Airbox question

[marcmax]

Lately I have had a desire to replace my black, plastic airbox with one of the older, aluminum clamshell airboxes. I just like the way it looks. Anyone have any comments, suggestions about possible performance issues, etc.

I realize they are not as convenient or easy to change an air filter with but I can put up with a little inconvenience to change the look.

[Rob Valdez 79 R65]

I think it has been said that the clam shell is not as efficient as the square box, but the round filters are less expensive than the round ones (I think).

If you get a clam shell, look for one with the perforated rear, for more air flow.
The trade-off, though, is more intake noise.

It was said that the perforated ones only came on the R90S (and possibly R100S & RS), but my '79 came with one from the factory.  





(Ignore the hole in the corner, I had that drilled for the crankcase breather exit hose)

[wilcom]

What a coincidence , I have a PITA clam shell that takes me 15 minutes to align the filter every time I change it. (then I figure it out, but never learn and agonize every time) Mine is a 79. Is the later one with the flat air filter a direct swap out???

[marcmax]

I don't know if it is a direct swap out but I guess I'm about to find out. I just bought one off of fleabay complete with the airbox mounted choke/enricher lever.

[nhmaf]

Part of the "hop up" tricks for the early R80/7 is to swap out the clamshell airbox for the flat one.  That, in combination with some other tweaks is apparently good for ~ 5 HP.   IF you really prefer the look of the clamshell, it's your bike and you're welcome to do what you like - the black flat square filter housings aren't that pretty, but they are functional.   I really hate trying to get the clamshell one back together on my R100/7.

[Rob Valdez 79 R65]

I really hate trying to get the clamshell one back together on my R100/7.

I guess since that is all I've known, it's not that big a deal to me.

It's easy to get the long bolt into the threaded boss - you just look through the left air-hole.

Yes, the nut on the top of the right half is tough, but the only time I need to mess with it is when I remove the gearbox.
The spanner from the factory toolkit fits it pretty well.  You just tighten the nut as far as you can with your fingers and then use the spanner for every half-flat, reversing the spanner every time.

[tvrla]

You'll have to change the starter cover as well. I can't imagine there being a HP change changing air filters. At least the HP figures when they changed the airbox didn't change on the R100s. I've made the change (flat to round) on my 1980 R100T but it was so long ago I don't recall what all I had to do. I think the same airtubes fit, but that's something to watch for.

As for needing the extra airholes in the back, I kind of doubt it. The R80 and R1007 didn't have the extra holes and they ran fine. As it is, the square airbox has two intake snorkles and they're pretty small - the older style intake is much larger to begin with.

[nhmaf]

But, the old clamshell airbox is drawing in relatively quiet air from behind the engine &under the seat.  The flat box/snorket is getting cooler air pushed in from the front when you are underway.   I'm sure that BMW wouldn't have changed the design if it wasn't some sort of improvement (HAHAHA!)

[Barry]

Snowbum makes the point that there is no significant ram effect to be had at any speed an airhead is capable of.  Cool air is good though.

The real purpose of the snorkels must have been to tune the inlet tracts for the same reason exhaust length is tuned to take advantage of reflected pressure pulses assisting combustion chamber filling in a particular rev range.

I wonder if any advantage was gained ?  Must be hard to tell as even though the R65 went from 45 HP to 50HP, valve sizes changed at the same time the snorkels were added.  

Claimed HP and torque figures didn't change at all for the R45 when the same change of air filter arrangements was made. What did change dramatically was the main jet size which dropped from 123 to 105. I make that a 34% reduction in jet area. It's always puzzled me that the same magnitude of jetting changes were not seen when other airheads adopted the snorkel air intakes.

[wirewrkr]

I don't know if anyone has mentioned this yet, but it's kind of important.
IF you decide to go forward with this, There is only one year that will fit your bike and that is of course from any 1979 model bike, (maybe some very late 78s, but I doubt it.)
It has to do with the breathers. in 79 the bike had a dual breather pipe setup, and the airbox was changed for 1 year to accommodate this.
The early airboxes will only work if you go backwards and restrict the breather!

[nhmaf]

What is best for more efficient fuel atomization, transport and combustion is to have higher inlet air speed (and of course cooler, denser air with more oxygen, too).   Trying to suck in air from behind is never as good as from the front.    The snorkel inlets are roughly the size of an adult human eyeball - so I say, flip your helmet visor up and ride with your eyes open facing front, and say if you detect any pressure difference on them that way as compared to turned around facing backwards (assuming you'd have to ride pillion for that part of the test) !!

 

[tvrla]

That's one thing to be facing forward in open air, but it's quite another to be down there, under the tank where those snorkels are. Take a good look - right in front of them there's a baffle connected to the starter cover. In front of that are frame tubes, Horn?, headlight assembly, forks, wheel/fender...

It's understood a lot of those things aren't direcly in front of the intakes (except for the starter cover dam) but they for sure are creating a bunch of turbulance!

...and say if you detect any pressure difference on them that way as compared to turned around facing backwards (assuming you'd have to ride pillion for that part of the test) !!

 

[Barry]

I agree it's logical to assume there would be some ram air effect from forward facing snorkels. Snowbum is even smarter than many people realise so when he says about the ram effect "if you are not doing over maybe 120 mph, forget trying to get RAM pressure". I'm inclined to believe him as I suspect he will have tested it or done the calculations or more probably he's done both. The nearest my own experience comes to getting a handle on this is working with pitot tube equations which actual do work on the ram effect principle. At 70 MPH I calculate a pitot tube generates in the region of 2" water gauge which is 0.5% of atmospheric pressure. So if Pitot calcs are a reasonable method of calculating the ram effect then there is some benefit but it's very small at legal speeds.

[nhmaf]

I also agree that the snorkel tubes' location is decidedly non-optimal if one were primarily interested in improving the ram air effect, too.   But, even with the reasonable cylinder displacement, the fairly low peak RPM limit also makes a quest for trying to turn an airhead's intake system into that of, for example, a Kawasaki ZX9's  or ZX6's ram air enhanced system will be likely to disappoint.  It just can't pump enough volume quickly enough due to the ~7K RPM limit, which is just midrange for the Kawasaki bikes.   Plus, the Kawasakis can go much further into triple digit speed territory than even the fastest airhead, which makes the air pressurization effects MUCH greater.  So, I agree, and always have agreed, that the benefits of the snorkels and flat airbox aren't huge or earth-shattering.  But, it is definitely a functional improvement over the old clamshell filter box - it just doesn't look so neat, tho.

[jg928s4]

I have a Starter cover and Aircleaner Clamshell if any one else is interested in Back Dating......