Group,
I hope that I can explain the formula where it makes sense and is useful
to you.
The idea is to calculate how much air the engine requires while covering
a mile. The cubic inches of air is then converted to square inches by
dividing by 1 mile. This gives you the size of the opening in the scoop.
Naturally this is not absolutely accurate because of the variables,
volumetric
efficiency, wheel slippage, etc.
The size of the column of air 1 mile long can be found by working
backwards from the distance covered (mile) divided by tire
circumference, equals the number of revolutions the wheel makes
multiplied by the gear ratio to calculate the revolutions the engine
makes while covering the distance, divided by 2 (since it takes 2
complete revolutions of the engine to complete the firing cycle for all
cylinders) multiplied by the cubic inches of the engine equals the cubic
inches of air pumped. Now divide this by the distance (mile) and you
have the size of the opening.
Here is the formula for my engine geared for the Lakes:
Mile (63,360") / tire circumference (86") x gear ratio (3.18) / 2 =
1,171.41 x engine size in cubic inches (304) = 356,108.64 cu. in. / mile
(63360") = 5.62 sq in.
I have put this formula in the computer so I can plug in gear ratios and
cubic inches and get instant results.
The higher the gear the smaller the size of the opening needed.
According to Tom Burkland, if the size is right for the engine, baffles
are not needed inside the scoop because the pressure is equalized to all
cylinders. He also said that it is wise to oversize a bit, to allow for
going through the gears, since the calculation is for top gear.
Tom, Redding CA, #216 D/CC
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