Ok, so I know people tend to suggest some fairly hair-brained ideas on a friday (Mr.W and his moon escalator for eg!!) but I genuinely think this is right??

The general opinion seems to be that to get a ram air effect you have to be knocking on the doors of at least 120mph or probably more? And as far as I can figure this is the speed that air would enter the intake on a bike.

So, if for example the intake on the bike was 30mm high by 200mm wide giving an intake area of 6000mm squared. Now if I fit two intakes that both measure 6000mm sq. (or just one really big one at 12,000mm sq.) and then narrow it down at some point along the ducting back to the original size of 6000mm sq, then at 65mph I will have 130 mph air??

I appreciate that you might not want 130mph air, and the eg is based on random figures so instead you could increase by 50% or whatever but surely the principal is right or am I talking Horlicks??

I think you need to get into some really complicated flow dynamics. The walls of your intake create friction that slows the air down creating a boundary effect and loads of turbulence. It is possible to design a throat that actually gives you no flow at all through it if you are not careful!

Caber

Isnt it the pressure of the air in the airbox that is what matters with the ram air effect - increase the pressure and you can burn more fuel and so get more power (in a similar but smaller way than turbocharging)???

Isn't this known as the venturi effect? Or something to do with Bernoulli's Principle?

Restricting the air flow (or fluid flow) results in an increase in speed. As said, it'd be difficult to test without some expensive software unless you try to rig up a wind speed indicator and get a desk fan out!

Sure is!!
Trouble is that 99% of the time all you'll actually be doing is introducing a restriction into the carbs/EFI and you'll loose power.

You will get faster airflow but you'll get restrictions and parasitic losses. Taken to extreme you'll end up with an infintely small amount of very high speed air!!! Which won't produce any power!!

Ram air works by compressing air in a funnel. The idea is that you're going so fast that you hit the air in front of the car so fast that it doesn't let the air in the funnel escape, so it gets forced down the funnel and compressed.

At slower vehicle speeds, you're going so slow that the "bubble" of slow air at the mouth of the intake simply diverts incoming air round the side of the car. The bubble of air is compressed, slightly, but it is only millibars above atmospheric.

when stationary, air can be considered incompressible. At 400 MPH or so, it is considered compressible. Aircraft aerodynamics are different to cars because of this. The 120 to 200 MPH range of bikes is just at the low end of noticable effect.

You can't compress air with a funnel you only increase it's velocity ----- reducing the the pressure.

To increase the the pressure you need to slow the air down by smoothly increasing the cross section area it flows through -- this is called a diffuser a good example is a NACA duct.

Also if you look at pictures of F1 or F5000 cars of the late 1970s and 1980s you will see goo examples of ram air ducts The air inlet is invariably a "pitot" style inlet but the cross sectional area of the duct then gradually increases to reduce the velocity of the flow and increase the pressure at the engine inlet trumpets.

http://www.autoracing1.com/Images/historic/big/marioF-5000.jpg

[Edited on 25/4/08 by britishtrident]

Plumb an extra MAP sensor in the airbox, hook it up to a multimeter and see if it has changed. on holland the saying is Meten is Weten which translated says: measuring is knowing!

grtz Thomas

I had 5 minutes to spare and the back of an envelope so worked out the max possible pressure gain at 150 kph --- is really tiny.

The equation used is a simplified arrangement of the what is usually called the flow energy equation.

(C1^2 /2) + p1v1 =(C2^2 /2) +p2v2


The main advantage of a ram air box is ensuring a smooth flow of cold air and the containment of back spray from the carb mouths.

Probably the most beneficial effect of a scoop lies not so much in the "ram air" effect, but rather in the relatively cool air the engine gets. There can be a very large difference in intake air temps under the bonnet and outside air.

Any scoop should be located in a high pressure area, such as at the base of the windscreen or the leading edge of the nose, or placed outside the boundary layer of any other bodywork.