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Discussion Starter #1
Does anyone understand the fluid dynamics/physics of the tiny snorkel size intake on the NineT? I've noticed the same phenomenon in cars, although they generally don't run at the high rpm that motorcycles achieve. Seems like the snorkel intake is smaller than even one of the throttle body intakes. Why is this?

If anyone has any design expertise and can explain this, I'd be grateful.
 

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Does anyone understand the fluid dynamics/physics of the tiny snorkel size intake on the NineT?.... Why is this?
Be careful what you ask for... Resonant Air boxes: Theory and Applications That gives an overview.
Tuned intakes offer a performance boost but also are used to quiet the intake. Basic reason the snorkel can be small is because it has pretty continuous, even flow. The throttle bodies take in air in "gulps". The airflow actually starts, stops,and can flow backwards (reversion) so the throttle bodies need to be bigger.
There's more discussion about this if you look at the airbox removal / individual filter threads.
 

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Have you seen the air filter? Pretty small. All my other bikes, including 650's, have larger filter elements.

At redline, it would be pulling the same amount of air as a typical 600ss, which have essentially 2X the redline. So really not that much air relative to other bikes.

Not sure if it has anything to do with the design, but a long narrow tube like that can have significant fluid momentum effects, which can provide elevated pressure in to the air box between cylinder suction pulses. The high velocity creates a "piston" of air in the tube which has momentum and will compress the air in the air box due to that momentum. All a guess...
 

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Discussion Starter #6
I guess the compressibility of air is a significant factor. I have a 3.5 liter truck with a large throttle body, 3" intake plumbing, but only a 1.5" airbox inlet. It probably never exceeds 4000 rpm, but the intake must be that size for a reason.
 

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chicagobob, I can't give a precise answer to your question, but maybe this is nevertheless interesting to you.

We tested a K&N Aircharger against a velocity stack.
It's a H-D-, not a BMW-engine, but both are two cylinders with 1200ccm.
The numbers are not important / we tested on a Superflow Dyno, a standard Dynojet Dyno showed 12-15% more.
Take a look at the curves; the red curve is the K&N Aircharger, the black curve is the velocity stack.





 

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Discussion Starter #8
Thanks, Tomaso. I had kind of the same experience trying velocity stacks on my carburetted Bonneville. Nice WOT, high RPM power increase, but difficulty in the midrange.
 

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Thanks, Tomaso. I had kind of the same experience trying velocity stacks on my carburetted Bonneville. Nice WOT, high RPM power increase, but difficulty in the midrange.
It's easy to be making apples to oranges comparisons and even easier to fool "butt dynos". Intake tuning is complex. It can be difficult to figure out if the change you made needs a jetting / mapping change or you inadvertently changed the inlet length and created a problem. CV carbs on the last pre-efi bikes could be finicky. Toss the restrictive airbox and install individual filters and you could get a huge mid-range flat spot / stutter no jetting changes would fix. Add a couple inches of straight tubing between the carb mouth and filter (moving the filters back and lengthening the inlet tract) and all would be well and we saw power gains. This proves length really does matter?:icon_scratch: Or do you say individual filters are bad because JUST installing them gave a bad result?:icon_scratch: Things get complicated and are frequently not what they appear to be .
 
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