Venting the head

tall951guy · Jun 26, 2014

Hi All:

I met a guy at an all British bike swap meet that mentioned venting the head, and said it was a good idea. He drilled and tapped the inlet rocker cover to accomplish this. If venting the head is a good idea, I think that I would rather drill out the center head steady hole and use a banjo bolt and oil line. It would be more hidden and less cluttered looking this way.

So! Is venting the head a good idea? Or, a waste of time?

comnoz · Jun 26, 2014

tall951guy said:
Hi All:

I met a guy at an all British bike swap meet that mentioned venting the head, and said it was a good idea. He drilled and tapped the inlet rocker cover to accomplish this. If venting the head is a good idea, I think that I would rather drill out the center head steady hole and use a banjo bolt and oil line. It would be more hidden and less cluttered looking this way.

So! Is venting the head a good idea? Or, a waste of time?

There is no blowby into the head so if you vent the head, any blowby that will come out that vent will have traveled up the pushrod tubes or oil drainback passage from the crankcase.

This does nothing to help let the oil drainback from the head as it should.

I would not recommend venting from the head. Jim

Hobot · Jun 26, 2014

Peel's exhaust eductor/extractor should suck enough over blow by to allow fresh air vent through restrictive filter. This is common feature if sump pumped out enough otherwise as comnoz says, like blowing up narrow water drain while rain piles water up on roof seams. Hm, maybe best to control Peel from sucking crank seals inside out by a head vent valve rather than exit, hm w/o restriction at exit side may help the sucker work at even idle rpm so no back wash valve needed, thanx.

ML · Jun 26, 2014

comnoz said:
There is no blowby into the head so if you vent the head, any blowby that will come out that vent will have traveled up the pushrod tubes or oil drainback passage from the crankcase.

This does nothing to help let the oil drainback from the head as it should.

I would not recommend venting from the head. Jim

Its kind of hard to argue with Jim's knowledge, but I do run a inlet cap vent. I experimented with clear hose at first to see if and what is happening. There is a very evident movement of gas indicating the rocker cavity is subject to pressure fluctuations. At the 6mm outlet some 800mm distance, the air flow can be witnessed against a strip of thin cassette tape. I understand on some 750 models, I think with the rear case breather at sustained high RPM, that venting the head caused oil starvation, but those circumstances would need to be investigated more thoroughly. On my 850, removing exhaust covers after a high speed run showed plenty of oil in the head as per normal.

Mick

comnoz · Jun 26, 2014

ML said:
comnoz said:

There is no blowby into the head so if you vent the head, any blowby that will come out that vent will have traveled up the pushrod tubes or oil drainback passage from the crankcase.

This does nothing to help let the oil drainback from the head as it should.

I would not recommend venting from the head. Jim

Click to expand...

Its kind of hard to argue with Jim's knowledge, but I do run a inlet cap vent. I experimented with clear hose at first to see if and what is happening. There is a very evident movement of gas indicating the rocker cavity is subject to pressure fluctuations. At the 6mm outlet some 800mm distance, the air flow can be witnessed against a strip of thin cassette tape. I understand on some 750 models, I think with the rear case breather at sustained high RPM, that venting the head caused oil starvation, but those circumstances would need to be investigated more thoroughly. On my 850, removing exhaust covers after a high speed run showed plenty of oil in the head as per normal.

Mick

There will be pressure pulses in the head when you put a hose on the valve cover. The pressure pulses do not originate in the head but travel up the pushrod tubes from the crankcase.
When I experimented with breather vents on the head I ended up with too much oil staying in the head as it could not drain down when the air was going up. Jim

Hobot · Jun 27, 2014

Don't forget the long torturous tiny uphill R side corner intake box path with TS case pressure venting up as oil is trying to go down, might tend to rise oil level and turbulence and smoke? Yet mick gets away with it as another engine breath monitoring feature so I'm confused. For a thought experiment - if the incompressible head oil flow, under dozens of PSI, was a greater volume than ring blow-by compressible gas volume, what would happen if head was only vent?

comnoz · Jun 27, 2014

hobot said:
Don't forget the long torturous tiny uphill R side corner intake box path with TS case pressure venting up as oil is trying to go down, might tend to rise oil level and turbulence and smoke? Yet mick gets away with it as another engine breath monitoring feature so I'm confused. For a thought experiment - if the incompressible head oil flow, under dozens of PSI, was a greater volume than ring blow-by compressible gas volume, what would happen if head was only vent?

I suspect it would be worse.
When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim

Hobot · Jun 27, 2014

Alrighty Jim thanks for another venting smoke fest for the rest of rest us to enjoy the easy way. I'm a bit concerned Peel's OIF may blow up or implode as only vent out right now is though the megaphone. Hm Peel will need more needle meters to find out, so I thankgoodness Ms Peel has a glass dial fetich too.

