You may prefer this to the ball check valve.

Plenty of room for a filter box and filter.

Paul Lamar

-randy echtinaw wrote:

Paul,
    I understand why NACA scoops won't work for cooling but in this case
the air intake is located towards the front of the plane with little
boundary layer buildup and it is always pulling a vacuum. If it would
work, a low profile NACA scoop would look much nicer than this snorkel.
I built two V-8 powered SkyBolts with a raised 1" x 4" scoop slightly
farther forward than this snorkel and it was sufficient for 350 hp.
Also, on the topic of filters, I used a small round filter mounted
vertically by the carb. Used a small flapper box, similar to a hot/cool
air diffuser box, hooked to this scoop. Unfiltered air went through the
center of the filter.
    Just food for thought.
Randy

I guess the secret is a good carb air scoop is not always pulling a vacuum.
A good carb air scoop has positive dynamic pressures of a few inches of water 
or one inch of Hg in it. I have a few hours in a Mooney and that is the case. 
You always get one or so inches of Hg when the ram air scoop valve is open.
I suspect this was known by the original inventors of the NACA scoop and that
is why they did not recommend it for carb air. It probably quits working slightly
above ambient (static) air pressure. BTW recently verified yet again
by Tracy Crook :)

Paul Lamar
 
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