Crankcase vent vacuum setup removal = bad

[Bill in OKC]

I like your diagram.  Don't forget that Ducati (and others) sell a few high performance bikes with ram air pressurized air boxes - not the Monster though AFAIK.  The pressure in the airbox will increase as the  bike's speed increases and so presumably when you need crankcase evacuation the most, you would have just the opposite.  I'm pretty sure Ducati engineers must have taken this into account when connecting the crank breather to the airbox if it meant gaining or losing a pony or two.

[chris1044]

I like your diagram.  Don't forget that Ducati (and others) sell a few high performance bikes with ram air pressurized air boxes - not the Monster though AFAIK.  The pressure in the airbox will increase as the  bike's speed increases and so presumably when you need crankcase evacuation the most, you would have just the opposite.  I'm pretty sure Ducati engineers must have taken this into account when connecting the crank breather to the airbox if it meant gaining or losing a pony or two.

Exactly.  As you turn the throttle, intake vacuum decreases.  So the notion that this system works entirely off vacuum is false when talking about higher RPM's.  However, as there clealy is air flowing through the air box during any engine RPM, and increasing as RPM increases, there is a venturi effect that would still pull vapors from the storage tank into the air box.

To accurately make a call on whether or not you're actually gaining any benifity from having this system on or off the bike, you'd need to know a few things.  Pressure in the crank case, pressure in the hose going to the air box (remember, vacuum is simply pressure less than atmospheric), and the calibration force of the reed valve.  As already noted, the pressure in the crank case is going to be consistantly changing;  it increases as the piston moves down the cylinder, and decreases as it moves up.  Thus, when RPM increases and the frequency of this pressure change becomes greater, the crankcase pressure would essentially "stabilize."  In any case, I'd almost bet my bike on the fact that the spring tension which the reed valve opens at is far less than the pressure needed to blow out the engine seals (when not excessively worn). 


IMO, there have been enough people who've piled on plenty of miles with the system removed that if you want to go with that route, then do just that.  But, for now, I'll keep mine hooked up.   If for some reason something on the system failed (say the tank cracked as I've seen on a few monsters I looked at), it'd probably come off as I'm sure the part is quite expensive

[ducatiz]

 If for some reason something on the system failed (say the tank cracked as I've seen on a few monsters I looked at), it'd probably come off as I'm sure the part is quite expensive

as the system is part of the emissions system, it's warranted under the 5-year emissions warranty..  it would be replaced if it cracked due to manufacture or design..

[erik822]

So after 5 pages of comments, the gist can be summed up thusly:

1. Leave the crankcase vent box thingy on your bike.
2. Who cares, take it off.

And the argument for each is that something in theory will happen, even though no one seems to have exact stats that show how much of something. They have numbers, charts and figures that dance around something and prove part of something. But no one seems to be able to definitively prove:

That taking the box off gains or loses horsepower or reliability.

It does, however, mess with the EPA-approved emissions controls. And since everyone leaves the charcoal canister on their bike, that would be reason enough to keep it on there. Oh wait, no one leaves that on there...

[battlecry]

Precisely...

[Howie]

Manifold vacuum, now called absolute pressure is highest when the throttle is closed and reduces as the throttle is opened, and, in some engines will have no vacuum at wide open throttle.

Ported vacuum is at the throttle plate and is 0"Hg when the throttle is closed and increases as the throttle is opened.

Venturi vacuum is the negative pressure (vacuum) created within the venturi (restriction) and increases with the speed of airflow.  Negative pressure in your air box is created by venturi vacuum.

[Langanobob]

Venturi vacuum is the negative pressure (vacuum) created within the venturi (restriction) and increases with the speed of airflow.  Negative pressure in your air box is created by venturi vacuum.

Howie, seems to me that air box vacuum is created by the pressure drop across the filter and pressure drop across the airbox inlet nozzle.  I thought that venturi vacuum is localized at the venturi?

Also, (aaargh) what about the Helmholtz (?) resonance that is supposed to create a positive airbox pressure due to reflected pressure waves from the intake valve closing and opening.  Wonder how those pressure waves interact with the crankcase pressure from pennyrobber's chart?  My suspicion is that the crankcase pressure itself is enough to sufficiently vent the crankcase, even against any positive pressure from the airbox pressure waves and that any vacuum effect from the air box is minimal.

Seems like someone here, maybe Speeddog, knows about the Helmholtz airbox pressure pulses.  I sure don't.

[Howie]

Howie, seems to me that air box vacuum is created by the pressure drop across the filter and pressure drop across the airbox inlet nozzle.  I thought that venturi vacuum is localized at the venturi?

Also, (aaargh) what about the Helmholtz (?) resonance that is supposed to create a positive airbox pressure due to reflected pressure waves from the intake valve closing and opening.  Wonder how those pressure waves interact with the crankcase pressure from pennyrobber's chart?  My suspicion is that the crankcase pressure itself is enough to sufficiently vent the crankcase, even against any positive pressure from the airbox pressure waves and that any vacuum effect from the air box is minimal.

Seems like someone here, maybe Speeddog, knows about the Helmholtz airbox pressure pulses.  I sure don't.

