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Crankcase breather filters

Old 10-31-2013, 02:30 PM
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Crankcase breather filters

Crankcase gases from the valve covers are routed to the air box, to a small box, vented to the air box mixed with intake air.

That crank case gas can effect air fuel ratio in other bikes (not a guess, confirmed with wide band meter)

I often find trace oil in the air box that came from the crankcase vent box.

Considering removing the vents from the air box... to individual filters....Outside of the intake stream.

I did a search and did not find this discussed or done... may be due to it was looked at and discounted as a waste, and additional complication not needed.
If so, i did not find that discussion here.

Thoughts?
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Old 10-31-2013, 03:29 PM
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seems like every four stroke I've had had a little bit of oil in the air box to be cleaned out when I was doing maintenance. not really sure why they do it that way maybe it's just an e.p.a thinh. when I changed my superhawk air filter the crankcase breather filters were trashed
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Old 10-31-2013, 04:23 PM
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Have a look at air box mods over the pond :-)
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Old 10-31-2013, 04:45 PM
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www.vtr1000.org • View topic - Airbox modding

I was thinking of the same thing... but only in the back of my head...
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Old 10-31-2013, 05:30 PM
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Thanks for the ideas.. But I've seen the air box mods done.. that is a different direction for a different desired outcome.

My concern and there for desire to learn for others prior trials.. is many bikes that have a oil vapor recover (not the VTR, it is not designed to coalesce and return liquid oil to the motor) do poorly when you remove the "box" that crank case breathers link to and use pod filters or vent to atmosphere.

Was hoping to learn from others.. but may just have to pick up two small filters and try it myself. Of build a central box that both lines run to and filter that.
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Old 10-31-2013, 05:44 PM
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Oh forgot to mention... it's discussed slightly in that thread just as an addition to making larger and less turbulent airbox. A couple of members cut out the whole junction and stick pod filters on like this:

51708K 1/8 Scale Black RC Nitro Engine Air Filter .21 + | eBay

and they're putting filters on the carb breathers as well...
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Old 11-04-2013, 10:27 AM
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For myself, most of the reason for doing this is to take as much as possible outside the airbox to maximise volume, not really about emissions, but there's a bonus to be had there as well, I guess.
I know the airbox volume is marginal- that;s easily proved by setting the cam timing 180 degrees out; the airbox doesn't have time to repressurise before the next cylinder's intake stroke 90 degrees later at anything over three or four thousand RPM.
I'd like to set up a big bang engine, just to see how it behaves on the road and track, but it would need almost twice the airbox volume and feed rate than we have as standard.

But the airbox is quite cluttered, and obtaining smooth laminar flows is going to be difficult.
So cleaning out the existing airbox and enlarging it is a way forward.

Then the snorkel size is the next problem- it simply won't flow more air.
Once you've enlarged that, it's all about getting clean, cold air in- which isn't going to happen with the intake arranged as it is as standard, with air being drawn in above a hot engine, and downstream of a hot oil cooler.......

But back to your original point, it's easy to use garden hosepipe or similar to run from the head breathers to a tee, and then to a filter- a ped or scooter air filter is about right- and all I did with the carb breathers is to wrap som filter foam around the end of the pipe and cable tie it on.
There's no gain from this as far as the carbs are concerned, but it then allows you to cut out the corners of the airbox and plastic weld a clean corner in- more volume for free....

Last edited by VTaRse; 11-04-2013 at 10:27 AM. Reason: speeling eroor
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Old 11-05-2013, 01:30 PM
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I've actually researched exactly this subject quite a bit and available info is limited. Routing both breathers to just an external filter (either individual of siamesed, and of course sealing the vacated airbox inputs) is not a good idea because it introduces positive pressure back into the airbox (which the vapor solids collection bottle minimizes if not also enables). The key to venting the crankcase is creating negative pressure (CCP)with check valves and filtered make-up air. Reduced CCP lowers windage and pumping losses and seal leakage among other things. I found some possibly check valves but have not tested them or finalized make-up air/filter and hose routing yet.

