REFLECTOR: AIR INTAKE DUCT

[Al Gietzen]

I have a tech. question.  I have a LYC. IO-360. I am replacing the Skeet
duct which runs from the belly scoop, through my K&N filter, through my
alternate air T, and into the throat of my fuel servo. The hose that is on
it now is 2 1/2". My A&P tells me it should be a minimum of 3" diameter, yet
the throat opening of the fuel servo is only 2 3/8". How can I determine
what is the right size to supply needed air flow?

Lou Stedman

 

Lou;

 

It’s all about pressure drop.  Pressure drop in the induction path directly
reduces the amount of air/fuel charge entering the cylinder. The throat of
the fuel servo is very short, so there is not a lot of frictional loss
there, and you want high velocity at that point for fuel dispersion, etc. It
sounds like you have a fairly torturous path for the induction air to travel
before getting to the intake manifold.

 

What is the right size? Well, that’s a bit complex.  You could measure the
pressure at the entrance to the fuel servo, and if it is more then 1” of HG
less than the pressure at the entrance to the scoop, the loss is probably
higher than you’d like.  Or you could do some analysis based on the engine
displacement, rpm, air flow velocity, and compute the pressure drop.  Not
very easy.  

 

But just a back of the envelope analysis - your engine would like to take in
180 cubic inches of air each revolution.  At 2700 rpm that’s 280 cubic ft
per min (cfm). You 2 ½” duct has an area of only 0.034 sq ft; so the
velocity in there would be 8230 ft/min, or about 95 mph. Depending on the
length of tube, that could be considerable friction and turbulence loss,
especially if the inner surface is not real smooth.  And does it also have
to make a turn? More loss.  How big a filter?  Pressure loss there as well.

 

A 3” diameter would slow the velocity by 30%; a big help.  3 ½” would be
even better.  Pressure losses generally go as the square of the velocity.

 

FWIW;

 

Al

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