You are correct about airbox vacuum, I was just trying to keep things simple.  It was also 1:47:14 AM here in Da Bronx.  Crankcase pressure will sufficiently vent the crankcase as long as the hose from the valve is not dumping into positive pressure, like in the old days before positive crankcase ventilation became mandatory on cars.  Back then, your car had an oil filler cap with a filter in it and a road draft tube that was cut in such a way that passing air would create negative pressure.   You could probably rig something like that on your Duc, but I imagine it would probably be quite ugly.  Engines actually prefer a slight negative crankcase pressure but achieving this on a Ducati engine without a ridiculously large crankcase would be quite a task, as illustrated by pennyrobber's chart.  In the case of our Ducs, the crankcase valve controls the amount of positive pressure.  The more you reduce the pressure where the vapors go, the more you can reduce crankcase pressure.    As far as your question about the Helmholtz (you spelled it right) resonator effect goes, I think you would need to put your air box on a flow bench to find out.  AFAIK, in the case of your bike the main purpose is noise canceling, but would have an effect.

Typically, a modern car engine PCV system consists of a hose from a valve cover to the air cleaner that will supply clean air as needed and another hose from the valve cover or crankcase to manifold vacuum through a small fixed orifice or PCV valve  to control crankcase pressure, and, if all is good you have a slight negative pressure.   

[Langanobob]

Typically, a modern car engine PCV system consists of a hose from a valve cover to the air cleaner that will supply clean air as needed and another hose from the valve cover or crankcase to manifold vacuum through a small fixed orifice or PCV valve  to control crankcase pressure, and, if all is good you have a slight negative pressure.

Howie, I have an old '66 Triumph Bonneville that used to be my daily rider.  It of course leaked oil and years ago I experimented with a small car type PCV valve from the crankcase to the intake manifold.   The Triumph motor came with some kind of really inefficient rotary valve for crankcase venting and IIRC I took it out all together and just ran a crankcase vent port straight to the PCV valve.   I thought it might foul plugs, but no.  It's worked perfectly for a long long time.  Amazingly the bike no longer leaks any oil at all during the summer when the motor is running a lot but during winter storage it still develops a good sized puddle under it. 

I've never quite grasped the concept of "If it ain't broke don't fix it" and I've thought about experimenting with something similar in the way of a PCV valve on my Monster but it probably will be a long time before it reaches the top of the priority list. 

[ducatiz]

Charcoal canister is not EPA it is for CARB. Everyone not in cali can legally remove it.  The crank vent is EPA.

[erik822]

Charcoal canister is not EPA it is for CARB. Everyone not in cali can legally remove it.  The crank vent is EPA.

Touché, Ducatiz

[Howie]

Howie, I have an old '66 Triumph Bonneville that used to be my daily rider.  It of course leaked oil and years ago I experimented with a small car type PCV valve from the crankcase to the intake manifold.   The Triumph motor came with some kind of really inefficient rotary valve for crankcase venting and IIRC I took it out all together and just ran a crankcase vent port straight to the PCV valve.   I thought it might foul plugs, but no.  It's worked perfectly for a long long time.  Amazingly the bike no longer leaks any oil at all during the summer when the motor is running a lot but during winter storage it still develops a good sized puddle under it. 

I've never quite grasped the concept of "If it ain't broke don't fix it" and I've thought about experimenting with something similar in the way of a PCV valve on my Monster but it probably will be a long time before it reaches the top of the priority list. 

Interesting.  Did, or was the Triumph supposed to be slight negative crankcase pressure out of the factory?  It seems to me like it could work, the hard part would be designing a liquid separator.  A properly sized fixed orifice might work as well, and would be simpler.  Because Ducati doesn't do it doesn't mean it can't be done.

[Langanobob]

Did, or was the Triumph supposed to be slight negative crankcase pressure out of the factory?

The original Triumph vertical twins were kind of interesting.  Both of the pistons went up and down together, one going up on the compression stroke while the other was going up on the exhaust stroke.  But coming down together they must've built up a heckuva crankcase pressure.   The engine was also dry sump, wherever that might fit into the equation.  As I recall the rotary valve was timed to be open when the pistons were coming down and closed coming up, or at least that's what common sense tells me, not so much memory as it was a long time ago last I had the motor apart.  I don't know if there was any cam timing type of overlap in the rotary valve operation.   So, I guess if that rotary valve was effectively closed it would pull a good vacuum with the pistons going up so that could've minimized the positive pressure build up going down.

But the end result was that there was positive crankcase pressure that made for the famous Britbike oil puddles.  After the PCV valve installation, no more puddles.

[Howie]

Thanks. I was familiar with all but the rotary valve.  I've often wondered why Ducati doesn't use dry sumps since this also helps prevent oil starvation

[ducatiz]

Thanks. i was familiar with all but the rotary valve.  I've often wondered why Ducati doesn't use dry sumps since this also helps prevent oil starvation

Ducati is probably in kind of a bind in terms of changing the engine design. 

On the one hand, Ducati purists would shit (and not just purists, even the weekend purists would shit).  Look what happened when they updated the frame on the 696.  Shit storm.  It's not like Ducati used a trellis frame on every bike, nor even desmo valves.  My 500GTL has neither and it runs great.  (ok. bastard child)

Second, they have sort of a religious thing going.  I mean about their engine design.  If htey changed anything drastic, it might be akin to admitting there is nothing particularly special about a transverse L-twin.  I mean, it's special to ME but engineering-wise it is kind of dated. 

I am sure there is a few dry sump engines in their design shop.  They have dozens of engines on stands for study and experimentation.  Probably built a few and then it wouldn't fly past the marketing guys.