E: Engine Not another breather thread!

Martyn Goodwin

Martyn Goodwin

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Non-VOC Member
davidd said:
Rex Bunn did a lot of work on motorcycle engine breathing. I do not have his book, but I think some of his articles are on line. I think Martyn has some experience with his breathers.

There appears to be a huge focus on breathers in the VOC. I just assume that 11 out of 10 breather problems are due to poor rings. It is so much easier to add breathers than fix the rings.

I often thought that an evacuation pump would be a better addition to the timing chest than an oil pump for most Vincent owners. The good pumps remove air and also allow an air bleeder to pull air in from an outside source if the crankcase pressure goes to far negative.

Alcohol and nitro engines have much less vacuum to worry about. The volume of fuel is very high and usually liquid when it hits the rings. The rings hydroplane over the fuel on the cylinder wall. This is why there is so much oil contamination with alcohol and nitro. Many dragsters use two oil tanks so they do not circulate the contaminated oil.

Part of the reason low crankcase pressures in racing are desirable is that you can then lower the ring pressures. I think that piston and ring friction is still 75% of the friction in the engine and that reductions of friction are good at making BHP.

David
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I had a "Bunn" breather fitted to my Comet t one stage - BTW they seem to be no longer available. It was during my early stages of ownership and learning when I was trying all sorts of breathers in an effort to reduce oil leaks - and then I realised that the secret was to eliminate the blow by with a round bore and snug rings - another story.

The Bunn breather is no more than a lightweight plastic PCV where the "valve" is a lightweight plastic "flapper". Easily reproduced by using a conventional PCV and mounting it horizontal so gravity has no effect on the movement of the "ball" inside it. Happy to provide photo's PM me if you want some.
 
Cyborg

Cyborg

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VOC Member
stu spalding said:
I remember reading somewhere that NASCAR engines are fitted with pumps to keep crankcase pressure down. Is there anything in this and if so, why? Cheers, Stu.
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My understanding from some reading I did a while back is that the drag racers started doing it simply to keep gaskets and seals from being blown out. They subsequently realized they were also achieving a horsepower gain along with it.... then realized they could run lower tension rings (maybe just oil rings, I don't recall for sure) and that gained them a little more horsepower... and the news started to spread.
 
H

highbury731

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VOC Member
The best example I know, of crank case pressure control being important for oil control and power is a sohc Norton single. They rely on rubber pads pressing on the top and bottom surface of the rockers to restrict oil flow out of the cam box. The crank shaft has a small diameter hole through the mainshaft which acts as a timed crank case breather. The motors pump oil out, especially from the cam box. Fastidious owners set the rubber 'seals' for heavy pressure to try and keep the oil inside the engine - and that costs power.
A more beneficial approach is to fit a flap valve to a much larger crank case vent, so that it runs under proper negative pressure. A smaller secondary flap valve between crank case and lower bevel chamber will enable the lower bevel chamber to run under negative pressure. That helps the oil drain from the cam box (they suffer from over-filling), and greatly reduces the flood of oil all over the engine and the rider's legs. And gives a signifcant power boost, especially if the owner has been using high pressure on those silly rubber cam box pads.....
Paul
 
Cyborg

Cyborg

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VOC Member
highbury731 said:
The best example I know, of crank case pressure control being important for oil control and power is a sohc Norton single. They rely on rubber pads pressing on the top and bottom surface of the rockers to restrict oil flow out of the cam box. The crank shaft has a small diameter hole through the mainshaft which acts as a timed crank case breather. The motors pump oil out, especially from the cam box. Fastidious owners set the rubber 'seals' for heavy pressure to try and keep the oil inside the engine - and that costs power.
A more beneficial approach is to fit a flap valve to a much larger crank case vent, so that it runs under proper negative pressure. A smaller secondary flap valve between crank case and lower bevel chamber will enable the lower bevel chamber to run under negative pressure. That helps the oil drain from the cam box (they suffer from over-filling), and greatly reduces the flood of oil all over the engine and the rider's legs. And gives a signifcant power boost, especially if the owner has been using high pressure on those silly rubber cam box pads.....
Paul
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Thank you for sharing that.

26025
 
N

Nigel Spaxman

Well Known and Active Forum User
VOC Member
I am happy with the operation of my timed breather. The only think it does dribble a bit of emulsified oil on the ground after a ride. Also I think a bit of mist comes out during a long ride. I have a hose going to the back axle to keep this mess off most of the bike. That seems to be the approach most people with the timed breather use. One thing I have heard of that I might try is running the breather into the exhaust pipe just ahead of the muffler. Has anyone here tried that? If the tube going into the pipe is made the right way it might cause a vacuum. Also the exhaust pipe is near the breather so it could be hooked up with a very short tube. The only problem would be if there was pressure in this line sometimes.
 
Simon Dinsdale

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Moderator
Nigel Spaxman said:
I am happy with the operation of my timed breather. The only think it does dribble a bit of emulsified oil on the ground after a ride. Also I think a bit of mist comes out during a long ride. I have a hose going to the back axle to keep this mess off most of the bike. That seems to be the approach most people with the timed breather use. One thing I have heard of that I might try is running the breather into the exhaust pipe just ahead of the muffler. Has anyone here tried that? If the tube going into the pipe is made the right way it might cause a vacuum. Also the exhaust pipe is near the breather so it could be hooked up with a very short tube. The only problem would be if there was pressure in this line sometimes.
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Paul Ennis tried running the timed breather pipe into the exhaust. He removed it fairly quickly and went back to std as he said the oil consumption shot up. Could the exhaust cause some sort of possible suction effect on the breather pipe?
 
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