore


From: gunn@watson.ibm.com

Date: Tue, 16 Apr 96 21:24:42 EDT

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 downstroke 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 frequncy at which the crankcase volume is changing. For a single cylinder, 360 deg parallel twin or any V-engine, this is just crank frequncy. 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.


From: MoPerfServ@aol.com

Date: Wed, 17 Apr 1996 00:58:45 -0400

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 which is probably pretty effective at keeping the crankcase negative. This reed valve works just like the PCV valve described.

Doug L


From: SDK1%SECND%NTS@go50.comp.pge.com

Date: Wed, 17 Apr 96 8:20:11 PDT

This is oh so timely. I blew out the clutch cover gasket on my SRX640 race motor Saturday. I'm pretty sure the cause was a pinched or insufficient crankcase ventilation. There was major carbon buildup on the top of the piston and head and the bike was using oil. The valve guide seals were nice and tight though, so I suspect the oil was coming up past the rings. On the SRX, one of the two crankcase vents goes to the oil tank. It seems like this would impede oil returning from the oil cooler but also help oil back into the motor/oil pump suction.

Stu Knigge

WSMC #8, AHRMA #324

sdk1@pge.com


Sent by:michael_moore@usa.pipeline.com (Michael A. Moore)

On Apr 16, 1996 17:37:47, 'DACHOWEN@raychem.com' wrote:

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 cant 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 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.

Even though you might be mistaken (hey, even I'M mistaken now and then) at least you are giving some thought to the subject, which is more than many people can say. Of course, if you keep thinking, you might get dangerous, so for the sake of humanity, stop now before your race bike gets any faster.

Cheers,

Michael Moore


From: "Tom Terry"

Date: Wed, 17 Apr 1996 14:03:49 -0700

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 the 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 with out 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

**************

On Apr 16, 11:48pm, Anthony Pagliaroli wrote:

>> At 11:56 PM 4/16/96 GMT, Michael A. Moore wrote:

> >Various inquiring minds wrote of crankase breathers and check valves....

> My question, 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


From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Wed, 17 Apr 1996 23:21:56 GMT

On Apr 16, 1996 23:48:56, 'Anthony Pagliaroli ' wrote:

>My question, 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 re 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.

Cheers,

Michael Moore


From: Joseph Facer

Date: Wed, 17 Apr 1996 19:07:47 -0700

Subject: Windage Part LXIX

Food for thought:

Yeah, you get a major pumping effect in the crankcase when you gotta single cylinder motor; thats how a two stroke works. And if you gotta 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 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 to our experience is either a carburator or perfume atomizer. They operate on Bernouilles principle; that 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 / JFmcrr@ix.netcom.com

AFM #237


From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Thu, 18 Apr 1996 03:11:28 GMT

On Apr 17, 1996 14:19:52, 'DACHOWEN@raychem.com' wrote:

>>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.

>ever hear of the Edsel? how about the Spruce Goose or the Titanic. what about Clear Pepsi?

>its a bad practice to blindly accept knowledge based on its acceptance by others. question everything.

>david.

**************

David - the Spruce Goose, Titanic and Edsel were not winners. I don't think you can honestly say the same about Cosworth, Ilmor, Renault, etc.

You are right that you should question things, but that shouldn't be done blindly either.

Michael Moore


From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Thu, 18 Apr 1996 00:45:18 GMT

On Apr 17, 1996 11:38:49, 'Matt Ammann ' wrote:

>My Question,

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

>matt

**************

Hello 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 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


From: mtiberio@wash-sunc.bbn.com

Date: Thu, 18 Apr 96 07:00:11 EDT

how refreshing to hear experiences (with documentation too) that are contrary to my experience, and (I might add) against safety rules (at least the exhaust suckers are).

My experience:

1) I never run check valves in my engine breather lines, early in my career when ever 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 labrynth 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 throuhgh this line). I then vent the output of this breather box to a catch can.

4) any motor that ran the output of its engibne 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 muct 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 nnotated with something like " and they retired in the 11th 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 behine me than experiment for a few potential HP and oil every damn thing except my crank.

mtiberio@bbn.com


From: MoPerfServ@aol.com

Date: Sat, 20 Apr 1996 09:45:36 -0400

At the AETC (SuperFlow) conference in '95 one of the attendees was asking about oil pressure loss in heavily evacuated crankcases.

They had noticed a 10 PSI loss, according to there gage.

Neal Williamson, (SuperFlow President) offered, in reply, that the oiling system relief valve was referenced to the crankcase pressure, and the pressure gage might be referenced to atmosphere. Everyone in the audience, who was thinking about the problem, went; DUHHH!!!!

The point is, that, on this particular drag car they were running 10 PSI crankcase vacuum! This was generated by the scavenge pumps.

