Drilling holes in the back of the airbox, R90/6

[banzaibob]

I had an R100S that had 10mm holes in the back of the airbox. I'd like to get a little more punch at 80-90mph on my R90/6. Any advantages to the ol' holes in the airbox routine for an R90/6? Thanks!

[typ181r90]

COOLER air, not more air, as alot of people seem to desire with this mod. I did the modification on my R90 and compared the airbox with an R100 I was riding with and it was noticeably cooler to the touch, though I didn't have anything on me to actually measure the temp. difference.

[20774]

Under what conditions do you need more punch at 80-90 mph??

I don't recall if this mod really had any affect other than to make more noise. If it does have an effect, you'll may see some improvements in one regime and degradations in performance in another. Just where I wouldn't know. I think, as a system, it's a zero-sum game.

[banzaibob]

I covered this in a post several weeks ago, it was about an R90/6 that I put a bunch of R100/S parts on including switching from a 10/32 final drive to an 11/32. Of course the 11/32 gives me a little more relaxed RPM a high speed but I think it now struggles a little bit to get out of its own way at anything above 75mph.

I have gone up from 150 to 152 main jets (the plugs were pretty "clean" and it knocked a bit on warm days). If I give it full throttle it will begrudgingly accelerate, but not on an uphill grade into the wind. Plus, I don't like asking it to do full throttle all the time. I think that's asking for other problems down the road.

I might add (here goes my credibility) that since it seemingly ran fine before the final drive swap, I have yet to do a comprehensive spring tune up on it yet. Due to my experience with other bikes, namely 4 cyl UJM's, Harleys, I think if I could safely give it 2-5% more air at 1/3 throttle (4300 rpm highway speed) that would solve my problem. Raising the needle a notch accompanied by 6-8 10mm holes in the back of the airbox? Do you think this would have your blessing? Again thanks!

[20774]

I don't think you need anyone's blessing to try something on your bike...it is your bike, of course! Give it a shot...see what happens...report back. You might want to consider make one change at a time so you know what caused the related effect. The only problem with drilling the airbox is you can't go back, other than to replace the part...use some duct tape!!

[typ181r90]

I bought a spare airbox on eBay for almost nothing which I cleaned and drilled out. This way I could always revert back to stock. What you're doing is pretty much like putting in a CAI in a car since the air for the intakes runs through the engine case, to get to the filter, and then the carbs on our BMWs. By drilling holes in the airbox you're just cooling down that air at the point before it reaches the intake snorkels. I didn't have to rejet for this mod, though I did rejet to lower temps from ethanol fuels and the exhaust (though the plugs were only the slightest bit lean when I changed the exhaust)

My set up right now on my R90/6 is a 34/11 rear drive, 155 main jets, 2.7 needle jets, needle clip at 2nd position. Exhaust are stock headers with dunstall replicas (with baffles). The bike easily gets over 90 and seems to top out around 110-115. I've never had the lag that you describe but I suppose that may have to do with the rear drive.

[Weasel]

Yabut drilling holes in the back of the airbox will get the air cleaner all wet when you have to ride through deep water in the neighborhood when it floods. With the air intake up top in the stock position, you'd be amazed at what you can ford, if necessary.

[8ninety8]

There must be a reason 16 factory drilled holes appear at the rear of the S airbox in '74. Thought about it for about three seconds. Must have at least as much to do with exiting hot air from the front of the case, over the diodes board, over the crankcase/starter area and on out the back, as it has to do with cooling the intake.

Louvers at the bottom of the front case cover, advance and points sealed, holes inside at the top of the cam cover. If you ride a collector bike and like the idea of a next owner collecting it, don't drill or modify anything.

Should water get into the top of the case at rubber dork backwards snorkle, while fording knee deep stream, (the picture of the bikini clad model standing on R90S in knee deep water comes to mind) weep holes at case sides and rear of trans top should eventually, in a hour or two, drain it out. IMHO. Ear protection elimates noise problem.

I too would like anyone owning an airhead, doing some mods, give an opinion. All of this is purely subjective according to how the rider either likes a stock bike, or likes to fiddle with a bike that when delivered had more then a few compromises. Also, building a bike from parts eliminates any worry about keeping anything stock. And we have marvelous museums with factory perfect examples to view. 2cents.

[toooldtocare]

My 74 R90S had the holes plugged with grey plastic plugs. Looked like something the factory did since they looked as old as the bike itself? Don't know why, unless BMW decided later it was not a good idea after all. However, I can see the advantages of cool air, but not the advantage of water entering the holes.

[amiles]

It seems that whatever airflow that you gain with the new vents will be lost from the area on the top of the engine. The path of least resistance would seem to be the new holes, less, if any, heated air would be moved from the top of the engine.

The original set-up draws some air from the vents on the front cover in addition to the snorkel. This air flow might well keep the temperature of the diode board and everything else in the cavity a bit cooler.

The thus heated air then going to the carbs via the air filter would help driveability particularly during warm up. During regular riding the warmer air would continue to contribute to the vaporization of the fuel, particularly on cooler days.

Now of course we all know that the "best" situation is to have the coldest air flowing without obstruction into the combustion chamber. (volumetric efficiency, don't you know) On a race track under narrow climactic conditions I won't argue this, However drivability (crisp throttle response) day to day under varied conditions will be better with a carefully designed air supply system that likely will have some specific restrictions to obtain this (stock set up).

Did you ever consider that the additional airbox vents may have been part of what required BMW to use the Del'Orto Accelerator pump equipped carbs to improve driveability avoiding a flat spot on hard acceleration that the CV carbs would have been unable to do?

Remember too that the /6 and the /S were targeted to different types of riders, everyday VS sporting oriented ones. "Whew"

[Weasel]

My apologies, I didn't know the S models had the holes...

[bikerfish1100]

Not only did the R90S models have holes (apparently, i have none to refernce to right now), but also later model 100S and 100RS models, at least thru '78.
meaning that they had nothing at all to do with the DellOrtos. Unless BMW just decided to carry them over for the next few years because they had a very large pile of tehm already drilled. not likely.
allowing more air to flow into the filter is not going to increase the air volume in the combustion chamber (that comes from carb jetting) so messing with your jetting because of those air holes is not the path to go.
i'm thinking the holes were only about providing cooler air to the carbs, which (i believe) combusts more readily due to compression capacity of colder vs warmer air. or something like that.

[20774]

Colder air is denser which should equate to more energy during the combustion process. That is if the temperature difference is really enough to make a difference. I mean, how much time does the air have to linger in the top of the engine case before being sucked through the filter into the engine? Seems like that's quite a bit of air being pumped through at 4000 RPM.

[toooldtocare]

Don't know about the R100S, but I do know that the R100RS Motorsport that I just sold did not have holes in the cover.

[8ninety8]

Measure from ground to airbox holes, approx. 23", 16" to front louvers, and imagine the hellacious downpour needed to flood the aircleaner/front electrics.

A stream covering almost the entire front wheel is something riders don't normaly encounter unless deliberately, and mounted on a dirt bike.

It would be fun to ask the designer from way back why it was originally done. Looks, function, or legendary imaginary power?