Nortiboy · Jun 27, 2014

This Atlas has 2 head vents and 2 crankcase vents going to a catch can. Dominated the track in the 80's. Opening everything to the atmosphere was the old way.

http://i1136.photobucket.com/albums/n48 ... 122722.jpg

Hobot · Jun 27, 2014

The Brit Iron and NOC bunch said the early Italian engines had large case vents in similar shape as ship deck funnels to get least pumping drag huffing and puffing. Hm what make the best smoke, single or multi grade, pumped up crude or synthetic?

Matchless · Jun 27, 2014

Comnoz wrote.......
I suspect it would be worse.
When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim

I have an early 1962 650 SS which is completely original. It has a banjo type breather fitted on the stud to the inlet rocker cover. This vents into the oil tank via a small bore pipe. On later models I think this was deleted. The only other breather is the timed one from the cam. This is vented to atmosphere (later bikes had a breather tower on the oil tank) I wonder why the factory fitted this rocker box vent?

xbacksideslider · Jun 27, 2014

comnoz said:
tall951guy said:

Hi All:

I met a guy at an all British bike swap meet that mentioned venting the head, and said it was a good idea. He drilled and tapped the inlet rocker cover to accomplish this. If venting the head is a good idea, I think that I would rather drill out the center head steady hole and use a banjo bolt and oil line. It would be more hidden and less cluttered looking this way.

So! Is venting the head a good idea? Or, a waste of time?

Click to expand...

There is no blowby into the head so if you vent the head, any blowby that will come out that vent will have traveled up the pushrod tubes or oil drainback passage from the crankcase.

This does nothing to help let the oil drainback from the head as it should.

I would not recommend venting from the head. Jim

Agreed, and I would add that you want a slight vacuum in the crank case to help control oil at the rings.
The old timed breather or a reed valve that induces a vacuum in the crank case is the way to go.

comnoz · Jun 27, 2014

Matchless said:
Comnoz wrote.......
I suspect it would be worse.
When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim

I have an early 1962 650 SS which is completely original. It has a banjo type breather fitted on the stud to the inlet rocker cover. This vents into the oil tank via a small bore pipe. On later models I think this was deleted. The only other breather is the timed one from the cam. This is vented to atmosphere (later bikes had a breather tower on the oil tank) I wonder why the factory fitted this rocker box vent?

I have seen that before. It was not used for very long -maybe it didn't work so well. Also the early engines needed all the help they could get in maintaining enough oil in the head.
Of course the 650 and smaller engines didn't have near the crankcase breathing problems that the later, bigger engines had. Jim

Torontonian · Jun 27, 2014

Not necessary. What is very necessary is a crankcase Reed Valve arrangement. Plus the head is "over-oiled". XS 650 one yes.

Matchless · Jun 27, 2014

Maybe the factory discontinued the head breather the same time the started to pressure feed the rockers. Before this the rocker feed was from the return, & a bit more hit & miss.

Triton Thrasher · Jun 29, 2014

tall951guy said:
a guy at an all British bike swap meet...........said it was a good idea.

I'd need better reason than that.

rx7171 · Jun 29, 2014

comnoz said:
hobot said:

Don't forget the long torturous tiny uphill R side corner intake box path with TS case pressure venting up as oil is trying to go down, might tend to rise oil level and turbulence and smoke? Yet mick gets away with it as another engine breath monitoring feature so I'm confused. For a thought experiment - if the incompressible head oil flow, under dozens of PSI, was a greater volume than ring blow-by compressible gas volume, what would happen if head was only vent?

Click to expand...

I suspect it would be worse.
When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim

I'm thinking the pressure pulses coming up the pushrod tubes provides pressure in the head that helps to push the oil fed to the rockers down the tiny drain hole back into the crank case.

comnoz · Jun 29, 2014

rx7171 said:
comnoz said:

hobot said:

Don't forget the long torturous tiny uphill R side corner intake box path with TS case pressure venting up as oil is trying to go down, might tend to rise oil level and turbulence and smoke? Yet mick gets away with it as another engine breath monitoring feature so I'm confused. For a thought experiment - if the incompressible head oil flow, under dozens of PSI, was a greater volume than ring blow-by compressible gas volume, what would happen if head was only vent?

Click to expand...

I suspect it would be worse.
When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim

Click to expand...

I'm thinking the pressure pulses coming up the pushrod tubes provides pressure in the head that helps to push the oil fed to the rockers down the tiny drain hole back into the crank case.

But the pressure pulses would be applied to the pushrod tubes and the tiny drainback holes equally. ...

rx7171 · Jun 29, 2014

I suspect it would be worse.
When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim[/quote]

I'm thinking the pressure pulses coming up the pushrod tubes provides pressure in the head that helps to push the oil fed to the rockers down the tiny drain hole back into the crank case.[/quote]

But the pressure pulses would be applied to the pushrod tubes and the tiny drainback holes equally. ...[/quote]

That sounds logical, however how do we understand what happened here. Beats me.

>> When I tried it, I was running the original timed camshaft breather on a stock engine and I added a vent to the rocker cover.
When running highway speed the head would flood with oil after a few miles and leave a real smoke trail from the oil going past the guides. After I removed it the problem went away. Jim<<

Venting the head

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