Up until maybe a few days ago i was on the "it doesn't work" boat.

Then I started talking to people, namely those that design racecars for a living.

Before I state what I think... or should I say his opinion, Id like to hear yours

 

I thought there wasnt a actual Ram Air effect till the speed of sound, but i think in most cases people just use it to bring fresh air to the engine.

 

Here is a link that I found a little while ago that is trying to disprove the function of ram air. http://www.vetteguru.com/ramair/

 

I disagree with it for 3 reasons.

1. sucuess we've had
2. who says the AIR in the intake track is going as fast as your car?
3. Who says you are compressing air?

 

That and how do you think spoilers and airplane wings work. Given speed aero forces can make things happen.. On a windy day stand in a ally way, you will find that the wind speed is faster there than in an open field. It's just a simple idea, it works just the same with a ram air system. have air moving in a large space compressed to a smaller space, and you have pressure buildup. Just try it once. It's an ausome effect.

 

thats exactly what i am (trying) to say. 70mph vehicle can mean MUCH greater air velocity in the intake track!

 

The article referenced above appears to be trying to deny the affect of "ram air" based on closed channel air flow. But that's not what ram air is. Consider the way they measure air speed in an airplane. They stick a pitot tube into the airstream, and measure the increase in pressure in the pitot tube. Knowing that pressure, you can calculate the air speed by using the velocity head equation.

If you work with the velocity head equation, it can be demonstrated that at 100mph the potential gain in pressure is about 0.18psi. That would translate into maybe 1% gain in air density, assuming you could "capture" the pressure gain with the inlet to the closed channel.... a well designed inlet might recover about 85% of the available pressure, while a simple "hole" might only recover 15%.

Using the above approach, with a WELL DESIGNED inlet, you might gain 1% HP at 100mph... or about 3HP on a stock engine. Since the velocity head is proportional to the square of velocity, you would only see a 1/4% increase at 50mph, and you would see a 2% increase at 141mph.

Then you need to face the issue of the actual velocity of the air at the inlet to the air intake tract. There is a "boundary layer" on the hood that generally provides a much lower air velocity the closer you get to the surface. That's why you see the Pro Stock air scoops sticking 15" above the hood, to get them out of the boundary layer.

It would be pretty easy to instrument a stock ram air setup with a pressure sensor (MAP type sensor would do) and measure the pressure in the airbox at road speeds, and compare it to the same setup sitting on a chassis dyno, with no fan.

The velocity of the air in the closed intake track is pretty much fixed by the displacement of the engine, rpm and volumetric efficiency. All you are doing with "ram air" is trying to increase the density of the air, so that at the same velocity it is flowing a larger mass of air.

 

Does 100mph of car velocity equate to 100mph air velocity in the intake tract though?

 

Ram air pressure is used to measure airspeed via a pitot tube, so ram air pressure exists, in spite of what we may read. Oops! I should have read Fred's post first. (Great minds etc... ) However, at normal vehicle speeds it's pretty much negligible, but if you can get to Mach 1 at sea level it's considerable.

The formula for ram air pressure is 1/2 density times velocity squared or Q = 1/2 x density x v^2. If you use sea level standard conditions, 50 mph is about .04 psi.

A few more speeds and approximate Ram Air Pressures:

70 mph = .08 psi
100 mph = .17
150 mph = .39
200 mph = .69
300 mph = 1.55
750 mph (about Mach 1) = 9.65

FWIW, "spoilers" disturb or "spoil" the flow over the wing so you don't get the low pressure on the top of the wing to create lift. Maybe you are thinking of "speed brakes" which create flat plate drag by sticking something into the airstream so that ram air pressure can exert a force on it.

A 8 ft^2 speed brake at 200 mph should give you 8 x 144 x .69 or about 795 lbs of drag. If you threw out the brakes at M1 it would be 11,000+ lbs. In that case the hydraulic cylinder(s) shouldn't be strong enough to fully extend the brakes. That's good. It keeps the "eyeballs out" negative g to a minimum in a fighter.

As was pointed out, expanding the area behind the inlet slows down the airflow and INCREASES the pressure. That's a lot how airboxes help. So 100 mph air entering an opening only stays at 100 mph if the cross sectional area remains the same as the inlet. If it increases, the air slows down and the pressure goes up. Vice versa also works.

Nextel Cup cars pick up air at the base of the windshield, which is probably the highest pressure area of the car (when it's travelling forward at high speed), so they may get a little more help. They also slow down and therefore expand the air into an airbox around the air cleaner.

Mainly you want pickup air in an area of the vehicle that is above atmospheric pressure and dump air (like from a radiator or intercooler or exhaust outlet) in an area that is below atmospheric pressure. That seems very intuitive, but people violate it all the time.

My $.02

 

And well worth it. Great post. Well I was meaning Car spoilers and airplane wings, but using them like that got the same point across as well.

 

It doesn't matter.

The air in the stock LT1 TB or MAF (for the sake of picking an example) is moving at about 220fps at max rpm.... or about 150mph. If the source of the air is "still", the air has to transition from "0" to 150mph as it enters the closed track. If you are moving at 100mph, and the inlet is correctly designed, the air only has to transition from 100mph to 150mph.... a net gain of 100mph in relative velocity at the transition.

But the air in the closed track (pipe) is still moving at 150mph.... it is just 1% denser.

 

I think the location of the ram air plays a big part also. If the inlet was placed in free air away from the vehicle at 100mph it would benefit from an extra 0.17psi of pressure as stated above but, due to the aerodynamics of the car you may see more or less. The front of the car and windshield areas probably will have an even higher pressures while the middle of the hood may even have a vacumm effect at higher speeds since the air goes over the nose of the car and straight over the hood.