R1200s - air inlet tract. Why so small?

[thedukeofhunslet]

Had the bike apart yesterday to clean it. Thought I would have the rear footrests off. While I had her naked (!) I couldn't help notice that the inlet tract has the smallest of openings. This is odd - given the size of the filter and that the body work could easily accommodate a larger neck. Its just the plastic tube and rubber flange doing all the restricting.
Now I'm sure BMW would only do this for a reason - it's not like this part is shared with another bike and they just used these parts to save on factory tool up.
So I would welcome any of you mathematical geniuses and induction theorists out there if any of you could explain that there is no point in me butchering remoulding a new inlet from bathroom silicone...
See what mean...

[boxerpan]

You can ask over on the Pelican but whether you would get a concensus or any sense is anyones guess

[madman]

Faster moving air, through the small part, would be at a lower pressure and suck more air in. When it gets to the wider part it would slow down and increase in pressure helping it push through the air filter.
I would assume that BMW had fluid dynamics people to design this, and that is what they came up with.

[sandbar]

Now I'm sure BMW would only do this for a reason

I think that there is also a 'noise' issue here as well. The snorkels, that manufacturers are using, do significantly reduce intake noise.

sandbar

[sproggy]

Faster moving air, through the small part, would be at a lower pressure and suck more air in. When it gets to the wider part it would slow down and increase in pressure helping it push through the air filter.

Isn't that the wrong way around? As the cross section of the snorkel increases the pressure will decrease for a given air speed. Is that Boyle's Law? It's someone's law, anyway.....

I think sandbar's suggestion of noise reduction is more likely to be correct - an 1100S with a lennies induct is considerably noisier than with the standard, restrictive snorkel. Maybe someone needs to design a new induct for the 1200 too. Or maybe what looks like a restrictively small opening isn't really.

[thedukeofhunslet]

You see, I'm no genius but it simply doesn't make sense unless as Sandbar says there is a noise issue.

One factor is known. By forcing air through a small aperture you increase the atmospheric pressure - this causes friction and hence creates heat. Heating the air up may assist fuel atomisation but immediatly reduces the amount of air supplied to the engine.
Cold air is generally better.

Given the size of the filter and throttle bodies I just don't see it - try sucking through 1 drinking straw and then through three and see the difference in the energy that you expel in drawing the same amount of air.

Perhaps the only way to solve this is with a Dyno.

[sproggy]

One factor is known. By forcing air through a small aperture you increase the atmospheric pressure - this causes friction and hence creates heat.

Nothing you or the bike does increases atmospheric pressure. Only the forces of nature (or God, if you're so inclined) can do that.

Forcing air through a small aperture at a fixed pressure (created by a combination of atmospheric and the function of the speed that you're travelling at) will tend to increase its speed, not its pressure. Anyway, because the entrance to the intake is the smallest cross section in the snorkel (judging by your photo) you're not actually accelerating the air through a restriction at all - that would be the case only if the intake was large and the cross section then reduced further back. This is just restricting the amount of air that's allowed in, probably as suggested as a secondary effect of reducing the amount of induction noise.

Creation of heat? Not measurably at the air speeds and pressures involved here.

The principle of ram air (forcing air down a forward-facing intake) is to increase the pressure inside the air box to above ambient, but in that case designers try to create as straight a path as possible without restrictions (this is why it's becoming common to route the air intake from the headlight area, where pressure is highest, straight through/past the headstock) and having a reduced cross section at the entry point is not good for air flow.

Heating the air up may assist fuel atomisation but immediatly reduces the amount of air supplied to the engine.
Cold air is generally better.

Heating up the air won't assist atomisation in an injected engine - this is controlled by the fuel pressure and injector design. Cold air is better because it's denser and contains more oxygen per cubic volume than warmer air.

Given the size of the filter and throttle bodies I just don't see it - try sucking through 1 drinking straw and then through three and see the difference in the energy that you expel in drawing the same amount of air.

But there is a limit to the "bigger is better" argument. Try sucking through 30 drinking straws. Other than the packaging difficulties (fitting them in your mouth) your body may not need the amount of air that can theoretically be sucked through 30 straws. Physical filter size could mean anything - depends how efficient it is at flowing air. Throttle body cross section is the nearest comparison although it's rarely that simple.

Perhaps the only way to solve this is with a Dyno.

Unfortunately that's unlikely to fully answer your question because you won't be able to replicate the forced-induction effect (if there is any) on a dyno. You may notice some difference with a larger intake but 'seat of the pants' testing on the road might prove to be more indicative, not to mention cheaper!

[thedukeofhunslet]

Thanks Sproggy.
Most illuminating. I was just looking at the pics of the Le Mans HP2 bike and that also has the same induction set up with a small intake on the offside of the fairing - so if it works on that then you can conclude without any science that there is a definate reason for the design.

[sproggy]

That's interesting, because racing bikes wouldn't have any concerns about induction noise! I guess either the standard intake allows enough air flow or there's something in the rules to say that the intake must be standard.




Or there's a second intake on the other side of the race bike......

[Archie]

Perhaps the only way to solve this is with a Dyno.

Unfortunately that's unlikely to fully answer your question because you won't be able to replicate the forced-induction effect (if there is any) on a dyno.

Actually Crescent have one with a 160mph fan attached just for that reason, see here

[sproggy]

Blimey, imagine standing in front of that.

PDQ just have a couple of small blowers that they directed towards the cylinders of my bike. Nothing towards the induct and nothing towards the oil cooler. Things got rather hot.....