Road draft tubes were used for more than fifty years to evacuate the crankcases of most trains, planes and automobiles; pretty much everything with an internal combustion engine (excluding boats) used one. They were usually made of metal, extending from high in the crankcase to just below the bottom of the engine, and ranged from 1/2 inch to an inch in diameter on most automobiles - slightly larger on trucks and bigger engines. They were successful at evacuating the crankcases and created a feature on the roads that you don't find today: the dreaded black stripe. As the vapors hit the roadway they left an oily sludge behind on the pavement that got worse as the engines aged and more oil was siphoned out along with the vapors. In a very ugly way, the black stripe was proof that the tubes were doing their job.
As I pointed out earlier, the tubes were used to remove excessive blow-by pressures from the case, thereby reducing oil leaks, windage and allowing the pistons to move more freely without high case pressure that acted to inhibit their movement. in addition, they removed harmful vapors from the engine case. Blow-by byproducts combine with oil and water vapor inside the case to form a host of noxious compounds, most of them acidic in nature, and these compounds attack the oil, turning it into an acidic sludge. The acids also effect internal engine components such as bearings and the like, greatly reducing their operational life. That's one reason the old pre-'60s cars didn't last all that long; 100,000 miles being pretty much the extent of an engine's usefulness.
The biggest problem with road draft tubes is (arguably) that they worked, and by the '40s and '50s they were sending tons of poisonous hydrocarbons into the atmosphere; a study done in the '50s, as noted above by TT, showed that between 20- and 40% of the pollution created by cars was emitted from the road draft tube
GM (and more precisely Cadillac), working with a system they had created in WW2 for their tank division, successfully routed the tube up to the intake where engine vacuum acted to more efficiently scavenge the crankcase than the (relatively) passive draft tube, which only worked when the vehicle was in motion, had. Additionally, the emissions were sent through a series of baffles and filters that removed oil and water vapors from the siphoned gasses before sending them back through the carburetor for re-burning. This not only reduced atmospheric pollution, but helped the engines last longer.
Now, I'm not a huge advocate of polluting the atmosphere. I appreciate the fact that Honda spent significant money and time to develop a system (BGSD) that not only helps clean the air, but makes the engine run more efficiently and last longer. I've made it a priority to ensure that the Blow-by Gas Scavenging Device is complete and fully functional on my bike as I believe it's the best thing for it and, after all, it's how Honda designed things to be...
My suggestion would be to retain the stock airbox and scavenging system both for the longevity of the bike and for a cleaner atmosphere; frankly, I've never been a big fan of the "empty triangle" and think it just looks silly. However, if you
insist on running pods on your bike my contention is that you need to provide some sort of crankcase scavenging device, be it a draft tube or whatever, so that the engine doesn't have to fight internal pressure and doesn't eat itself up from stewing in its own juices. Anecdotal evidence such as "I took my breather tube clean off and it didn't make a difference" is simply based on ignorance as to what is really happening down inside the engine, and is no proof at all.
If you decide to use a road draft tube, it should be routed straight down from the breather with no kinks or filters, which would defeat the purpose of the tube. It should extend an inch or so below the bottom of the engine into the slipstream provided by the bike's motion. For increased scavenging, the tip of the tube can be cut in a 45 degree angle, with the angle facing toward the rear of the bike.
JMHO
