Re: Warm vs Cool Air

["Rich Wagner" <drsavage@domain.elided>]

Kim wrote:

>My old Moto Guzzi motorcycle didn't have filters of any type on the twin
>40mm delortos.  There were just a couple of velocity stacks with some
>screening on the ends suitable for stopping small birds and big rocks.

Then Scott wrote:

>The
>problem is that the mesh occludes a significant portion of the intake
>area, reducing the amount of air that can flow past it.  (K&N filters,
>to pick one example, also use mesh to hold the fiber elements in place,
>but the deep pleating around the perimeter means that the available
>surface area is so much higher that it evens out.  Or, if you lose 25%
>of your surface area to the mesh, then simply make the surface area 200%
>bigger and you've still got 50% more flow area available.)  And consider
>that fine dust such as can be found at the apex of any corner on a road
>circuit (kicked up when the guy in front of you drops a wheel in the
>dirt) is, when mixed with hot motor oil, a very effective grinding
>paste.  

Back in my motorcycle mechanic days, we used to see lots of the Yamaha
racers running without airboxes or filters during races.  Due to the
increase in maximum manifold absolute pressure, they all felt an increase
in power using this technique.  They all felt something else too: 
significant loss of compression by the end of the race day.  There is
enough dust in clean air to abrade the inside of any cylinder.  Some type
of filter is absolutely necessary if you want your engine to last any
length of time.

Scott's assertion that screens constrict the intake airflow to some degree
is intuitively correct.  In real practice, it's not the case.  A screen on
the downstream side of the filter is a very important device.  It actually
improves the airflow downstream, making it more uniform, reducing
large-scale turbulence, and reducing the chance of flow separation (VERY
IMPORTANT) in the intake runner.  I won't go into the details of how it
works here, but the beneficial effects of a screen far outweigh the slight
decrease in cross-sectional area that it imposes.

Kim also asks, refering to velocity stacks:

>Or is it just the shape of the stack that facilitates
>good breathing, i.e. the venturi effect?  Every velocity stack I have seen
>is tulip or bell shaped.

The tulip shape is necessary to reduce separation around the lip of the
stack.  As air flows into the stack, it does so from all directions.  If
the stack is just a straight pipe, the air flowing in perpendicular from
the side, next to the lip, will seperate as it makes its turn to flow into
the pipe.  By providing a curved surface, the seperation can be reduced,
providing better flow.


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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