Paul:
Are you aware of an research done on the effects of using induction
tubing of square or rectangular x-section?

You've mentioned using rectangular exhaust tubing in the past, I
know, but I've been thinking that fabrication of intake systems
would be easier if one could make critical components from sheet
goods instead of buying/making all sorts of sophisticated bends from
stainless or aluminum tubing.

R. Berglund

It should work. For a given cross sectional area there is more skin
friction
drag.

I found a much cheaper source for tubing.
http://www.siliconeintakes.com/

Half the price of Burns stainless.

Paul Lamar

R. Berglund:

Round duct is preferred over rectangular duct for moving air.
Rectangular duct weighs more, for the same flow rate, than round duct
does. It's not a matter of just matching the area of the round tube
to the area of the rectangular tube to get the same flow rate. Like
an airplane with sharp intersections between the wing and the
fuselage the interior corners of the rectangular tube suffer from
interference drag.

Now, while I'm typing about flow rate and drag, any kind of directional
change in the intake system will increase restriction and decrease flow
rate. The most efficient intake system will have straight round pipe
straight into the engine ports. No elbows or turns of any kind.

A bellmouth at the inlet of the tubes will reduce pressure loss in
the system as well also contributing to improved flow rate.

I've seen a lot of pictures of intake systems on Wankel powered aircraft
with pretty swooping bends across the top of the engine or tight elbows
right at the intake ports. Both of these are really bad news for the
air flow. Elbows, no matter how pretty they are, cause turbulence in
the airflow, using up energy, and resulting in restricted flow into the
engine.

If a change in direction can not be avoided it is best to use a mandrel
bent round elbow with a throat radius equal to the diameter of the tube.
The elbow should be followed by straight tubing have a length equal
to at least three times the diameter of the tube to allow the air
flow time to straighten out after being upset by the elbow.

If a required change in direction is more than 90 degrees it is best
to use two elbows with a length of straight between them. Best
practice is to divide any direction change into two equal elbows
with sufficient straight between them to allow the airflow time to
straighten out.

Mark LaPierre


We just went over this in Fluid Mechanics. It takes 25-40 diameters
of length for a turbulent flow to stabilize and around 100 for the laminar
flow. The breakpoint is at a Reynolds number of about 2300.

Chris Cosman

Some intake manifolds see Mach .4.

Paul Lamar


For flow measurement, the standard is 10 pipe diameters upstream and 5
diameters down stream. For a lot of gas flow elements, they put
straighteners
in the pipe to shorten the upstream length, for especially gas flows.

Dale Davies



Yes, if you put a center line splitter in the tubing bend you can reduce
the required length of straight for the flow to settle down to less than
half what it would be without the splitter.  The turn, even with the
splitter, will still consume a great deal more energy than the same
length of straight pipe.  For maximum efficiency turns of any kind
should be avoided or at least minimized.

 Mark LaPierre