Bunn CCB systems were available but frankly I think the guy is a bit FOS; go to

Bunn Breather Kits
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Old 11-05-2013, 02:40 PM
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Originally Posted by skokievtr View Post
Routing both breathers to just an external filter (either individual of siamesed, and of course sealing the vacated airbox inputs) is not a good idea because it introduces positive pressure back into the airbox (which the vapor solids collection bottle minimizes if not also enables).
Not sure I understand what you're saying; how can piping the crankcase into an external filter introduce positive pressure back into the airbox, when the two systems aren't connected?

BTW, I think the theory behind the white plastic catch tank normally fitted in the airbox is that as one piston goes down, increasing crankcase pressure, depending on which piston is going down, the other one is going up.
That's simplistic; if the engine was a parallel twin with a 180 crank it would be exactly true, but with a vee twin with a 90 degree crank the pistons aren't exactly opposite. But looking at the "worse" point (maximum crankcase pressure, minimum crankcase volume), with one piston fully down and the other one partially down, compared to the point where the pistons are at their highest positions - minimum crankcase pressure, maximum crankcase volume, with the difference in crankcase volume roughly equal to the plastic bottle volume. I haven't measured it, but this feels right.

So the bottle effectively acts as a pressure reservoir where the difference between the maximum volume inside the engine and under the pistons and the minimum volume can be "stored" until the engine moves past that point. it isn't fully sealed, of course, as there is a small hole venting into the airbox. Thi is small enough not to materially affect the airbox pulses and pressures, messing up the fuelling and jetting requirements, but allows the engine to regulate it's internal effective mean pressure to a normal pressure, even though this varies during the crank cycle.

Please take note that at neither extreme point are both pistons simultaneously fully up or down; when one is fully up the other is only partially up or down. So we're not dealing with a difference in volume (if pressures were always at atmospheric, which isn't the case either) of 1000cc's of difference in crankcase volume difference between max volume and min volume.

Now, the above would be all there is to it if air was not a compressible gas, to prove my theory all you have to do is completely fill the crankcase with a liquid and slowly turn the engine over through a full 720 degree crank revolution. Excess liquid would then have to be pushed up into the white plastic bottle, and because the liquid isn't compressible to all extents and purposes, you could exactly measure how large the plastic bottle would need to be to completely absorb the difference in crankcase volume. The volume inside the pipework also acts as part of the reservoir volume.

But it doesn't have to be as large as that, because the gas can compress a little, as opposed to a liquid, with no real performance loss.

Check this theory out by blocking both breathers while the engine is running, and I'd expect that the tickover speed would decrease.


Right, last point; you won't get much drop in crankcase pressure with just one-way valves, because there's always a little pressure being introduced into the engine by gas leaking past the rings.
Even if there wasn't, the difference in pressure would only be at the most the difference between the max and min volumes, assuming gas inside the engine was at a steady one atmosphere, rather than varying rapidly as the pistons fly up and down.

To get a real pressure drop you need to pump gas out of the cases to lower the average pressure inside the engine; but of course that takes power, electrical or mechanical, and that need for power negates the pumping gains (or really, reduced losses) caused when the engine has to pump air around within the cases and pipework.
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Old 11-06-2013, 07:00 PM
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Originally Posted by VTaRse View Post
Not sure I understand what you're saying; how can piping the crankcase into an external filter introduce positive pressure back into the airbox, when the two systems aren't connected?
I meant crankcase not airbox.

And the "catch bottle" in the air box fed by the cylinder head breather tubes (which serve as crank case breathers) is exactly and only that, a catch bottle to settle-out oil vapor "solids", and minimize the effect crankcase positive and negative pressure pulses has on carburation.

