How Does the Twin Cam Engine Venting System Work?

[Ohio HD]

It's a super simple system. Just not always super good at what it does. oil level too high, it effects the breather output by mixing oil that's being blown around the rocker box assembly, then making it to your catch can, or TB.

Some motors have a very clean vent system. Mostly that's due to that motor having an extremely good ring seal.

The attached images are just a walk through the system, as to how the crank case introduces the pressure to the heads, and then how the breather system evacuates the crank, now head pressure.



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[Ohio HD]

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[billbuilds]

     Good explanation and photos. I was just check out the breather layout in the SM the other day. Almost installed the umbrella valves incorrectly using the stamped steel units. No, they don't go the way they come packaged.       
     Did those Vulvan rocker arm supports come chamfered like that or is that your work? They're very pleasant to the eye.

[Ohio HD]

No, not my doing. They make them as you see them. All of the Vulcan parts that I've used (parts they made) are very high quality.

[Wookie3011]

Thanks Ohio HD, I am still trying to memorize oil flow and paths as well as pressure paths in the TwinCam engine. If I understand how it works and can visualize it, then working on and trouble shooting is easier. It really helps when it's all laid out like this. The shop manual has it all in several pages in a way that makes it hard to visualize. Vulcan has outstanding performance parts. Super nice guy to. 

[FLDavetrain]

So with all installed correctly and still copious amounts of oil pouring out once at full temp implies rocker boxes are overwhelmed w oil that's not draining

[Don D]

Lifters, oil pressure, and volume play roles in how much oil is delivered to the top. Drain back holes in both the rocker supports and heads are in the mix getting this oil back to the sump / tank. Ring seal, if compromised, adds to crankcase pressure and if excessive interferes with the oil separation process which is done under the umbrella valves.

[Thermodyne]

The breather system works reasonably well, so long as the engine is mechanically sound.  There is actually a slight vacuum on the crankcase when all is working well.

But if you lose ring seal and the blow by increases, the increased volume of gases being vented will carry more oil out too.  And the engine is going to lose ring seal over time, no way to stop that.

Another issue that increases oil carry over is raising the oil pressure.  Higher oil pressures increase the amount of oil in the upper and lower areas of the engine. And that can easily overwork the simple little oil separator set up in the breather as well as exceed the capacity of the drains in the rocker supports.

If it only slobbers out on the big slab, it's usually fixable by drilling out the drain holes.  But when they get to slobbering all the time, it usually requires more work.       

[60Gunner]

Thing is,it's a compounding issue. The stock setup is marginally good enough...for a stock bike.
There's way better alternatives. Crankcase breathing has never been a strong suit for Harleys.
Use a good external reed valve. They close quicker and don't allow ANY air back in that builds pressure. They also don't dry out and fail to seal at all.
I don't get a drop of oil out of mine. My umbrella valves are removed. The stamped housings are still there, empty, to aid in oil separation.

Drill the return holes under them to 1/8".

[Ohio HD]

This is what comes from my catch can on my 124" TC. I'm running a stone stock breather system, and see no oil coming from the breathers. I also have extremely good ring seal, which has more to do with excessive oil expulsion than the breather mechanism. Put more oil into the rocker box than the oil drains can keep up with, you'll see oil expulsion no matter what you use as a breather valve.

This is after a weekend of hard riding, some moisture from condensation, and a minuscule amount of oil trace from oil mist that is generated by high RPM running.

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The breathers in my current 124", stock late model OEM.

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[Ohio HD]

This is the breather system that I'll use in the 124" TC I'm building now. The OEM style assembly removed, and will use an S&S 50-8122 breather valve connected to the upper rocker box, then fed to the catch can. Of course one valve on each head.

This motor will see a lot of high RPM use. This setup was recommended to me by the guy that just did my heads. He knows a lot more about high performance HD's than I ever will. Inexpensive and works.

The Vulcan breathers I have will go into the old 124" motor.


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[cheech]

Are you plugging what you call the "pressure exit hole" in the head or rocker bridge or just at the OEM breather holes where the air cleaner mounts?

[Ohio HD]

Actually this AM I decided to use parts from the early TC breather, the cast aluminum parts. Strip the valve and foam out. Drill and tap the drain hole with a 1/4-18 NPT tap and use a flush plug to block off the exit port all together. I'll use green Loctite to insure it never movies.

But I can't believe I don't have a 1/4-18 NPT tap. I have to get one, and will pick up the flush plugs in aluminum just to keep the materials the same.



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[jsachs1]

This is the breather system that I'll use in the 124" TC I'm building now. The OEM style assembly removed, and will use an S&S 50-8122 breather valve connected to the upper rocker box, then fed to the catch can. Of course one valve on each head.

This motor will see a lot of high RPM use. This setup was recommended to me by the guy that just did my heads. He knows a lot more about high performance HD's than I ever will. Inexpensive and works.

The Vulcan breathers I have will go into the old 124" motor.


