The formula is MPH^2*0.0000176.
Air is considered uncompressible at low speeds, so the pressure isn't as much as you would think. It certainly isn't going to supercharge the engine.
But don't think it won't produce horsepower. When the intake valve opens the piston pulls a vacuum compared to atmospheric pressure. Depending on the engine size and the intake plumbing, you can see -.5 to -2.0 psi.
There are three ways to help cope with this.
1. Supercharger. Pushing the air in under pressure is always a good thing.
2. Intake ram air tuning. When the intake valve closes, the air rushing in the intake has no place to go and causes a reverse pressure wave. A tuned stack can take advantage of this to reflect the wave back down the intake just as the intake valve is opening to push air into the cylinder. Only works at one RPM.
3. Ram air intake. When you look at the massive intake on an NHRA pro stocker, you would think he must be getting 50 psi on the top end. Nope. The size is relative to the amount of air the engine needs. The larger you get, the more drag and the drag exceeds the horsepower created. Don?t ignore that .396 psi at 150 mph! The engine is gasping for air with no ram air, and the ram air can take that slight negative pressure and turn it slightly positive. Horsepower and speed are never gained in huge amounts, but done in very small increments of work. Ram air is one of those dyno killers. You can't duplicate the air flow over the car into the intake on a dyno.
From Sumners web page:
http://purplesagetradingpost.com/sumner/bvillecar/bville-tburkland-1.html
John Burk presented the following two posts on Landracing.com in regards as how you figure the scoop inlet opening size for a motor:
For calculating the size of the scoop inlet you need to find how many inches you go in two
turns of the engine . Say you have a 300 ci 200 mph roadster with a 3.5:1 rear and 30"
tires . In two turns of the engine the wheels turn .57 times (2 divided by 3.5) . The tires
are 94.2" around (30 x 3.14). So in two turns of the engine the roadster moves 53.7"
(94.2 x .57) . The 300 ci x 53.7" long bar of air that got scooped up in 2 turns of the
engine has a cross-section of 5.6 sq. in. (300 ci divided by 53.7") . The opening in the scoop
should be 2.67" dia. or anything with 5.6 sq. in. This is for an unblown engine and for a
trans with a 1:1 high gear . We'll forget about details like tire growth and engine
volumetric efficiency but we should add 5% (5.6 + 5% = 5.88 sq. in.) for the supercharge
effect we've gained at 200 mph .
(I then asked how you figure the drop of air speed in the inlet (scoop) if the cross-section of the inlet increases towards the motor)
Sum
If you want the air to slow from 200 mph to 40 mph (1/5 the speed) the area needs to
increase 5 times . If the inlet is 5 sq.in. it should increase to 25 sq. in.
If the inlet is 2 1/2" round (4.9 sq.in.) 5 times that is 24.53 sq.in. and the dia of that is
5.59".
The area of a circle is dia.x dia.x .785
The dia of an area is the area divided by .785 and the square root of that (divide before
you hit the square root button).
By the way I think the inlet needs to be radiused for the lower gears when all the air
doesn't come from straight ahead .