As to the rest of your "distortation" (not dissertation), I have no further comment.
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Old 11-07-2013, 04:43 AM
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Ah, that explains it, lol

I re-read my post, I did go off on one, there....
I was thinking out loud, really. The idea of having a steady negative crankcase pressure is a good one, but gets very complicated with the volume of the sealed cases changing rapidly all of the time. Because air is so elastic it isn't going to behave like a liquid, and most of my points would only be valid on a low speed engine.

Has anyone ever done the dyno back-to back with lowered crankcase pressure systems?
Is there any real gain?
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Old 11-07-2013, 08:53 AM
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Crankcase breather relocation, free power! - Triumph Forum: Triumph Rat Motorcycle Forums

Similar discussion on the Triumph Bonneville forum awhile back.
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Old 11-07-2013, 10:23 AM
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Nice link there! While it is only one source, it is the first reasonable explanation for the internal bottle/oil catch I've seen!

My thoughts on this whole process were, why would Honda spend the effort and take up the internal volume of the airbox when a crankcase filter seems much easier? Up to this point, I only had these reasons:
1) They didn't want to have another maintenance item with two more filters needing to be replaced, so jammed everything in one box.
2) Somehow the internal crankcase air pressure balanced out from the way their hoses were routed.
3) The oil blow back was too messy to deal with and easier to contain in the airbox.

The crankcase pressure just didn't quite add up because of the way it was all vented... the two cases aren't directly connected because of the hole in the bottle... adding turbulence and varied pressures into the airbox. I can't imagine with the varied engine speeds that you could get any kind of consistent negative pressure with the previously discussed compressible and fluid nature of air...

The first post in that link mentions that emissions requirements are much easier to meet if the somewhat dirty crankcase vapors and fed back into the carbs to be burned off. This makes the most sense to me so far...
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Old 11-07-2013, 03:32 PM
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I just bought three of these filters. One for my Superhawk Chopper project, one for the Superhawk and being that cheap, and one will come in handy for some other project down the road.

After getting some ideas from here and the VTR1000.org site, I'm putting an oversize K&N filter in the lid at an angle. Pictures of everything that has been done, in a new thread to come. Bike has great power at all RPM's, but I want to see what giving the airbox more volume would do to this bike. I built a small metal tank for the breather and both crankcase hoses to attach to. Just have to figure out a place to put it, without it looking out of place.

Last edited by CruxGNZ; 11-07-2013 at 03:34 PM. Reason: spleling
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Old 11-07-2013, 04:00 PM
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9mm surely isn't big enough... do you plan on just drilling out the flange?
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Old 11-07-2013, 05:49 PM
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Yeah, thought about that afterwards. I probably should've got the 12mm, but I'm crafty and will figure out a way to make it work.
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Old 11-07-2013, 07:21 PM
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Originally Posted by superbeater View Post
Crankcase breather relocation, free power! - Triumph Forum: Triumph Rat Motorcycle Forums

Similar discussion on the Triumph Bonneville forum awhile back.
see post 14 on page2 i think..

good use for pair valves

"Hey Forchetto!

Good write up! Makes a lot of sense! Some drag racing cars I used to work on would evacuate the crankcase with vacuum from the exhaust headers. A hose ran from each valve cover to the header collector. Convenient way to get rid of the blow-by oil and fumes.

By the way, who make the breather that you are using?

"
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Old 11-07-2013, 08:38 PM
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Originally Posted by sjscicluna View Post
see post 14 on page2 i think..

good use for pair valves

"Hey Forchetto!

Good write up! Makes a lot of sense! Some drag racing cars I used to work on would evacuate the crankcase with vacuum from the exhaust headers. A hose ran from each valve cover to the header collector. Convenient way to get rid of the blow-by oil and fumes.

By the way, who make the breather that you are using?

"
There's two different setups going on here. The one you're talking about for dragracing uses a venturi effect of the exhaust to pull air from the crankcase using a PCV valve.
Then there is the one from the Triumph site which uses the pistons up and down movements and a valve to pump out the air.

I can't remember, did we figure out if the Triumph setup will work on our 90 degree V-Twins? I think it was a no.
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