I did a lot of breather testing on Onan engines, (42ci boxer, flathead) and when a reed valve breather was used, you could see the crankcase pulses shift toward the negative.

The average was negative but there were still positive pulses.

The benefit to all of this was for oil control, (inconclusive) and leakage control.

Any power gains were too small to be measured.

Keep in mind that these generator engines are low comp., 3600 RPM things and not Grumpy Jenkins 9000 RPM Pro-Stockers.

About the negative pressure not letting the piston rise; remember it helps the piston fall, net ZERO.

Allowing the gases to flow in and out freely would generate more work, (two stroke compression releases allow a cylinder to pump in and out freely creating more engine braking, and your engine turns over slower with the throttle open than it does with it closed.)

Grist for thought

Doug L


From: michael_moore@usa.pipeline.com (Michael A. Moore)

Date: Sat, 20 Apr 1996 12:56:25 -0400

On Apr 20, 1996 09:22:18, 'MELOVELO@aol.com' wrote:

>Mike, thanks for the info, I'm wondering if you run your exhaust (sucker) hose through a catch can first, or do you go straight from crankcase to exhaust?

>Mike Gillette

**************

Hello Mike,

On the Laverda I run the hose direct to the exhaust pipe. Keep in mind that I run a high pipe, so there is about 12" of rise from the engine to the pipe, so if any oil gets into the hose (no sign of oil in the 750s breather hose to date) the hose tends to act as the catch tank. I'd have to look, but there may be enough volume in the hose to meet the catchtank requirement, presuming you can't convince tech that the exhaust pipe will hold enough. :-) AFM tech is generally pretty tight on this stuff, and no one complained at the 3 races I made last season. There would be nothing wrong with running into a sealed catch tank first, and it is probably a good idea if your vent hose runs downhill. I don't think that some of the concerns about oil retention I've read are that important. Your crankcase pressure should pump down pretty well after the first few cycles, greatly eliminating the likelihood of oil being forced out the breather (little or no pressure to do so). The amount of additional vacuum from the .125" or so hole in the pipe is deliberately small to limit the chance of oil being pulled from the engine. My tuner has done this setup on a number of bikes (street, RR and MX/XC) and race car engines (D Sports with ZX10) and has never seen oil come out of the pipe.

I think that if you had a case of massive engine pressurization (stuck rings maybe) to where oil is gushing out your vent pipe, you are likely to notice it only after your catch tank has overflowed and put you down anyway.

Depending on the location of the vent pipe, it might be worthwhile to avoid the high, long wheelies. I've heard of people having the oil puke out of a standard bike just because it is all collected around the case breather and is coming out due to gravity when doing a "balance point" block-long wheelie.

Michael Moore


From: Calducati@aol.com

Date: Sat, 20 Apr 1996 19:37:20 -0400

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, bu 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 commbustion 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 sucessful.

Please keep up the good work everyone. All this information just makes me enjoy my duck more.

Thank you.

David 93 900ss


From: GUNN@watson.ibm.com

Date: Sat, 20 Apr 96 22:45:17 EDT

Subject: Effect of crankcase vacuum on piston ring seal.

I find this very surprising, given that the only thing the ring knows about is the DIFFERENCE in pressure between combustion chamber and crankcase. If we take the combustion pressure to be about 1000 psi, and the crankcase vacuum to be 10" Hg = 5 psi, are we to understand that an increase in differential pressure from 1000 to 1005 psi causes a dramatic decrease in blowby? If this is to be believed, there must be measurements of blowby with and without suction that support it.

Ian Gunn.


From: dpaddock@jlc.net (David Paddock)

Date: Sun, 29 Sep 1996 21:38:19 -0500

Re crankcase breathing: the whole idea is to exhaust the space below the piston(s) from the products of combustion which blow by the piston rings. You can appreciate that after a couple of power strokes the pressure in this space will rise if it weren't vented. When you think about it, the pressure would eventually rise to the pressure of the combustion process taking place on the other side of the piston which would cause the engine to stop. (As a practical matter, the engine would be blowing oil out of every place it could and the cases would explode long before the piston pressures equalized.)

The ideal situation is to have a perfect vacuum in the cases so that the piston doesn't have to act as an air compressor in its downstroke. This is lost Work which doesn't appear at the output of the crankshaft. The perfect breather would expel the blowby gas while simutaneously providing this vacuum. A properly-timed breather is intended to do just that. A. Golland in his seminal work, "Goldie", describes the development of the breather system for the Gold Star which started out as a check valve a la PCV-style. The BSA factory found that crankcase vacuum steadily decreased from its maximum at 2000 rpm to zero (atmospheric pressure) at around 6000 rpm because the breather disc element oscillations couldn't keep up with increasing engine speeds. At engine revs greater than 6000 the pressure went positive, causing heavy oil loss from the breather pipe. BSA attempted to improve the situation by relocating the device from the crankcase wall to the timing cover; this helped a bit but even with further modification the matter was still unsatisfactory, and the scheme was discarded in 1954 in favor of a rotary, timed breather which was already in use on the BSA twins. And which is entirely satisfactory.

