Ok, am I just stupid as it doesn't make sense to me.

When the piston goes down this creates crank case pressure that pushes against the piston and robs power.
Opening up the breather allows this pressure to escape more easily.
As the piston rises a vacuum is created in the crank case that pulls against the piston and robs power.
Opening up the breather allows air to get back in and therefore reduces the pull on the piston.
Surely not allowing air back in will negate this and rob power? mmm

It requires energy to over come a vacuum just as it requires energy to create pressure.
Think suction cup off a piece of glass.
You need energy to push it on and energy to pull it off.

Am I way off here??

 

It sounds almost like more trouble than its actually worth, having to worry about the valve. Although, a combination of the two would definately make me different!

 

I'm in Maui honeymooning with my wife. It's 30 Celsius here and awesome. Going home sunday night though [:@]

Still finding a few minutes to visit the KF though, she hates that!

 

The pushing air out and pulling it back in is what robs power. If there is a vacuum inside the case, there's no power robbing. There's not a complete vacuum of course, but if the piston is fully at the bottom of the down stroke, that would be about atmospheric pressure since any pressure greater than atmosphere will escape out the PCV valve. As the piston rises to the top of the stroke, the PCV valve closes and that creates the vacuum as the piston rises. Once at the top of the stroke, the resulting vacuum will act like a spring and actually help to pull the piston back down. So any losses from pulling the "spring" on the up stroke, are recovered on the down stroke. The result is that the energy loss/gain is a wash.

Contrast to the case without the PCV in which air is constantly being pushed out and sucked back in, there is loss of energy in that case.

 

Hey Brian, what do you think of combining your vent mod with this mod? best of both worlds? Might have to get a bigger PCV valve i would think so it would fit (i think the tubing i got is 5/8ths)

 

Ok, does that not also apply to the pressure built up on a down stroke?
It helps push the piston back up thus no net loss?
(I accept that heat is generated when a gas is compressed. (Boyles law I think) And that heat is wasted energy.)
(some of this energy may be recovered when the gas expands back to it's original volume)

A constant pressure or vacuum should havenegligible net effect.

It also occurs to me that with the valve installed, this pressure is released once, on the first down stroke, and because the valve stops any air entering the system there is no air to compress on the second down stroke.

I think that ultimatly the freer the system is the more efficient it will be. Not working against a pressure or a vacuum at any part of the stroke. You can't compress a gas and recover all the stored energy nor can you work against a vacuum and recover all the energy. There will be loss.

A bigger diameter hose long enough to not let crud into the crankcase. Looped up nice and high and left dangling into the airbox should be enough.

Idid some reading and basically discovered the purpose of removing the pressure from the crankcase is not about saving the energy used to compress the air in there, but more about removing the gases that squeeze past the piston. These gases will not only cause the oil to need replacing more often, but cause the pressure to build up and eventually cause a seal failure or something like that. The idea of piping these gases, (unburnt fuel etc) back into the intake is so the engine runs more efficently, by getting a second chance to burn these gases, and it releases less emmissions. Older engines just vented to the atmosphere. The PCV valve is there to make this flow only one way and to stop a backfire causing excess pressure in the crankcase.

I would be very intertested in seeing the dyno results of ,
1. Standard
2. Opened up
3. PCV valve installed.
All other factors the same.

I think there wouldn't be much in it either way but my money would be on number 2. By a nose.

 

Personally, with all the time i have spent around motors.... i dont see that much of a gain from opening the vent up, yea im sure it will help, and if you were racing u would want to, but real world, real bike situations... your not really gonna notice any of the vent mods.... Marty sold the pcv to me down in FL... so thats the one i did... If you open the hole in the air box up for the vent and leave it at that, that should be good, so what if you increase the vent hose diameter.... its still the same sized hole coming out of the crankcase itself... thats what i think at least, alot of the mods to the bike are not that big of a gain, its just fun to mess with....

 

Yep, Agreed. We are just tinkering.

I was just enjoying the mental challenge of the science behind it all.
And I don't want to even go down the hole pipe length, average diameter, opening diameter, gas flow thing.
There is a few hours wittled away.

 

Yep..... we are all just addicted to modding, anyone else hear from the wife "You just cant leave well enough alone..." ?? lol.... This is my first bike so its fun to see what its all about..... I like taking things about seeing how they work and putting them back together.....

 

I'm taking the middle road on this one.

If it aint broke, don't fix it.