These requirements are spelled out in the installation manual.
The airbox performance is alluded to in chapter 16. i.e. it says that "performance data as specified..... can only be warranted by employment of the genuine Rotax airbox". That just means 100hp is only guaranteed when using the box. It just happens to be about a 10hp drop without it.
The requirements for the vent lines are in a caution in 15.1.1:
"
The float chamber venting lines (3) lines have to be routed into a ram-air and vacuum free zone or into the airbox, according to the requirements and release of BRP-Powertrain. See section 16. These lines must not be routed into the slipstream or down the firewall."
And the most important:
"Pressure differences between intake pressure and pressure in the carburetor chambers may lead to engine malfunction due to incorrect fuel supply."
Fig 59 shows how the vents should be routed, to nipples on the airbox.
The wording is a little vague but it generally means the carb vents have to be at the same atmospheric pressure as the venturis - that means in the same atmosphere inside the filters, not just near them on the outside. It's not a huge deal at sea level and lower altitudes, which is why this is not very well known and most of the time doesn't lead to way too rich running.
LS
I am still interested in where the documentation is that indicates a 10% HP loss if the Rotax induction air box is not used ( I was involved in testing of installations both with and without that did not indicate it was nearly that much)?
As for vagueness of what to do with the vent lines...I don't think it is vague at all. It specifically says "they have to be routed into a ram-air and vacuum free zone or into the airbox". It does not say attached to nipples on the air box
or air filters.
Oh dear... Interesting. If it were me, I'd talk to Vans about it and see what their take is. That definitely contravenes the installation manual......
PS: or maybe they get around it by splitting hairs on "....routed into a ram-air and vacuum free zone OR into the airbox.....". But the possibility of a pressure differential between the venturi area and outside the filter is real due to the drag of the air filters. And it definitely causes the carburettors to run rich (plus the mod is too cheap and easy to do
).
I'll bring my camera out to the hangar today and take a pic of the mod. I use the same K&N's that it looks like Vans uses too.
PPS: this a requirement of the carburettor installations and for proper operation of them so, no, there'd of course be no adverse affects at lower altitudes. The mixture would be correct at all times. I suspect the pressure differential that does arise at sea level is less than what arises at higher alts, maybe that's why it's less noticeable. Or maybe the carbs just accidentally run lean enough down there for it not to show considerably on the EGT's and plugs?
LS
I don't think the current RV-12 engine installation is contravenes to the current installation manual. Remember it says "they have to be routed into a ram-air and vacuum free zone or into the air box". There needs to be evidence that it is not routed to a ram air and vacuum free zone to justify the need for the mod. that you propose.
I have flown an RV-12 round trip from Portland to Oshkosh (about 1750 statute miles each way) at 11,000-12,000 ft altitude range for a good majority of the route, and saw no indication of excessive richness in either the EGT's or the condition of the plugs after the trip.
I am interested in the information you have posted, but one problem I have with the information is the generalization that all airplanes with a Rotax 912 that are not using the (very expensive) Rotax induction air box, should have this modification. I believe it is a modification because it is not documented in the install manual. I am not meaning to be critical...making generalizations is a mistake made all the time...even between the different RV models.
My point is...just because your airplane needed a mod. such as this, doesn't mean an RV-12 does.
I would be interested in seeing photos of what filters you are using (from your previous description it sounded like you were using the round flat style K&N filter. The RV-12 (and what is shipped as standard equipment with a new 912 engine) uses the conical K&N filter that is manufactured by K&N exclusively for Rotax.
I would also be interested in having more information about what airplane this described problem existed on (photos of the engine installation would be great). There is a wide range of airplanes (and their associated engine installations), that use the 912 Rotax. The Kitfox for example...a totally different cowling and associated cooling airflow style. I imagine there is a lot of difference as far as air pressure balance and flow going on within the cowling than on an RV-12 or a lot of other airplanes.
Your info has developed a curiosity and I plan to investigate it further, but I think it is incorrect to generalize that an RV-12 should have this mod done. It may be totally appropriate to log on to a discussion group catering to teh model airplane you fly and suggest that they all do it (at least they would have somewhat similar engine/cowling installations) but I would still need to see evidence that it is not possibly a problem that exists because of something specific to your airplane...
...I have been in this business for a long time...one thing I have learned...Every time I think I can deduce what would be right, I find out how much I don't know.