Drilling the vent is going to lessen pumping loss. It's got nothing to do with breathing.

I cannot imagine there's much to be gained by drilling it out. I suppose it might be measureable, but I can't imagine it's useful compared to the amount of work and risk involved.

 

Someone hook up a vacuum pump on that line and lets see if we get anywhere near the big % increase the auto racers get..

 

This is one of the most questionable least thought out modifications I've ever seen for this bike. No account has been taken of anything to do with the actual vent system, just how big a vent opening is needed, just stick a drill in and make something bigger. Clearly Bill did his homework, but we don't know what it was, he just said don't drill. Here is what I found to make a thought out choice.

That vent, even with the blockage appears to be plenty adequate to allow for the ventilation of the air being pumped by the engine. Here are the cases. It shows the maze of blockages down stream through which the mist must pass before ever entering the vent tube. I'm thinking in this case the factory engineers did their homework, since inadequate venting would pressurize the cases to the point where oil would possibly be pushed out of any joint, possibly push out seals, and past rings if it was that severe. So later here I will present what I learned in general.



In the image you can see the lip, but you can also see the tube port is open on both sides for good flow there. Then there is the risk of drilling into the actual gear box portion of the case, which opens it up to a whirring mass of gears stirring up a really heavy oil mist to blow up the tube. And still no one knows for sure how much actual cross sectional area is needed for either displacement engine to breathe adequately. Oh, did you notice the low point in the passage with the return hose in the left side case? They clearly believe there will be condensed oil getting back there to drain. They must have felt that little shelf was of value and likely calculated the sizing too.

A better possible choice would be to use negative crank case pressure by installing a PCV valve like automobiles have for decades now. It is a known modification for the heavier breathing KLR650 and I've considered it for my 650 which breaths through a similar size baffled port in the cam cover, but the baffling is poorer allowing more heavy oil mist to pass through it.

Here is the argument for a PCV valve from AGCO Automotive:
Key point, no driling and you getting negative crank case pressure. Question, will it work adequately to do the work?

Now about the PCV valve on a motorcycle research. Check out the discussion on Thumper Talk which still leaves the question open if an automotive PCV will work? The XR650R reed valve makes a point that negative pressure is desirable from Honda's point of view and the ZX12 mod begs the question a bit more. Still more research needed, maybe some experimentation by someone with the proper gauging... I don't have it. So I can't answer the question, will it work?

Then there is the argument to leave the engine case vent alone, that the flow will be adequate. I did some research on the flow, how much will be pushed out and drawn in based on the 350 cc displacement. 350cc is converted to Liters. .350L per revolution with 10,000 r/m ends up with 3500 L/m (.350L/.0001min=3500L/min)which converts to 123 cfm air flow. With a bit more research I found a chart that gave flow through a sand blasting nozzle but only went up to 1/4". The interesting part, though, is that 1/4" nozzle allows 120 cfm air flow, so clearly a 1/2" orifice in the motorcycle cases would allow more flow than that.

The case casting passage set up appears to be more than equal to the flow of the orifice and that means not a flow blockage. The length of the fitting and tubing will provide some restriction, but being less than a foot it is likely not enough to restrict flow to below that of a 1/4" orifice. Any restriction would have to be at the actual air box and as you might note, 1/4" allows adequate air flow, so chances are unless it is less than 1/4" it is not an issue. (I don't know how big it is, didn't look, the bike is 120miles away, but that is the research that has been left undone as well as what the flow may be in the cases themselves.)

So the question really is if it is at all worth it to risk dumping shavings into the cases and possibly the gear box, which would only happen if you drilled through both the step and the passage wall, at all?

Check my math and my thought process, let me know if there are any math or flow errors. This is, after all, the part where this modification is thought out. The conversation, is it actually needed? It may be desirable to have more flow capability than the case needs, but it appears that is done with the present engine design.

It seems the answer is the engine case vent is adequate. From what Bill says, and I cannot verify, but others here could, the air box has possibly too small a hole for the venting. Maybe the air box engineers are the ones who made the design issue for whatever reason. Wouldn't it be interesting to know?

 

I will translate KLX678's post for our reading impaired - DON'T DRILL YER CRANKCASE VENT THINGY!

 

For the rest of you, now you know possibly why you should not do so. If you're like me you do want to know why when it comes to things you work with.

 

I drilled (well messed up drilled and then had to fix) at the air box..

Anyone doing this mod, i highly recommend taking the airbox out before you go to it with a drill. Otherwise the angle will be off, as the frame gets in the way.

Though the hole at the airbox is 1/2" if you feel from the inside, its only a tiny hole, hence Bill's request to drill it out. Again, he said DO NOT drill the crankcase, but the airbox.

 

I don't know if it is possible, but is there enough room to stick a heated rod or drill bit to melt it open bigger from the inside of the air box? I don't have my bike here to look. I was thinking my brother said he did that on his 06 (or 07, I forget which).

 

Why not reroute the line entirely and keep it out of the airbox? I mean, if you're going to go through all that trouble, just get a piece of heater hose put one of these: http://www.scooterpartshouse.com/ima...01-016/500.jpg on the end and call it a day?

Also, great post above there KLX678. Seeing the cross section helped me see that it's not really much of a restriction. The math was a bonus.

 

I also tried that as a prior method to the drilling. Took a 1/2" drill heated it up til it was red hot, and using a pair of vice grips put it in the hole. Problem with that was a 1/2" was too big, and it was touching the sides of the extruding tube and started melting the entire connection pipe. Plus the airbox plastic is quite thick, so the drill piece would have to sit there for an extended period, making the damage even worse.

You maybe able to get away with it, with a little more patience than i had, starting with a smaller awl or drill bit & go bigger and bigger in incremental steps.

I think the best thing is just take the extra hour or so, and undo the filter while its inside the bike. You'll need a stubby phillips also for a couple of the screws, and get it out of there, so you have unadulterated access.

 

That was going to be my other option, you can pick these up from a local auto parts store for about $10 - $15

But what stopped me, was my bike often gets caked in mud, and this filter would get clogged and filty every ride out in the muddy trails.. It would just become a pain to keep clean. Plus i'd already damaged my airbox enough to not be able to just plug it... I had to come up with a way to be able to reseal the connector.