I don't doubt that opening the engine to the atmosphere will decrease oil loss due to internal pressure build-up but the horsepower wasted sucking in and blowing out the air is inherently unacceptable. If the breather isn't doing its job look for blockage, leaks, and/or improper breather timing.


From: FALCO@CCIT.ARIZONA.EDU

Date: Mon, 30 Sep 1996 08:04:02 -0700 (MST)

On September 29 David Paddock wrote:

> ... The ideal situation is to have a perfect vacuum in the cases so that the piston doesn't have to act as an air compressor in its downstroke. This is lost Work which doesn't appear at the output of the crankshaft. The perfect breather would expel the blowby gas while simutaneously providing this vacuum...

**************

I agree with David. However, an even "more ideal" situation -- albeit an unrealistic one to achieve--would be if it could somehow be arranged that atmos. pressure were in the cases during the upstroke, and David's vacuum were created under the piston only when it reached TDC. Otherwise the piston, which has a surface area of ~9 in^2 in the case of a 500cc, would have to travel the entire length of its stroke against a force of ~130 pounds (9x14.7=132). Although, I hasten to add, since there would be no pumping losses for the air in the crankcase--since there wouldn't be any air in the crankcase--it would get all of this back on the downstroke. However, if our new "more ideal" breather could somehow, without pumping losses, introduce atmos. pressure air into the crankcase for the upstroke, and suck it all out again for the downstroke, there would be another ~30 h.p. available at the crank of our 500cc engine at 5000 rpm. And, all without oil loss, since the crankcase never would be subjected to pressures above atmos.

Although this magic breathing scheme would be impossible to achieve, it does illustrate the number of horsepower rattling around inside the engine unable to get out. Even if a timed breather achieves only 10% of this ideal of atmos. pressure for the trip toward TDC, and vacuum on the way back down, that's a very useful additional 3 h.p. available for free. Venting the crankcase straight to atmosphere is another way to keep the oil in, but it also keeps these extra h.p. in.

Charles Falco


From: "A.K Holmes"

Date: Tue, 1 Oct 96 12:09:06 BST

> ................. Even if a timed breather achieves only 10% of this ideal of atmos. pressure for the trip toward TDC, and vacuum on the way back down, that's a very useful additional 3 h.p. available for free. Venting the crankcase straight to atmosphere is another way to keep the oil in, but it also keeps these extra h.p. in.

> Charles Falco

**************

The timed breather employed on the Gold Star, A10 and early Commandos probably produces the opposite effect to Charles' ideal breather. If it's working properly, there is a partial vacuum on the upward piston stroke, and on the downward stroke this is cancelled out by blow-by, so that there is atmospheric pressure on the downward stroke, or even positive pressure if the breather outlet is too small and the gas can't escape fast enough. I think we've been through all this on brit-iron before, but the ideal practical solution is to use the escaping exhaust gasses to scavenge the crankcase and produce negative crankcase pressure throughout the cycle. However this is difficult to achieve, and involves unsightly plumbing to the exhaust system, so the next best alternative is to do what most classic racers do and fit a very large open breather pipe, and let the piston suck in and blow out air as it pleases. There is no power loss with this system as long as the breather is of a sufficiently large diameter, so that no significant pressure difference builds up between the crankcase and the atmosphere. The problem with many old Brit bikes is that the breather is much too small. On the early Norton Commando, the crankcase has to breath through four tiny 1/8" diameter holes in the camshaft, equivalent in area to one 1/4" hole. At 6,000 rpm the engine has 5 milliseconds to pump 750cc of air through a 1/4" hole. Someone on brit-iron suggested the following experiment. Try sucking and blowing through an empty ball-pen tube, then try the same thing with the tube from the centre of a toilet roll. I think you'll find it's much easier in the second case. There was an article in Classic Bike a few years ago about a vintage racing Gold Star. The only significant power gain they were able to achieve was by modifying the breather to the large open pipe type. If I remember correctly, this gave a 3 bhp increase in peak power, and lowered the piston crown temperature (I don't know how they measured this) enabling a main jet two sizes smaller to be used. I don't know to what extent the power gain came from the improved breathing versus the fact that they nolonger had to run an over-rich mixture to keep the engine cool.

Andy Holmes (pha59@elec.keele.ac.uk)




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