Be sure to measure whatever you build.
All is takes is a length of clear plastic tubing and some water to make a manometer.

An old post of mine:
------
Heard of the 'Ram Air' effect where the speed of the air hitting the vehicle
builds up pressure in a forward facing air duct and helps 'supercharge' your
engine a bit. ?

Well, at 'typical' street speeds the effect is very very small.

The formula for that is:

Inches of water pressure from Ram Air = (mph) times (mph) divided by 2025.

So at 70 mph the ram air effect is:
(70 mph) times (70 mph) divided by 2025 = 2.4 inches of water.

So the very best possible air inlet tube designed for Ram Air would actually
have a positive pressure of 2.4 inches of water.

How much pressure is an inch of water ?

One psi of pressure is equal to about 28 inches of water.

You can get this by dividing the cubic inches in one cubic foot ( 12 x 12 x
12 ) by the weight of one cubic foot of water ( 62.4 lbs).

The air pressure around us is averages about 14.5 psi at typical heights above
sea level.

So another way of looking at it is that the air pressure around us is about
400 inches of water. ( 14.5 x 28).

So the best Ram Air system at 70 mph gives you a boost of about 2/400 or one
half of one percent.
Small.

Ram air started in airplanes where it means more.
An airplane going 300 mph
gets a boost of:

300 x 300 divided by 2025 = 44 inches

This is a percentage boost of 44/400 or about 11%

Want a challenge?
Consider your tires.
Some strange facts:
At 70 mph the center of your tire is moving 70 mph.
The bottom of your tire where it 'grabs' the pavement is moving at 0.1 mph
The top of your tire is moving forward at 140 mph.
If you could put a 'Ram Air' inlet right at the top of the tires (where it is
horribly dirty and wet) you could get a significant air pressure boost.....