How Does the Twin Cam Engine Venting System Work?

[Ohio HD]

The location of where venting is tapped matters. There are many places that have a lot of oil windage that are not suitable to use to relieve pressure without guarding or other means of oil separation. These conditions are prevalent all the Harley big twins even on motors that have no issues.

We're assuming valving is connected using the OEM flow location. What I plan to do is not how the OEM system was designed. Hence at the beginning of the post I show the OEM system function and flow path.

[Don D]

I read all of that but in my opinion there is a lot of oil splash out of the top of the covers, your proposed connection point , right? This is probably why sachs drills the heads, he can comment on that.

[Ohio HD]

I read all of that but in my opinion there is a lot of oil splash out of the top of the covers, your proposed connection point , right? This is probably why sachs drills the heads, he can comment on that.

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

Pretty sure that John and I are on the same page as far as drilling the connection into the rocker cover.
This also recommended to me by Jim Libonati. Both who have been involved with racing HD motors and building them. Both for over 40 years.


My purpose is not to better the OEM system for oil control, rather to insure all pressure in the rocker box is relived. This is why I said "We're assuming valving is connected using the OEM flow location" for the discussion of valve types.

The motor I'm building is not a highway cruiser. I expect to have less than perfect "cleanliness" when running this motor hard. 

[Don D]

All makes sense, appreciate tye clarification.

[60Gunner]

"You get oil expelling when the head can't drain fast enough, or is being over loaded by internal motor issues or components."

The location of where venting is tapped matters. There are many places that have a lot of oil windage that are not suitable to use to relieve pressure without guarding or other means of oil separation. These conditions are prevalent all the Harley big twins even on motors that have no issues.

This is right on what I was getting it. It DOES matter.

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.       

Not entirely true either. My DT pump and plate has adjustable oil pressure as I'm sure you know and it's set up currently to 42psi or roughly 10psi higher than stock. I don't get a drop out of my breathers partly because of the improved crankcase vacuum of the reed valves and no oil pooling in the heads or rocker boxes but mostly because of the dual port scavenging of the pump and a plate which also maintains a constant vacuum on those scavenge ports.
Another thing I noticed the other day changing oil. It took an extra 6+oz to get to the full mark than it use to because I drained 6+oz more. Oil that is no longer being left in the sump for the crank to swim in. Yes I fill my oil to the full mark now.
And a benefit of improved pressure is setting my idle to 920rpm when engine temps reach 250 to he!p with cooling without fear of starving the top end of oil.