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I started using a set up like that in 2000 on some of my customer builds. I mounted a rt. angle adaptor over the center of the intake rocker arm. I put a PVC valve into the adaptor, a length of hose and let it work. There is a very
small amount of power gained. I run this system on my Buell.
John

[Ohio HD]

I started using a set up like that in 2000 on some of my customer builds. I mounted a rt. angle adaptor over the center of the intake rocker arm. I put a PVC valve into the adaptor, a length of hose and let it work. There is a very
small amount of power gained. I run this system on my Buell.
John

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    Jim said the same, a little insurance that you're relieving the pressure.

[turboprop]

Actually this AM I decided to use parts from the early TC breather, the cast aluminum parts. Strip the valve and foam out. Drill and tap the drain hole with a 1/4-18 NPT tap and use a flush plug to block off the exit port all together. I'll use green Loctite to insure it never movies.

But I can't believe I don't have a 1/4-18 NPT tap. I have to get one, and will pick up the flush plugs in aluminum just to keep the materials the same.



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I think you plan to run the AC from HPI. Have you considered hunting it with button head bolts (not center drilled) and a piece of aluminum round stock instead of the vented standoffs from HPI? 

[Ohio HD]

That's correct, The V2 is still the plan. I have the original internal and the external breather standoffs. I'm going to use the standoffs for internal vent to TB and those bolts. They're hollow, but by blocking the breather inside the rocker box, I don't have to deal with sludge coming out the backing plate bolts when I have the need to remove them.

That's what made me decide to go that direction this AM, seal it from the inside. Just to eliminate the need to clean and dry the threads in the head when I remove the air breather. Then I can apply blue Loctite and put it back together without getting the threaded hole dried out so the Loctite adheres.



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[kd]

I started using a set up like that in 2000 on some of my customer builds. I mounted a rt. angle adaptor over the center of the intake rocker arm. I put a PVC valve into the adaptor, a length of hose and let it work. There is a very
small amount of power gained. I run this system on my Buell.
John

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    Jim said the same, a little insurance that you're relieving the pressure.

If you are using an automotive PCV valve test it first.  For the most part, they are not a positive seal and will actually breath both ways. (one way is more like a leak)  If you want to hold negative crankcase pressure it may not perform the way you want.

[Ohio HD]

If you are using an automotive PCV valve test it first.  For the most part, they are not a positive seal and will actually breath both ways. (one way is more like a leak)  If you want to hold negative crankcase pressure it may not perform the way you want.

S&S one way check valves.


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[kd]

Yeah, I just dropped into the thread and it looked like PCV valves were on the menu too.  rbabos did some testing a few years and ran a bit of a thread.  They were not all the same.  I am sure the S&S units are called one way check valves for a reason.

[Ohio HD]

Yep, you can only get flow through the white side to the black side. The other direction they flat close.

[Ohio HD]

A really positive point to using these valves, if one does get gunked up for some reason. You can change it in five minutes. The OEM valves you have to open the rocker box, buy gaskets, etc....  just to look at it.

[rigidthumper]

Are you going to drill & tap the head, or the rockerbox?

[turboprop]

If you are using an automotive PCV valve test it first.  For the most part, they are not a positive seal and will actually breath both ways. (one way is more like a leak)  If you want to hold negative crankcase pressure it may not perform the way you want.

S&S one way check valves.


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How do you intend to route the lines and what are they going to?

Have you seen the video by Tom Reiser on using the oil tank as a catch can and then venting the tank? He wants to drill the trans case (M8 application) for the oil return. There are other ways to do this. For example on an earlier TC the trans case has a plugged hole on top right behind the engine. The vent line could go into that and then drill/tap the oil spout for a vent line. I have been thinking of leveraging this against the Torque-A-Hoe bike and eliminating the external catch can. Only drawback I see is not being able to monitor what is coming out of the vents, but it should result in a much cleaner exterior, specially after extended high speed touring.

[Ohio HD]

If you are using an automotive PCV valve test it first.  For the most part, they are not a positive seal and will actually breath both ways. (one way is more like a leak)  If you want to hold negative crankcase pressure it may not perform the way you want.

S&S one way check valves.


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How do you intend to route the lines and what are they going to?

Have you seen the video by Tom Reiser on using the oil tank as a catch can and then venting the tank? He wants to drill the trans case (M8 application) for the oil return. There are other ways to do this. For example on an earlier TC the trans case has a plugged hole on top right behind the engine. The vent line could go into that and then drill/tap the oil spout for a vent line. I have been thinking of leveraging this against the Torque-A-Hoe bike and eliminating the external catch can. Only drawback I see is not being able to monitor what is coming out of the vents, but it should result in a much cleaner exterior, specially after extended high speed touring.

That concept of taking the discharge from the heads to essentially the oil pan via the transmission case has me thinking. It would be a nice clean system regarding making it a closed system as it really should be. I agree about being in the blind as far as being able to see what happens at times. Currently I planned to go from heads to catch can.

I had always planned to drill and tap the oil inlet opening at the transmission oil filler neck while everything is apart. That way if I did decide to vent from the transmission case, I just remove a cap and connect a hose.

I have a few questions about what I can do with and around the transmission case. To the point I bought a damaged six speed case to learn from. I should have that on Tuesday next week. Pinwall Cycle is in Ohio, and ships pretty quick.

Updated: they already shipped yesterday, and it should be here today.

I'm going to look for the TR video, thanks for the heads up.